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Chemistry Lab Equipment & Procedures
Identification & Usage Guide
by David Arrow
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Equipment
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Lab
Procedures
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Equipment
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Chem Lab EquipmentAlphabetical Listing
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Click on an object name to learn about it.
Chem Lab EquipmentCategorical Listing
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Lab Procedures
13. Methods of Separation
A. Filtration
G. Separatory Funnel
H. Electrolysis
* Some of the above links require an internet connection
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Beakers
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Beakers are used to measure approximate volumes of liquids. They are not to be used to measure exact amounts as they are not very precise and often not accurate.
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Beakers
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Beakers should not be used to measure exact amounts; but rather to mix, transport, and hold chemicals while they react.
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Beakers
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The volume of liquid should be read from the bottom of the meniscus.
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Beakers
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White
Patch
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Beakers should be labeled by writing in PENCIL on the white patch. This can be easily erased.
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Labeling Glassware
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Beakers
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Beakers are named according to their capacity. Pictured above are a 150mL beaker and a 250mL beaker. (Notice that this is usually more than is marked to measure on the beaker.)
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Erlenmeyer Flask
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Like beakers, Erlenmeyer Flasks are not used to measure exact volumes. Their triangular sides allow for swirling and mixing of chemicals with less chance of splashing out.
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Erlenmeyer Flask
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Use Erlenmeyer flasks when filtering to catch the filtrate or when dispensing a liquid with a burette. The conical shape of the flask will keep the liquid from splashing out more than a beaker.
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Erlenmeyer Flask
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Erlenmeyers are also used instead of beakers when a solution needs to be stored. Rubber stoppers can be placed in the opening.
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Florence Flask (Boiling Flask)
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Florence flasks are also called boiling flask. They have a rounded body with flat bottom and a small cylindrical opening.
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Florence Flask (Boiling Flask)
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Florence flasks are used when a liquid needs to be boiled vigorously. Their smaller opening prevents the liquid from splashing out when boiling, and they are generally more resistant to cracking due to high temperatures
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Volumetric Flasks
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Volumetric Flasks are flasks that are designed to accurately measure only one specific volume of liquid (100 mL, 250mL, 500mL, etc depending on size). Thus, they only have one mark on them.
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Volumetric Flasks
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Volumetric Flasks are typically used when preparing a solution of a known concentration. These flasks should not be used to heat or store substances.
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Volumetric Flasks
Meniscus
Markonflask
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BottomofMeniscus
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Liquid should be added until the bottom of the meniscus is just above (but touches) the mark on the flask. A medicine dropper is usually used for the last few milliters. A stopper is placed on top to shake.
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Preparing Solutions: Step 1
Do the Math
Volumetric Flasks are usually used to prepare a solution of a specific concentration. Molarity is typically used by chemists to measure concentration.
Molarity = moles of solute
liters of solution
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1. Use the molarity formula and do the math to determine the needed moles of solute
Then, Convert the moles of solute into grams to determine how much you will need to measure
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Preparing Solutions: Step 2
Pouring Solute into Flask
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2. Measure out the proper amount of solute and place it into the proper volumetric flask
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Preparing Solutions: Step 3
Adding Water to Finalize Solution
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3. Add deionized water to the flask until you have the desired amount of solution. Be sure not overfill.
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Micro-Well Tray (ChemPlate)
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This is a plastic tray that contains 12 numbered wells and one large well. This tray is used to mix chemicals and view reactions on a small scale.
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Micro-Well Tray.. Keep Track!
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Graduated Cylinders
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Graduated cylinders are useful for measuring and dispensing a measured amount of liquid volume. Generally, the smaller cylinder, the more precise the measurement; so use the smallest cylinder possible.
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Graduated Cylinders
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Most cylinders will have a plastic bumper on them. This is used to prevent the cylinder from breaking if it tips over. The bumper should be kept near the top of the cylinder.
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Graduated Cylinders
Read from bottom
A Correct reading
is 52.7 mL
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When measuring the volume in a graduated cylinder, always read from the bottom of the meniscus.
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Pipettes
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Pipettes are very precise for measuring and dispensing a desired volume. They are more precise than graduated cylinders and should be used when a high degree of precision is needed.
