IGCSE Chemistry 0620

Topic: Acids, bases and salts - Preparation of salts

Objective: Describe the preparation of insoluble salts by precipitation

This section details the method for preparing insoluble salts through a precipitation reaction. Precipitation is a chemical reaction where two soluble ionic compounds react to form an insoluble ionic compound, which then separates from the solution as a solid.

Principle of Precipitation

When a solution containing ions of a soluble salt is mixed with a solution containing ions of another soluble salt, the ions can combine to form an insoluble salt. This insoluble salt will then come out of the solution as a precipitate.

Materials Required

The following materials are typically required for the preparation of insoluble salts by precipitation:

• Two soluble salts (e.g., sodium chloride (NaCl) and silver nitrate (AgNO₃))
• Two test tubes
• Droppers
• Test tube rack
• Beaker (for mixing)
• Stirring rod

Procedure

1. Using a dropper, add a few drops of one soluble salt solution to a test tube.
2. Using a separate dropper, add a few drops of the second soluble salt solution to a second test tube.
3. Carefully add the contents of one test tube to the other test tube.
4. Observe the mixture for any change, particularly the formation of a solid (precipitate).
5. If a precipitate forms, it can be collected by filtration.

Chemical Equation Example

A common example is the reaction between sodium chloride (NaCl) and silver nitrate (AgNO₃) to form silver chloride (AgCl), which is an insoluble white solid.

The balanced chemical equation is:

$AgNO_3(aq) + NaCl(aq) \rightarrow AgCl(s) + NaNO_3(aq)$

Important Considerations

• The concentration of the solutions can affect the amount of precipitate formed.
• The rate of mixing can also influence the size of the precipitate particles.
• The precipitate can be collected by filtration.

Filtration of the Precipitate

If a precipitate is formed, it can be separated from the solution by filtration.

Step	Description
Set up	Place a funnel in the mouth of a test tube.
Filtration	Pour the mixture through the funnel. The solid precipitate will be trapped on the filter paper, while the liquid (filtrate) will pass through.
Washing	Wash the precipitate with a small amount of distilled water to remove any remaining impurities.
Drying	Dry the precipitate in an oven or in air.

Safety Precautions

Always wear safety goggles when carrying out chemical experiments. Handle chemicals with care and avoid contact with skin and eyes. Dispose of chemical waste properly as instructed by your teacher.