Which among the following is a chemical change?

A chemical change involves the formation of one or more new substances through a chemical reaction [2]. When lemon juice is added to tea, the citric acid reacts with the tea's polyphenols (tannins), causing a change in the chemical structure and resulting in a visible color change [t1][t7]. This alteration in chemical composition signifies a chemical change. In contrast, a wet towel drying is a physical change because water simply evaporates from liquid to gas without forming a new substance [t2]. Hot air rising over a radiator is a physical process of convection driven by density differences [t3]. Brewing coffee by passing steam through grounds is primarily a physical extraction process where soluble compounds are dissolved into the water [t5][t8]. Therefore, the addition of lemon juice to tea is the only option involving a chemical reaction that produces new substances.

Sources

1. [1] Science-Class VII . NCERT(Revised ed 2025) > Chapter 5: Changes Around Us: Physical and Chemical > In a Nutshell > p. 68
2. [2] Science-Class VII . NCERT(Revised ed 2025) > Chapter 5: Changes Around Us: Physical and Chemical > Fig. 5.2: Blowing air in (a) tap water; (b) lime water > p. 60