Question

Write any 4 observations in an activity which may suggest that a chemical reaction has taken place. Give an example in support of your answer.

Answer 1

Observations that suggest that a chemical reaction has taken place are:
1. Evolution of gas
Eg. When we add zinc to dilute sulphuric acid, we will observe the bubbles of hydrogen has being formed around zinc granules.
2. Formation of precipitate
Eg. When lead nitrate solution in added to potassium iodide solution, a yellow precipitate of lead iodide is formed.
3. Change in colour
Eg. Chemical reaction between sulphur dioxide gas and acidified potassium dichromate solution is characterised by a change in colour from Orange to green.
4. Change in state
Eg. Combustion reaction of candle wax is characterised by a change in state from solid to liquid and gas as wax is solid, melted wax formed by combustion of wax is liquid at room temperature whereas carbon dioxide produced by the combustion of wax is a gas.

Answer 2

some examples are
1 evolution of a gas
a In reactions of metals with acids, H2 gas is evolved which can be detected by a pop sound upon burning.
b. In decomposition of calcium carbonate by heating, CO2 is evolved which can be detected by lime water turning milky white.

2. formation of precipitate
a. In reaction of Barium chloride and potassium sulphate, a precipitate of barium sulphate is formed.

3. Change of colour
a. in some titration reactions, we often see change of colour of indicator as a signal of completion of reaction.
b. In reaction of Copper sulphate with zinc, the blue colour of the solution disappears due to formation of zinc sulphate.

4. Change in temperature
a. some reactions involving acid base neutralisation causes evolution of heat which can be detected by change in temperature.