Basic Information About Common Laboratory Operations

A.  Handling of Reagents

Reagent solutions are usually kept in bottles in the laboratory.  When pouring the contents, the stopper should be laid with its flat side on the table to avoid contamination and it should be guided by a glass rod held against the lip of the first container.  Pipettes or droppers are also used to get the required amount.  Always use a clean pipette or dropper for every reagent.  For solid reagents, dry and clean spatula should be used to spoon out the required amount.

B.  Heating shall amount of Liquid

To heat small amount of liquid in a test tube, fill the test tube half-full and hold it with a test tube holder.  Heat the liquid by holding the test tube holder at 45o angle and moving it back and forth over the flame for even heating.  Never look into the test tube that has been heated or is being heated and never point the tube towards anyone.

C.  Precipitation, Decantation and Filtration

Precipitation is the process in which a soluble substance is separated from its solution as a solid by means of chemical reagent.  The insoluble solid formed is called precipitate.  This can be separated from the solution by allowing the precipitate to settle down at the bottom of the container and the supernatant liquid is carefully poured off without disturbing the solid particles.  This method of separation is called decantation.

D.  Preparation of Solution and Reagents

When a solid is to be dissolved in water or another solvent, the solid is added in small amount into the liquid while the mixture is stirred with a glass rod.

For liquid reagents especially in the dilution of acids, the acid is slowly added to the water with constant stirring.  Don’t ever attempt to add the water to the acid.

E.  Evaporation

Evaporation is a physical method of separation of soluble solute from a solution.

  1. Direct Evaporation – the process of separation in which the liquid is removed by the application of heat in which the sample to be evaporated is in the evaporating dish, the dish is in direct contact with the flame.  Direct evaluation is done in aqueous solutions.
  2. Indirect Evaporation – this is done by the removal of liquid in such a way that the container is not in direct contact with the flame.  It is usually placed in a boiling water bath, steam bath or heating mantle.  This is done to remove solvent which are flammable or those that decompose when direct is applied.
  3. Spontaneous Evaporation – this type of evaporation is the removal of a liquid without the application of heat,  This is usually done to remove volatile solvents,

Practice using direct and indirect evaporation of 10 ml of saturated NACl solution.  For spontaneous evaporation, dissolve a pinch of benzoic acid in 10 ml ether.  Set aside.  Observe at the end of the laboratory period.  Caution: Ether is flammable, therefore work on this procedure away from an open flame.

F. Determination of Odor

To determine the odor of a substance, gently fan the vapors towards your nostrils.

Describe the odor of the following samples: vinegar, acetone, ammonia, amyl acetate, alcohol.

G. Determination of Acidity and Basicity using litmus paper

Place pieces of blue and red litmus paper on a watch glass.  Dip one end of stirring rod into a test tube containing the acid and touch it to both colors.  Wash and dry the same glass rod, then do the same for a basic solution.

To test whether the solution is neutral, use both red and blue litmus paper.