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Pipettes
Graduated Pipette
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Volumetric Pipette
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Both volumetric and graduated pipettes are used. Pipettes come in many different sizes. The smallest pipette that can hold the desired volume should be chosen. For example, if you need 9.00 mL, use a 10mL graduated pipette instead of a 25mL pipette.
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Pipettes
Many marks on a graduated pipette
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Single mark on a volumetric pipette
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Both volumetric and graduated pipettes are used. A volumetric pipette usually has a bulb in the middle and is only marked to measure one specific volume. Graduated pipettes have many graduations on them to measure many volumes.
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Pipette Bulb
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Pipette bulbs are used to suction a liquid up into a pipette when measuring a liquid volume with a pipette.
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Pipette Bulb
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Pipette bulbs are used to suction a liquid up into a pipette when measuring a liquid volume with a pipette.
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Using a Pipette Bulb
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Burette
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A burette is a long, graduated cylinder with a stopcock at the bottom to control the dispensing of the liquid from the burette. The stopcock can be adjusted to dispense the liquid quickly or drop by drop.
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Performing a Titration
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Burettes are usually used in titrations. A titration is simply a controlled chemical reaction where the amounts of solutions mixed are carefully measured. The purpose is almost always to find the concentration of an unknown solution.
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One solution concentration is known and used to determine the concentration of a solution whose concentration is not known.
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Performing a Titration
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Medicine Dropper Pippette
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Small pipette used to dispense liquids drop by drop. Often used with a volumetric flask to finish filling.
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How to Use a Medicine Dropper
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Metal Spatula
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Small metal scoop used for transferring small amounts of chemicals. This is not to be used to stir chemicals as the iron in the spatula will react with many chemicals.
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Plastic Spoon
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Small metal scoop used for transferring small amounts of chemicals. This can also be used to mix or stir chemicals.
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Glass Stirring Rod
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Small, solid glass rod. This is used to stir and mix chemicals during a reaction. Glass will not react with most chemicals, so this is the best choice for stirring.
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ParaFilm
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ParaFilm is a transparent, heavy-duty plastic that is tacky and stretchible. It is used to cover test tubes when shaking to protect skin. It can also be used to seal glassware when storing a solution overnight.
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Using ParaFilm
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Crucible
Image: maxpixel.net
Image: www..sciencecompany.com
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Image: http://nobel.scas.bcit.ca
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A crucible is a vessel that is used to hold materials while they are being heated to extremely high temperatures. They are usually made out of clay, ceramic, or porcelain that can withstand high temperatures.
They are most often used for holding metals that are heated to their melting point.
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Triangle, Clay
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Image: http://myriverside.sd43.bc.ca
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A clay triangle is used to suspend a crucible while it is being heated. They are made out of ceramic material that is capable of withstanding high temperatures.
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Crucible Tongs
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These tongs are good for many general uses such as picking up crucibles and grasping metals to be heated in a flame. These tongs are not to be used to hold beakers or test tubes.
See the following items for proper equipment for beakers.
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Beaker Tongs
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These tongs have a wider shape and rubber coating to enable them to pick up beakers.
Since rubber is flammable, these should NOT be put in an open flame!
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Tongs, Hot Beaker
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These tongs have a heat resistant padding on the end and are used to pick up beakers that are very hot (such as those on hot plates or those just heated by bunsen burner).
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Test Tube Holder
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This instrument is the one that should be used for holding a test tube while you are heating it in a flame. Remember, tongs and forceps should NOT be used for holding test tubes.
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Test Tube Holder
Clamps near top
of test tube
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Grasp test tube
holder at end
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The test tube is placed gently in the holders so that the clamps are near the top of the test tube. The test tube holder should be held at the end and not squeezed at the middle.
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Test Tube Brush
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These brushes are used for cleaning out test tubes and small glassware.
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Forceps (Tweezers)
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These are a good multipurpose-use instruments for grasping items. Due to the short handle, they should not be used for holding an object in a flame.
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Ring Stand
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This stand can be used with a ring clamp (for filtering or heating beakers) of a clamp for holding test tubes.
