Question

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

Answer 1

Chemical change:

• A chemical change is a permanent change in which new substances are formed.
• The chemical change is permanent and cannot be reversed by a simple physical method.
• A chemical change involves an exchange of energy in the form of heat or light.
• For example, rusting of iron, burning of wood, respiration, etc.

Physical change:

• A physical change is a change in which no new substance is formed.
• A physical change is temporary and reversible.
• In physical change, the chemical composition of the original substance is not altered.
• For example, drying wet clothes, breaking a glass tumbler, drying paints, etc.

Example 1:

• On heating, crystals of potassium nitrate (${\mathrm{KNO}}_3$) melt.
• Hence, it is a physical change.
• The molten potassium nitrate then decomposes to form potassium nitrite (${\mathrm{KNO}}_2$) and oxygen (${\mathrm{O}}_2$) and hence it is a chemical change.

Example 2

• On heating zinc hydroxide ($\mathrm{Zn}{\left(\mathrm{OH}\right)}_2$) it changes into zinc oxide($\mathrm{ZnO}$) and water vapours.
• Thus, it is a chemical change because new products are formed during heating.

$\mathrm{Zn}{\left(\mathrm{OH}\right)}_{2\left(\mathrm{s}\right)}\stackrel{∆}{\to }{\mathrm{ZnO}}_{\left(\mathrm{s}\right)}+{\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{l}\right)}$

• Zinc oxide formed is yellow when hot but becomes white on cooling without any change in composition.
• Hence, it is a physical change.

$$\begin{gathered} {\mathrm{ZnO}}_{\left(\mathrm{s}\right)}\to {\mathrm{ZnO}}_{\left(\mathrm{s}\right)} \\ \mathrm{Yellow}\mathrm{White} \end{gathered}$$