Question

Is burning of a substance always a chemical change? Explain how burning of a candle involves both physical and chemical change.

Answer 1

To answer this question, you must recall the differences between a chemical change and a physical change. Changes that are reversible and involve change only in the physical appearance are known as physical changes. Chemical changes are usually irreversible changes

Complete step by step solution:
- Burning can be said to be the rapid and energetic oxidation of a fuel. The fuel, usually hydrocarbons, uses up oxygen from its surroundings and forms carbon dioxide and water. The water produced evaporates as water vapour as soon as it is formed due to the high temperature of the flame. Since a chemical reaction takes place, thus, we can say that burning is always a chemical change.
- We know that candles are made up of wax. Wax is a long chain hydrocarbon. So the burning of a candle is nothing but the combustion of a hydrocarbon, in this case, wax. This combustion involves changes in the chemical structure of the wax and the wax forms carbon dioxide and water vapour. So, this is the chemical change part of the burning of a candle.
- As the candle continues to burn, the solid wax melts and forms liquid wax. This is a change only in the physical state of the wax and thus is a physical change.

Note: We know that there are two types of changes that generally occur in matter, namely physical changes and chemical changes. Physical changes are the changes that take place when a substance boils, melts, condenses or precipitates and only the physical properties of the substance are altered. Chemical changes generally involve the occurrence of chemical reactions.