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Funnel
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Funnel made of plastic (glass funnels are also available). Used for holding filter paper when filtering (see next few slides).
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Filter Paper
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Filter paper is used to separate a solid from a liquid. It comes with different sized pores. The paper with smaller pores will filter out more particles, but also takes longer for the liquid to pass through the paper.
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Filter Paper
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1. Fold filter paper in half
2. Fold filter paper in half again
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3. Pull back one ply of the paper so that it forms
a cone shape.
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Filtering Set-up
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1.Placefoldedfilterpaperinfunnel
2.Placefunnelinringstand
3.UseErlenmeyerFlasktocatchfiltrate
4.Adjustheightofringclampsothatfunneltipisinsideofflask
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Filtering Set-up
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5. Slowly pour in solution to be filtered
6. Try to only pour in liquid, keeping most of the solid out of filter.
7. Do not allow liquid level to reach above edge of filter paper.
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Filtering Video
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If video does not play, click here or go to https://www.youtube.com/watch?v=jbVxg8SFY38
Decanting
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Solid Stays
in Beaker
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Only Liquid is
Poured Out
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A quicker alternative to filtering is decanting to separate a liquid from a substance. When decanting, you first let the solid settle to the bottom and then carefully pour off only the liquid, leaving the solid behind.
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Decanting
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Stirring Rod
Held Touching
Spout of
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A glass stirring rod should be held at the spout of the beaker. This blocks the solid from escaping the beaker as well as controls the flow of liquid. Next, the solid is rinsed with DI water and decanted again.
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Decanting
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Decanting is not as complete and as pure as filtration (some of the solid sometimes falls through), but it is much faster. However, with the rinsing step and practice, it can be an efficient way to separate substances.
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Decanting Video
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….Only need to watch video up to 5:58
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If video does not play, click here or go t https://www.youtube.com/watch?v=S09SA_QApZ4&start=318&end=358
Centrifugation
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A centrifuge is a device used to separate mixtures. The sample is spun at high speeds until the components of the mixture form distinct layers in the vial.
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Blood Before
Centrifugation
Blood After
Centrifugation
Centrifugation
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A centrifuge is effective when the mixture you are separating contains substances with different densities, such as a solid dispersed in a liquid. Such as separating…
• the pulp from orange juice
• cream from raw milk
• blood cells from plasma
At above right, you see plasma layer and separate, dark colored blood cell layer after centrifugation
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Centrifugation
Blood Sample
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Plasma
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White Blood Cells
The centrifugal force exerted by the spinning causes the more dense substances to be pulled to the outside (bottom) of the tube, while the least dense substances remain at the top
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Red Blood Cells
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Centrifugationhttps://www.youtube.com/watch?v=KEXWd3_fM94
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A Follow this link to watch a video of centrifugation
Paper Chromatography
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In paper chromatography, a liquid mixture is separated by placing some absorbent paper containing a small sample of the mixture in a solvent. As the solvent rises up the paper by capillary action, the mixture is separated into its different components
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Paper Chromatography
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Absorbent
Paper
Those most attracted to solvent rise up the farthest
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Chromatography works because the different substances in the mixture (ink) have different levels of attraction to the solvent.
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Less attracted inks “fall out” sooner
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Inks in mixture are attracted to solvent
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Solvent rises up paper due to capillary action
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Paper Chromatography
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Paper chromatography can be used to separate a mixture containing at least some polar compounds.
There must be a significant difference in the polarity of the substances in the mixture you are trying to separate. This assures that there will be a difference in the attraction to the solvent.
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There are many different types of chromatography that are commonly used in analytical labs to analyze and identify samples.
Paper chromatography is often used to separate inks or pigments in a mixture, but is also used to separate amino acids or to determine the presence of certain drugs or hormones in a sample.
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Paper Chromatography
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If video does not play, click here or go to https://www.youtube.com/watch?v=3ZP_E0eTmMU
Extraction by Separatory Funnel
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A separatory funnel can be used to separate two immiscible liquids. Liquids are generally immiscible if one is polar and the other is nonpolar.
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Extraction by Separatory Funnel
Since the layers don’t mix, you will be able to see a dividing line between the two liquids.
