Two reactants in a flask at room temperature are producing bubbles of a gas that turn limewater milky. The reactants could be

The production of bubbles that turn limewater milky is the standard chemical test for carbon dioxide (CO2) [3]. When carbon dioxide reacts with limewater (calcium hydroxide), it forms insoluble calcium carbonate, which creates a milky or cloudy appearance [3]. According to chemical principles, all metal carbonates react with acids to produce a salt, water, and carbon dioxide gas [2]. Specifically, the reaction between magnesium carbonate and hydrochloric acid yields magnesium chloride, water, and carbon dioxide [1]. In contrast, the reaction of zinc with hydrochloric acid produces hydrogen gas, which does not turn limewater milky. Methane and oxygen require ignition for combustion and do not react spontaneously at room temperature to produce bubbles. Copper is less reactive than hydrogen and does not react with dilute hydrochloric acid at room temperature.

Sources

1. [3] 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. 61
2. [1] Science , class X (NCERT 2025 ed.) > Chapter 2: Acids, Bases and Salts > 2.1.6 Reaction of a Non-metallic Oxide with Base > p. 22
3. [2] Science , class X (NCERT 2025 ed.) > Chapter 2: Acids, Bases and Salts > Activity 2.5 > p. 21