Look at Fig. 6.1 which shows three situations (a) a burning candle (b) an extinguished candle (c) melting the wax.

Which of these shows a reversible change and why?

(c) Melting wax shows a reversible change. This is because it is a physical process, where only the physical state of wax changes from solid to liquid, and can be reversed back on cooling.

● A burning candle(a) and (b) an extinguished candle show irreversible changes. This is because these involve a change in chemical properties of the candle and wax. When a candle is burned, the wax around the wick melts and is converted to fumes. This change is chemical in nature and therefore cannot be reversed.

● Similarly, an extinguished candle can be lightened up again, but that will be the start of a new combustion process, not the reversal of fumes that were produced during burning to flame again. Thus, this change also cannot be reversed.