The gas which turns limewater milky is

The gas that turns limewater milky is carbon dioxide (CO2). Limewater is an aqueous solution of calcium hydroxide [Ca(OH)2] [3]. When carbon dioxide is bubbled through this solution, it reacts to form calcium carbonate (CaCO3) and water [2]. Calcium carbonate is an insoluble white precipitate that remains suspended in the liquid, giving it a characteristic 'milky' or cloudy appearance [1]. This reaction is a standard chemical test used to identify the presence of carbon dioxide gas [1]. While other gases like carbon monoxide, ammonia, and nitrogen dioxide have distinct chemical properties, they do not produce this specific insoluble carbonate precipitate with calcium hydroxide. If carbon dioxide continues to be bubbled through the milky solution, it may eventually turn clear again as soluble calcium bicarbonate forms, but the initial milky stage is the definitive test for CO2.

Sources

1. [1] 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. [3] Science , class X (NCERT 2025 ed.) > Chapter 5: Life Processes > Activity 5.4 > p. 87
3. [2] 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