Question

When a candle burns, both physical and chemical changes take place. Identify these changes. Give another example of a familiar process in which both the chemical and physical changes take place.

Answer 1

Hint: Candles are made up of wax and wax upon combustion give mixtures of gases. Physical change does not involve change in chemical composition of a compound. A process where a compound is undergoing change in state as well as a chemical change can be used as an example of both chemical and physical changes occuring at the same time.

Complete answer:
- We know that a candle is made up of wax. Wax is just a long chain alkane. So, when a candle burns, combustion of wax occurs. This change involves change in chemical structure and can be expressed by the following reaction.
\[wax\xrightarrow{\Delta }C{O_2} + {H_2}\]
So, this is a chemical change occuring during the burning of candles.
- Now, as the candle burns, we see that wax also gets converted into its liquid form.
\[wa{x_{(solid)}}\xrightarrow{\Delta }wa{x_{(liquid)}}\]
In this change, we can see that the chemical composition of wax is the same, just the physical state of wax is changed. So, this change can be characterised as a physical change.
- Now, the use of LPG gas is an example of a process which involves both physical and chemical changes.
- The gas is in a liquid state in the cylinder. Now, as the gas gets out from the tube and comes at the burner, it is in a gaseous state. So, a change from liquid state to gas state occurs here hence this is a physical change.
- As LPG gas burns in the presence of atmospheric oxygen, it gets converted into carbon dioxide and hydrogen gas. So, a chemical reaction occurs during this process and hence it is an example of a chemical change.

Additional Information:
- A Chemical change is a change that involves change in chemical composition of a compound. This type of a change can also be expressed by reaction.
- Physical change is characterized by change in physical state if the compound.

Note: Note that in the burning of candles, no solid or liquid products are formed, that does not mean that any reaction has not occured. The products can also be gaseous. Note that a change can be characterized either as chemical or physical change. So, do not think about any other change.