Question

Give two examples in which a physical change and a chemical change occur simultaneously.

Answer 1

Two examples in which a physical change and a chemical change occur simultaneously are:

Burning of candle:

• Melting of wax: When a candle is burnt, melting of wax takes place. This is a physical process as there is no formation of a new chemical substance. It involves only a change in the state of the wax from solid to liquid form.
• Burning of the wick of candle: Melting of wax is accompanied by burning of the wick of the candle which is a chemical change as the wick is burnt to release Carbon dioxide $\left({\mathrm{CO}}_2\right)$, heat and light.

Thermal decomposition of Zinc carbonate:

• Zinc carbonate $\left({\mathrm{ZnCO}}_3\right)$ decomposes into Zinc oxide and Carbon dioxide on heating.
• As new substances are formed (zinc oxide and carbon dioxide) which have a new chemical composition and properties than zinc carbonate, it is a chemical change.
• $\underset{(\mathrm{Zinc}\mathrm{carbonate})}{{\mathrm{ZnCO}}_3\left(\mathrm{s}\right)}\stackrel{∆}{\to }\underset{(\mathrm{Zinc}\mathrm{oxide})}{\mathrm{ZnO}\left(\mathrm{s}\right)}+\underset{(\mathrm{Carbon}\mathrm{dioxide})}{{\mathrm{CO}}_2\left(\mathrm{g}\right)}$
• At the same time, zinc oxide is yellow when hot and white when cool. This change in colour is a physical change as it involves only colour change without the formation of any new chemical substances.
• $\underset{\underset{\left(\mathrm{Yellow}\right)}{(\mathrm{Zinc}\mathrm{oxide})}}{\mathrm{ZnO}\left(\mathrm{s}\right)}\underset{\mathrm{Hot}}{\overset{\mathrm{Cool}}{\rightleftarrows }}\underset{\underset{\left(\mathrm{White}\right)}{(\mathrm{Zinc}\mathrm{oxide})}}{\mathrm{ZnO}\left(\mathrm{s}\right)}$

Physical change: A physical change involves the formation of no new substance and there is no change in the chemical composition of the substance undergoing the change.

Chemical change: A chemical change involves the formation of one or more new substances and there is a change in the chemical composition of the substance undergoing the change.