Page 124 - ICSE Chemistry 8

3. Forma on of precipitate: Some reac ons are characterised by the forma on of precipitate. A
precipitate is an insoluble solid formed during a chemical reac on. It is indicated by a downwards
arrow ( ).

Examples: Precipitate
• When hydrogen sulphide gas is passed through blue- A solid substance separated
from a solu on is called a
coloured copper sulphate solu on, a black precipitate of
precipitate.
copper sulphide is formed.
CuSO (aq) + H S(g) CuS(s)  + H SO (aq)
4
4
2
2
Copper sulphate Hydrogen sulphide Copper sulphide Sulphuric acid
(blue) (black ppt)
• When ammonium hydroxide and iron sulphate solu ons are mixed together, a dirty-green
precipitate of iron hydroxide is obtained.
2NH OH(aq) + FeSO (aq) Fe(OH) (s)  + (NH ) SO (aq)
2
4
4
4 2
4
Ammonium hydroxide Iron sulphate Iron hydroxide Ammonium sulphate
(dirty-green precipitate)
• When sodium chloride and silver nitrate solu ons are mixed together, a white precipitate of
silver chloride is formed.
NaCl(aq) + AgNO (aq) AgCl(s)  + NaNO (aq)
3
3
Sodium chloride Silver nitrate Silver chloride Sodium nitrate
(curdy white ppt)
Activity 6.3

Aim: To show the forma on of a precipitate during a chemical reac on
Materials required: Sodium sulphate solu on, barium chloride solu on, two test tubes
Procedure
• Take a solu on of sodium sulphate in a test tube and a solu on of barium
chloride in another test tube.

• Mix the two solu ons.
Observa on: Sodium sulphate and barium chloride react to form a white precipitate
of barium sulphate. a white
Na SO (aq) + BaCl (aq) BaSO (s) + 2NaCl(aq) precipitate
4
2
4
2
Sodium sulphate Barium chloride Barium sulphate Sodium chloride
(dense white ppt)
Conclusion: Mixing the solu ons of sodium sulphate and barium sulphate leads to the forma on of a
white precipitate.

4. Change of state: In many chemical reac ons, a change of physical state is observed.

Examples:
• Hydrogen and oxygen gases combine together to form water which is a liquid.
2H (g) + O (g) 2H O(l)
2
2
2
Hydrogen Oxygen Water

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