Question

While teaching the topic of physical and chemical changes, Mahima's teacher listed the conditions that take place during a change. He then asked Mahima to identify the conditions based on which it can be said with certainty that a particular change is chemical in nature. Among the changes that are listed by the teacher, which changes should be selected by Mahima?

• A. Change in state
• B. Change in colour
• C. Evolution of a gas
• D. Change in temperature

Answer 1

The correct option is C

Evolution of a gas

Explanation for the correct answer:-

Option (C) Evolution of a gas

• Whenever gas is being evolved in any chemical reaction we can say that it is a chemical change.
• This is because in any chemical reaction whenever gas is evolved as a byproduct we can call that reaction as a chemical change in nature.
• For example when we burn fuel, Carbon dioxide gas is always released as a byproduct: $\underset{\mathrm{Methane}}{{\mathrm{CH}}_4}+\underset{\mathrm{Oxygen}}{2{\mathrm{O}}_2}\to \underset{\mathrm{Carbon}\mathrm{dioxide}}{{\mathrm{CO}}_2\left(\mathrm{g}\right)(\uparrow )}+\underset{\mathrm{Water}}{2{\mathrm{H}}_2\mathrm{O}}$
• Thus Mahima should choose this option as it perfectly describes a chemical change.
• Hence it s the correct option.

Explanation for the incorrect answer:-

Option (A) Change in state

• Change in the state can occur in both chemical and physical changes, thus we cannot call change in state to be chemical change only.
• For example, When we freeze water it changes its state from liquid to solid.
• Again when the ice starts to melt, it changes its state from solid to liquid.
• Thus it also becomes a physical change as we can reverse the process.
• Hence it is the incorrect option.

Option (B) Change in colour

• Change in colour can be both chemical and physical change.
• When we heat an Iron rod, it changes its colour to red.
• But again when we treat the same Iron rod with Copper sulphate solution(which is blue in colour), it will change its colour to green due to the formation of Iron(II) sulphate.

$$\begin{gathered} \underset{\mathrm{Iron}}{2\mathrm{Fe}\left(\mathrm{s}\right)}+\underset{\mathrm{Oxygen}}{{\mathrm{O}}_2\left(\mathrm{g}\right)}\stackrel{∆}{\to }\underset{\mathrm{Ferrous}\mathrm{oxide}}{2\mathrm{FeO}\left(\mathrm{s}\right)} \\ \underset{\mathrm{Iron}}{4\mathrm{Fe}\left(\mathrm{s}\right)}+\underset{\mathrm{Oxygen}}{3{\mathrm{O}}_2\left(\mathrm{g}\right)}\stackrel{∆}{\to }\underset{\mathrm{Ferric}\mathrm{oxide}}{3{\mathrm{Fe}}_2{\mathrm{O}}_3\left(\mathrm{s}\right)} \\ \underset{\mathrm{Iron}}{3\mathrm{Fe}\left(\mathrm{s}\right)}+\underset{\mathrm{Oxygen}}{2{\mathrm{O}}_2\left(\mathrm{g}\right)}\stackrel{∆}{\to }\underset{\mathrm{Iron}(\mathrm{II},\mathrm{III})\mathrm{oxide}}{{\mathrm{Fe}}_3{\mathrm{O}}_4\left(\mathrm{s}\right)} \end{gathered}$$

$\underset{\mathrm{Iron}}{\mathrm{Fe}\left(\mathrm{s}\right)}+\underset{\mathrm{Copper}\mathrm{sulphate}}{{\mathrm{CuSO}}_4\left(\mathrm{aq}\right)\left(\mathrm{blue}\right)}\to \underset{\mathrm{Ferrous}\mathrm{sulphate}}{{\mathrm{FeSO}}_4\left(\mathrm{aq}\right)\left(\mathrm{green}\right)}+\underset{\mathrm{Copper}}{\mathrm{Cu}\left(\mathrm{s}\right)}$

Option (D) Change in temperature

• Change in the tem[erature can happen in both chemical as well as physical change.
• Thus, it is the incorrect option.

Therefore, option (C) Evolution of a gas is the correct answer.