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Since you can see the division, use the funnel to drain one liquid out first and leave the other.
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Extraction by Separatory Funnel
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If video does not play, click here or go to https://www.youtube.com/watch?v=EFiFPoOzqtk
Distillation
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Distillation is a process that can be used to separate 2 liquids that are mixed together. In the example pictured, a mixture of liquid ethanol and water are being separated.
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Distillation
The mixture of liquids is
heated until one begins to boil
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Distillation
The mixture of liquids is
heated until one begins to boil
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In this case, the ethanol
begins to boil when the temp
gets to 78º C
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Distillation
The mixture of liquids is
heated until one begins to boil
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In this case, the ethanol
begins to boil when the temp
gets to 78º C
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As more heat is added, the
Temp stays at 78 º C while all
of the ethanol evaporates and
the water all remains as liquid
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Distillation
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Cold water is run through
a double-wall
condensation tube
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When the ethanol vapor hits
The cold wall of the tube,
It condenses back into liquid.
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Distillation
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Only
Waer
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The original flask will contain
only water and all of the ethanol
will be in the collection beaker
Ethanol
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When the bubbling stops
and the temp starts to rise, the
Distillation is complete
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Distillation
Distillation is used when separating a mixture of 2 miscible liquids that have significantly different boiling points
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Antifreeze is a mix of ethylene glycol and water
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Ethylene glycol & water
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Acetone boils at 56ºC , so it boils well before water
Ethylene glycol boils at 197 ºC, so water boils off first, & Ethylene glycol is left
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Distillation
Note the differentboiling points of each substance
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Distillation is used to separate crude oil into its many different components
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Distillation Video (Internet required)
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If video does not play, click here or go to https://www.youtube.com/watch?v=74rOJ1GFbC0
Electrolysis
Electrolysis of Water
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Some compounds can be separated into the elements by passing electricity through them. This process is know as electrolysis.
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Electrolysis
When electricity is passed through a conducting substance, the energy from the electricity can break apart the chemical bonds.
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In this picture, sodium chloride, NaCl, is being broken down into sodium and chlorine
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Cl
Na
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Electrolysis - https://www.youtube.com/watch?v=i9xS9t-KMpc
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In the video, you are seeing lead bromide being broken down chemically into lead and bromine
Lead bromide Lead + Bromine
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Hot Plate & Magnetic Stirrer
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Hot plates are used to do a controlled heating of a substance, such as boiling off water. Most also have a magnetic stirrer feature that can be used to stir a solution for a long period of time.
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Evaporating Dish
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Evaporating dishes are made of porcelain that is chemically resistant and very heat tolerant. They can withstand temperatures of over 1000 °C.
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Using an Evaporating Dish
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Non-flammable liquids are often evaporated by placing an evaporating dish on a ring clamp and heated with a bunsen burner.
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Using an Evaporating Dish
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Watch Glass
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A watch glass is a contact lens-shaped piece of glass used to either (1)evaporate a liquid or (2) as a cover for a beaker that is being heated.
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Watch Glass
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A watch glass is often used when evaporating a solution so that any residual material or precipitate can be easily seen. The watch glass can be placed on top of a boiling beaker to speed up evaporation.
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Watch Glass
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A watch glass can be used as a beaker cover. The glass keeps out impurities, but does not seal the beaker completely. Thus, evaporation is still allowed to occur within the beaker.
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Drying Oven
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Drying ovens are used to keep a chemical in a controlled, warm environment. They are most often used to evaporate all water from a substance or solution.
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Crystallization
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A solid solute dissolved in a liquid can often be separated by crystallization. In this process, the liquid is allowed to evaporate, leaving the solid behind crystallized on the container.
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Info
Crystallization - https://www.youtube.com/watch?v=-fGD4NHgp6c
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Click here if video won’t play
The video is a time lapse photagraphy of saltwater evaporating. Only the water evaporates, leaving the salt behind. Note the cubic shape of the salt crystals as they form.
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Crystallization
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Crystallized
Solid
(on inside surface
of flask)
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Note that the crystallized solid is usually “stuck” to the container and is difficult to scrape out. In a quantitative lab, the flask mass is first measured empty, then the flask is measured after crystallization (with crystals in it). The difference between the 2 is the crystal mass.
