Question

Write one chemical equation each for the chemical reaction in which the following have taken place:
(i) Change in colour
(ii) Change in temperature
(iii) Formation of precipitate
Mention colour change/temperature change (rise/fall)/compound precipitated along with equation.

Answer 1

Change in colour:
Solution: A chemical reaction that causes a colour change is the heating of copper carbonate:
$$\text{CuCO}_3 \xrightarrow{\text{heat}} \text{CuO} + \text{CO}_2.$$
- Observation: Copper carbonate (green) changes to copper oxide (black).
Change in temperature:
Solution: An example is the exothermic reaction of quicklime with water:
$$\text{CaO} + \text{H}_2\text{O} \rightarrow \text{Ca(OH)}_2 + \text{Heat}.$$
- Observation: The reaction releases heat, causing a rise in temperature.
Formation of precipitate:
Solution: An example is the reaction between sodium sulfate and barium chloride:
$$\text{Na}_2\text{SO}_4 + \text{BaCl}_2 \rightarrow \text{BaSO}_4 \downarrow + 2\text{NaCl}.$$
- Observation: A white precipitate of barium sulfate ($\text{BaSO}_4$) is formed.