Question

Classify the following change as chemical and physical change:

Passing of electric current through water and water breaking down into hydrogen and oxygen gases

Answer 1

Passing of electric current through Water $\left({\mathrm{H}}_2\mathrm{O}\right)$ and water breaking down into Hydrogen $\left({\mathrm{H}}_2\right)$ and Oxygen $\left({\mathrm{O}}_2\right)$ gases is a chemical change.

• This is because two new chemical substances i.e.; hydrogen and oxygen gases with different chemical composition and chemical properties than water are formed from it.
• $\underset{\left(\mathrm{Water}\right)}{2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)}\stackrel{\mathrm{Electric}\mathrm{current}}{\to }\underset{\left(\mathrm{Hydrogen}\right)}{2{\mathrm{H}}_2\left(\mathrm{g}\right)}+\underset{\left(\mathrm{Oxygen}\right)}{{\mathrm{O}}_2\left(\mathrm{g}\right)}$
• It involves energy changes as energy is consumed from the electric current to break the bonds between hydrogen and oxygen of water.

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

• It is a permanent change and irreversible change as the substance undergoing change can not be brought back to its original state.
• Physical as well as chemical properties of the substance change during a chemical change and energy is either consumed when bonds are broken or released when chemical bonds are formed. Examples: Rusting of Iron $\left(\mathrm{Fe}\right)$, Burning of wood.

Physical change: A change in which no new substance is formed and there is no change in the chemical composition of the substance undergoing the change.

• It is a temporary change and reversible change as most of the times, the substance can be brought back to its original state.
• Only physical properties like hardness, colour etc. are changed during a physical change and energy is neither consumed nor released as chemical bonds are not involved in the physical change. Examples: Conversion of Water $\left({\mathrm{H}}_2\mathrm{O}\right)$ from solid to liquid form, melting of wax.