MassCrystals = Massflask+crystals - Massflask
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Striker
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A striker contains a small piece of flint that rests against a rough metal. Grinding the flint creates a spark which is used to ignite flammable gases (such as in a bunsen burner)
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Bunsen Burner
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A bunsen burner controls an exothermic (energy is given off) chemical reaction between methane gas and oxygen. The chemical equation for this reaction is
CH4 + 2O2 CO2 + 2H2O
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Bunsen Burner
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The reaction produces water and carbon dioxide along with a tremendous amount of heat. We use the heat produced from this reaction to heat other objects.
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4 Parts of the Bunsen Burner
-turningbarreladjusts
flametype(blue,yellow)
1.Barrel
Clickoneachofthe4partsoftheburnertolearnaboutit.
2.Rubber Tubing
•connectstogasjet
•methanegasflowsthrough
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3.Air Intake Hole
•controlstheamountof
oxygeninmix
•affectstypeofflame
(yelloworblue)
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4.Gas Control Valve
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•controlstheamountof
methane
•adjustsheightofflame
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How to Light a Bunsen Burner
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Standard Flame of a Bunsen Burner
Outer cone
(Dark blue)
Inner cone
(Light blue)
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Wire Gauze Pad
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A metal mesh wire containing a heat resistant pad made of ceramic fibers. This pad can be used as a “hot plate” to set hot items on, but it is most often used with a ring stand to heat beakers.
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Set up for Heating a Beaker
1.Placewiregauzepadontopofringclamp.
2.Adjustringclampsothatitisabout2inchesaboveburner.
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Set up for Heating a Beaker
3.Placebeakerontopofwirepad.Besurethepadisflatandthebeakerisstable.
4.Removetheburnerfromunderthebeakerandlighttheburner.
5.Placetheburnerunderneaththebeaker.
6.Adjusttheflameheightsothattipofinnerlightblueconeisalmosttouchingbottomofbeaker.
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Spectrophotometer
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How to
use the
Spec 20
This spectrophotometer passes a specific wavelength of light through a solution sample and measures how much of that light is absorbed. It is used for determining the concentration of a solution.
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Electronic Balance
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Electronic balances are used for measuring the mass of an object. One advantage of using electronic balances is the tare button, which subtracts any mass on the balance. They are also much faster to use and easier to read.
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Electronic Balance
Level
Bubble
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Electronic balances are very sensitive and need to be level to work properly. Even leaning on the table or counter will mess up the reading. Some are equipped with a level bubble to show when it is level.
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Electronic Balance
Properly leveled balance
Bubble outside of circle
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Adjusting
the legs
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The back “legs” on such a balance need to be turned to make the balance level. The balance is level when the bubble is inside of the circle on the bubble level.
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Using an Electronic Balance
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Use the TARE button carefully! Make sure the balance reads “0.00” before you place the object(s) that you desire to measure on it.
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Using the Electronic Balance
Properly leveled balance
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Use the TAP method to get the amount of chemical desired without going over. Chemicals should NEVER be dumped back into the reagent bottle once they touch another container.
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Dial-O-Gram Balance
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Dial-O-Gram balances are very accurate at measuring the mass of an object. Although they are well-designed and much faster than triple-beams, they can be difficult to read. When available, electronic balances are preferable.
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Triangular File
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Triangular files are metal files that can be used for a variety of purposes. In addition to filing off corrosion from metals, they are also often used to etch a groove in glass tube in order to cut it to desired size.
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Litmus Paper
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Litmus paper is paper that has been treated with litmus indicator solution and is used to determine whether a liquid is an acid or a base. Red paper will turn blue in a base and blue paper turns red in an acid.
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Universal Indicator Paper
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Like litmus paper, universal indicator paper will turn different colors in acids and bases. The advantage to this paper is that it will indicate whether the acid or base is weak or strong. However, it is not very good at distinguishing solutions that are neutral or weakly acidic or basic.
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Wash Bottle
Wash bottles usually are filled with deionized water. They are usually used as “rinsing” bottles to rinse off equipment. We often use them to rinse off probes such as pH probes between uses.
