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Laboratory Laboratory Operations Operations

and and TechniquesTechniques

OBJECTIVESOBJECTIVESDemonstrate proper techniques in

transferring solid chemicals, transferring liquid reagents, heating, separating a solid from a liquid, preparing saturated solutions and determining odors of substances;

Appreciate the need to observe proper laboratory techniques to avoid accidents and achieve maximum efficiency.

TRANSFERRING SOLID CHEMICALS

Solid chemicals are generally kept in reagent bottles. Stoppers of reagent bottles should be laid with the flat side on the table to prevent contamination. If the stopper has winged top, it is held between two fingers and never laid on the table. Dark colored reagent bottles are used to protect some chemicals from bright light.

Solid chemicals are spooned out from the reagent bottle using a clean and dry spatula. A dirty spatula will not only contaminate the portion taken but also what is left in the bottle.

 Use a piece of paper to transfer solids to a small mouthed container like a test tube. The technique of transferring solids to a test tube are as follows:

Get a piece of clean paper. Fold it lengthwise. Cut it such that it is about 2cm longer than the test tube and slightly wider than its diameter. Place the solid near the edge of the paper. Hold the test tube in a horizontal position.

Then slide the paper into the test tube until it touches the bottom. Incline the test tube and tap the paper gently. In this way, the solid is deposited at the bottom of the test tube and not along the sides. Practice the technique of transferring solids to a test tube using common table salt.

TRANSFERRING LIQUID REAGENTS

To transfer a liquid reagent to another container, you may do the following techniques:When pouring a liquid into a wide mouthed container, have the neck of the reagent bottle touch the rim of the receiving vessel.

Use a glass rod or a funnel when transferring a liquid into a narrow mouthed container. The glass rod or funnel directs the flow of the liquid and prevents it from spilling down the side of the container.

If only small amounts of liquid reagent are needed, a pipette or a medicine dropper may be used. A pipette is a glass tube calibrated to deliver specific volume of liquids. Perfect the technique of transferring a liquid from a reagent bottle to a beaker, a graduated cylinder, and a test tube. Use water as the liquid.

HEATING LIQUIDS

◦When heating a small amount of liquid in a test tube, use a test tube holder. Hold the test tube at an angle of 45° and heat its content by passing it back and forth over the flame. Never look directly into the test tube that is being heated. Never point it toward anyone. Do not cover the test tube with a rubber or cork stopper while heating. The test tube might break due to the build up of pressure.

Use a beaker of a flask when heating a large amount of liquid. Wire gauze is placed over an iron ring to prevent direct heating. Glassware like beakers, flasks, and the evaporating dish should not be heated directly. If an iron clamp is used to hold the container, it should not be fastened too tightly to prevent breakage. Glass expands when heated.

To heat a test tube that is mounted on an iron stand, hold the burner and pass the flame underneath. The test tube is always clamped at the upper portion.

SEPARATING A SOLID FROM A LIQUID

In the laboratory, it is sometimes necessary to separate a solid from a liquid. The commonly used separation methods are the following:◦Filtration

Filtration is the process of separating solid particles from a liquid by pouring the mixture through filter paper. The first step in filtration is the preparation of the filter paper. Get a piece of filter paper and fold it into halves. Then fold this into quarters. Cut it to form a circle. Open the filter paper such that you have one fold on one side and three folds on the other side. Moisten the paper so that it fits snugly into the funnel. The rim of the filter paper should be lower than the rim of the funnel. The liquid that passes through the filter paper is called the filtrate.

DecantationWhen a solid is heavy and has a

tendency to settle, the separation is accomplished by decantation. This is the process of allowing the solid particles to settle, then carefully pouring off the liquid, referred to as the supernatant liquid. Practice filtration and decantation using a 25-mL mixture of charcoal and water. Compare the extent of separation of the solid and the liquid in the two processes

PREPARING SATURATED SOLUTIONS

In some experiments, you will be required to prepare saturated solutions. A saturated solution is an example of a dynamic equilibrium where the rate of dissolution is equal to the rate of crystallization.Solute + solvent ↔ solution

You can prepare a saturated solution by continually adding the solute to a given quantity of solvent until no more of the solid dissolves, even after constant stirring.

DETERMINING ODORDETERMINING ODOR Determining odor involves some danger since many substances are poisonous. To do this, open the reagent bottle and hold it about 13cm from your face. Fan very gently the air above the bottle toward you and inhale the air. Do not breathe in heavily, just sniff.

Thank You!!