White powders

Many compounds both harmless and deadly look identical. Often these substance take the form of white powders. It is up to the forensic chemist to analyse and identify such powders.

Below is an activity that will introduce you to the properties of some common household powders.
You are provided with a selection of known white powders. Powders may be prepared individually as shown on the right.

Place a sample of each powder in the well, as shown on the right,and conduct each test.
Start with corn starch and its reaction with iodine, then progress onto, sugar, citric acid, baking powder and salt.

 
Continue reacting each powder with vinegar. Notice that some powders will produce a gas, as shown on the right.

 
Heating the powders can get a little smelly and messy. Use aluminium foil to create a small container that you can hold the powder over the flame with. Record if your powder melts, burns, or nothing happens to it.

 

Universal indicator can also tell you something about the chemical nature of the powder. The green indicator will change colour according to the chemical nature of the powder.

Blue indicates that the powder is a base. Red indicates that it is an acid.
   
Powder Reaction with iodine Reaction with vinegar Reaction with baking soda water Reaction to heat Dissolves in water Colour change with Universal indicator
Corn starch            
Alum crystals            
Ground sugar            
Citric acid powder            
Baking soda            
Salt (sodium chloride)            

You are now provided with a mixture of unknown powders. Identify the powders in the mixture. Draw up your own table to record observations of the tests you conduct.

Salt is hard to identify. Use your knowledge of precipitates to suggest one possible way of identifying salt in a mixture of salt and baking powder.