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Revised by:
David Arrow of West York High School
Based on an original powerpoint by: THE SCIENCE LEARNING CENTER
THE UNIVERSITY OF MICHIGAN-DEARBORN - Angela Nolte, Dawn Wisniewski, Saadia Yunus, and Ruth Dusenbery
General Introduction
The Spectronic 20 (Spec 20) is a particular model of spectrophotometer.
Spectrophotometers are instruments that measure the amount of light absorbed by solutions.
The purpose is to ultimately determine the concentration of a solution based on how much light is absorbed
What Spectrophotometers Do:
More
If video does not play, click here or go to https://www.youtube.com/watch?v=pxC6F7bK8CU
What Spectrophotometers Do:
Spectrometers pass light of a specific wavelength (color) through a sample
Some of the light gets absorbed by the sample and some passes through it. The detector senses the light that passes through the sample
ON
OFF
Beer’s Law
The more concentrated a colored solution is, the darker it appears.
The more concentrated solutions appear darker because they absorb more light.
More Concentrated
Beer’s Law
There is a directly proportional relationship between the absorbance and the concentration of a solution. (the higher the concentration, the higher the absorbance will be)
Beer’s Law: Building a Calibration Curve
.08
.16
Standard #Concent.(Molarity)Absorb.1234
First,asetofsolutionsispreparedwhoseconcentrationisknown(calledstandards).
.24
.32
Beer’s Law: Building a Calibration Curve
.08
.16
Standard #Concent.(Molarity)Absorb.1234
First,asetofsolutionsispreparedwhoseconcentrationisknown(calledstandards).
Next,aspectrophotometerisusedtomeasuretheabsorbanceofeachofthosestandardsolutions.
.24
.32
.52
.13
.42
.27
Beer’s Law: Building a Calibration Curve
Then, those points are plotted on a graph and a best-fit line is established showing the relationship between concentration and absorbance of the standard solutions
This is called a calibration curve
Concentration vs Absorbance of Standard Solutions
Plotted Absorbances
Of Standard Solutions
Beer’s Law: Using a Calibration Curve
?
Nowtheconcentrationofasolutionofunknownconcentrationcanbedetermined.
Theabsorbanceoftheunknownismeasured.
Theconcentrationoftheunknowncannowbedeterminedfromthecalibrationcurve.
Absorbance
Of Unknown
.37
.21 M
Using a Spectrophotometer:
More
If video does not play, click here or go to https://www.youtube.com/watch?v=QdufRwbkeKo
Choosing the Wavelength
When measuring the absorbance of your sample, you want to choose a wavelength of light that is absorbed well by the sample you are analyzing
Absorbance of
CuSO4
Notice that CuSO4 allows blue and purples to pass through, but absorbs red light well
Using a wavelength of 600-650 nm works well for ______CuSO4
Choosing the Wavelength
When measuring the absorbance of your sample, you want to choose a wavelength of light that is absorbed well by the sample you are analyzing
Absorbance of
CoSO4
Notice that CoSO4 allows red and yellow to pass through, but absorbs blue-green light well
Using a wavelength of 475- 550 nm works well for ______CoSO4
Set Wavelength
790
The wavelength selector is on the top right side of the instrument.
The dial is turned to select which wavelength of light will be passed through your sample.
810
830
Cuvettes
All readings are done in cuvettes, which resemble small glass test tubes, but are made from higher quality glass.
Usually you will need two cuvettes to take the readings -- one to hold the water blank and one to hold the sample solution.
Preparing the Cuvettes
Always rinse out your cuvettes first with water
Next, pour a small amount of your sample in the cuvette and rinse the cuvette out with your sample
This way, your cuvette does not need to be dry. If it is wet with a few drops of your sample, it will not dilute your sample.
Filling Volume
Fill with sample to about here
When pouring a liquid into the cuvette, the solution must fill the cuvette to a sufficient height so that the internal light beam passes through the solution in the cuvette, and not through air.
Beam of light passes through about here
Air Bubbles
Errors may occur in a reading if air bubbles are present in the solution.
