Working With Liquids Lab Key

Working With Liquids Lab Key

1. Decanting a liquid:

Decanting is simply pouring off a liquid without loosing any of the more dense substance (usually an insoluble solid) in the bottom of the container. Chemists are most often after the substance at the BOTTOM of the container.
There is a limit to the amount of liquid that can be decanted. It is most important that NONE of the substance at the bottom of the container is lost in decanting.

2. Reading liquid volume:

Attraction between molecules of the same substance is called cohesion. Attraction between molecules of diffferent substances is called adhesion.
Because of its high adhesion force, water will climb the sides of its container, causing the surface to curve upward slightly. This happens in all containers, no matter the diameter. When viewed from the side, this curve is called a meniscus.
Scientific measurements are made by reading the BOTTOM of this curve.
Mercury is one of the only room temperature liquids whose cohesive forces are greater than the adhesion force, causing its surface to curve downward in a tube.

3. Transferring a liquid with a pipet:

A volumetric pipet only has one graduation mark. These pipets are designed to measure only one volume. The drawings below are of a volumetric pipet.
A Mohr pipet has marks similar to a graduated cylinder, to measure different volumes with the same pipet.
A third type of pipet, the dropper pipet (commonly called a medicine dropper) is designed to deliver small amounts (drops) of liquid. Most dropper pipets have a bulb as part of the dropper and do not have graduation marks.

Draw liquid into a pipet with a pipet bulb, NEVER by mouth like a straw. BUT...do not draw the liquid all the way up into the bulb! When liquid has been drawn ABOVE the needed volume, quickly remove the bulb and place a finger over the top to hold in the liquid. Roll the finger SLIGHTLY to allow liquid to slowly drain from the pipet to adjust the volume. This takes practice. When adjusted to the correct volume, remove your finger completely and allow the liquid to freely flow out of the pipet. DO NOT remove the liquid remaining in the tip.

4. Filtering to recover an insoluble solid from solution:

Gravity Filtration
Gravity filtration is the most common type used in basic science labs.
Support the funnel with a ring stand. If the ring is too big to hold the funnel, set a wire triangle across the ring and put the funnel through the triangle.
NEVER allow the filtering sample to go over the top of the paper. This allows solute to escape around the filter and increases experimental error.

Vacuum Filtration
Vacuum filtration reduces filtration time.
The filter funnel is prepared in the usualy way, then attached to a filtering flask (near right). This flask has a nipple to attach a hose from a vacuum pump.
Many lab water facuets have aspirators (far right) attached for this purpose. First, the filter flask and the aspirator are connected with rubber tubing. When the faucet is turned on, a stream of water flows past the nipple on the aspirator, pulling air out of the flask, producing a vacuum. While this vacuum is fairly weak (about 24 mm Hg), it is enough for most chemical and biological lab filtrations.

5. Using a separatory funnel:

If liquids are mixed, but not mutually soluble, they can be separated by allowing them to set in a separatory funnel until layers form. As long as the liquids have different densities, they will form layers - with the most dense layer at the bottom.
The liquids can be drained from the bottom of the separatory funnel one at a time.

6. Boiling to recover a soluble substance from solution:

A solution containing a solid substance dissolved in a liquid can be separated by evaporation; leaving the solid behind.
It is important to reduce the heat when the liquid level gets low. As the solid crystallizes, high heat will cause it to spatter - which may lead to some of the solid popping out of the beaker - increasing experimental error.
A solution of soluble liquids can be separated by distillation.
The liquid with the lowest boiling point will begin to turn into a vapor first. The temperature will not change until all the first liquid has been removed. When temperature begins to change, it is time to change the collecting vessel to catch the next liquid to boil.
The vapor travels through the transfer tube, which is surrounded by the condensing tube, to the collecting vessel. The condensing tube has water circulating through it to cool (condense) the vapor back into a liquid.

DO NOT reduce heat by adjusting the gas flow to the burner. To heat "slowly", physically move the burner back and forth under the container! (diagram at right)
To keep a container from boiling over, move the burner from under the container until it "settles down".
Boiling chips may be used.

7. Boiling point determination:

Boiling point is formally defined as the temperature at which the vapor pressure of a liquid is equal to atmospheric pressure. In practical terms, the boiling point of a liquid is the temperature at which it is converted into a gas (vapor).
The diagram on the right shows an apparatus that uses a capillary tube for boiling point determination. When the capillary tube is placed in the liquid, open end down, surface tension keeps the liquid from rising into its small diameter opening. That means that the capillary tube contains air at atmospheric pressure when the test begins.

TWO assumptions are made:

The temperature of the test liquid will be the SAME as the temperature of the hot water bath.

The boiling point temperature of the test liquid will the LESS than the boiling point temperature of water.

Suspend the apparatus in a hot water bath (a beaker of water which is to be heated).

No part of the apparatus should touch any part of the beaker! You want to measure the temperature of the water, NOT the beaker!

Begin heating the water bath, gently stir the water, and read the temperature.

When bubbles begin coming out of the capillary tube - Watch closely! They will be small! - turn off the heat source.

Continue stirring the water and reading its temperature.

When bubbles stop coming out of the capillary tube, the vapor pressure of the test liquid is equal to atmospheric pressure - that's the boiling point temperature!
Pure liquids have very specific boiling points, so an exact boiling point determination can be used to identify an unknown liquid.