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Q102 (CDS-I/2020) Science & Technology › Basic Science (Physics, Chemistry, Biology) › Basic chemical principles Answer Verified

When air is blown from the mouth into a test tube containing limewater, the limewater turns milky. This is due to the presence of

Result
Your answer: —  Â·  Correct: C
Explanation

When air is blown from the mouth into a test tube containing limewater, the solution turns milky due to the presence of carbon dioxide (CO2). Limewater is a solution of calcium hydroxide [Ca(OH)2]. Exhaled breath contains a higher concentration of carbon dioxide compared to inhaled air because CO2 is a byproduct of cellular respiration [4]. When CO2 reacts with calcium hydroxide, it forms calcium carbonate (CaCO3) and water [1]. Calcium carbonate is an insoluble white solid that precipitates out of the solution, creating the characteristic 'milky' or cloudy appearance. This reaction is the standard chemical test for identifying carbon dioxide gas [1]. If excess CO2 is passed through the solution, the milkiness eventually disappears as the insoluble calcium carbonate converts into soluble calcium hydrogencarbonate [3].

Sources

  1. [2] Science-Class VII . NCERT(Revised ed 2025) > Chapter 9: Life Processes in Animals > Activity 9.3: Let us explore > p. 131
  2. [4] Science-Class VII . NCERT(Revised ed 2025) > Chapter 10: Life Processes in Plants > 10.4 Do Plants Respire? > p. 149
  3. [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
  4. [3] Science , class X (NCERT 2025 ed.) > Chapter 2: Acids, Bases and Salts > Activity 2.5 > p. 21
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