Before reading any sample (even a blank), you must remove all air bubbles.
Removing Air Bubbles
Removing air bubblescan be done by
tapping the bottom of the cuvette to dislodge the bubbles.
If tapping does not work, then cover the top of the cuvette with Parafilm (a stretchy plastic covering) and slowly invert the cuvette several times until all the bubbles are removed.
Kimwipes
The proper material used to clean the surface of a cuvette is called a Kimwipe.
It is important to clean the outside, lower portion of a cuvette before taking any readings. Fingerprints, liquid droplets, and smudges on the cuvette surface may absorb the light and affect your absorbance readings
Clean With Kimwipes
Touch only
This top part
of cuvette
Wipe the outside of the cuvette with a Kimwipe.
After cleaning the cuvettes, handle them by their tops only. Don’t touch the lower portion of the glass, or you’ll have to clean it again.
Inserting the Sample
Vertical Index Mark
To assure reproducible positioning in the sample chamber, the cuvette has a vertical index mark (line or arrow) near its top.
When inserted properly, the vertical index mark on the cuvette must be exactly aligned with the small nub on the top of the sample holder.
Nub
Final Step
Close the cover to the sample chamber, then make your reading.
If the cover is not closed, stray light can enter and give false readings.
Summary – Inserting a Cuvette
Rinse the inside several times with the solution that is going to be measured.
Fill the cuvette to the writing.
Remove any air bubbles by tapping or inverting.
Clean the outside surface with Kimwipes.
Gently insert the cuvette into the sample chamber.
Align the vertical index mark on the cuvette to the nub in holder.
Close the chamber cover before taking a reading.
Learning to Read the Meter
Before you begin to work with sample solutions, it is wise to learn how to read the Spec 20’s scale for light absorption.
The meter simultaneously indicates Absorbance (the amount of light absorbed by the sample) on the lower scale and Percent Transmittance (the portion of light passing through the sample) on the top scale.
Absorbance and Transmittance are opposites. 100% Transmittance means 0 absorbance
Learning to Read the Meter
Top scale is read to the right.
The top scale (%T) is divided into increments of constant size, and must be read from left to right. This linear scale is easy to read.
The bottom Absorbancescale has increments between tick marks which vary across the scale and must be read from right to left. This non-linear scale is more difficult to read accurately.
Bottom scale is read to the left.
Learning to Read the Meter
Avoid Parallax Error
Parallax is an error in reading an instrument caused by the observer looking at an angle.
To avoid parallax error when reading the meter, your head must be directly in front of the needle (as shown here).
Learning to Read the Meter
Avoid Parallax Error
Adjust viewing angle
until the needle and its
shadow on the mirror
are aligned.
If you see the reflection of the needle in the mirror, then you are not looking straight on.
Remove Cuvette After Reading
Always remove the cuvette from the sample holder as soon as the necessary adjustment or reading has been completed.
Leaving the cuvette in the sample holder for an extended period can damage the light sensing device.
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Miscibility -
Liquids are said to be “miscible” if they mix together to form a homogeneous solution.
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“Ring”
dividing
layers of
2 liquids
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Ethanol & Water Mix
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Miscible
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Polarity -
A molecule is polar if it has a positive side and a negative side
δ-
Negative Side
Electrons are evenly spread
(no build-up of
electrons anywhere)
O
H
H
Water
Positive Side
δ+
F
F
δ+
“Ring”
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Negative Side
(build-up of
electrons on
this end)
δ-
δ+
H
F
No positive or negative end
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Positive Side
(loss of
electrons on
this end)
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Nonpolar
Polar
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Differences in electronegativity and shape determine if the electrons in a molecule are spread evenly or not. If they are evenly spread, the molecule is nonpolar. If they “get pulled” to one side, it is polar.
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Capillary Action -
Refers to a liquid rising up a narrow tube-like space to due to an attraction to the tube
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“Ring”
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Adhesion
- how strongly a
substance is attracted
to another surface
Cohesion
- how strongly a
substance is
attracted to itself
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“Higher polarity means higher rising” - the more polar the molecule it is, the more it will be attracted to a tube with charges. Therefore, the higher it will rise up the tube.
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