When air is blown from mouth into a test tube containing limewater, the limewater turns milky. This is due to the presence of
- A.water vapour
- B.oxygen
- C.carbon dioxide
- D.carbon monoxide
Correct answer
Explanation
The correct answer is C, carbon dioxide. The air we breathe out carries carbon dioxide, which reacts with limewater to form a white, insoluble solid that makes it look milky.
Limewater is a clear solution of calcium hydroxide, Ca(OH)₂. When carbon dioxide passes through it, calcium carbonate forms: Ca(OH)₂ + CO₂ → CaCO₃ + H₂O. Calcium carbonate does not dissolve in water, so it stays suspended as fine white particles. Exhaled air has about 4% carbon dioxide compared with about 0.04% in the air we breathe in, so the change is quick. If much more carbon dioxide is passed, the milkiness disappears because soluble calcium hydrogencarbonate forms.
Option A is wrong because water vapour does not react with limewater. Option B is wrong because oxygen gives no such reaction. Option D is wrong because carbon monoxide is not exhaled and does not turn limewater milky.
Exam tip: limewater turns milky with CO₂ (CaCO₃ forms) and clears again with excess CO₂ (Ca(HCO₃)₂ forms).
Practice Questions
View allWhich one of the following plant tissues has dead cells ?
- A.Epidermis
- B.Parenchyma
- C.Collenchyma
- D.Sclerenchyma
Show answer
Sclerenchyma cells are dead at maturity, with thick walls hardened by lignin. Sclerenchyma is the tissue that makes plants hard and stiff.
Read the full explanation →Imagine a current-carrying straight conductor with magnetic field of lines in anti-clockwise direction. Then the direction of current is determined by
- A.the Right-Hand Thumb rule and it would be in the downward direction.
- B.the Left-Hand Thumb rule and it would be in the downward direction.
- C.the Right-Hand Thumb rule and it would be in the upward direction.
- D.the Left-Hand Thumb rule and it would be in the upward direction.
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Read the full explanation →Which one of the following statements is true for sound waves propagating in air?
- A.Sound is an electromagnetic wave and transverse in nature
- B.Sound is a mechanical wave and longitudinal in nature
- C.Sound is a mechanical wave and transverse in nature
- D.Sound is an electromagnetic wave and longitudinal in nature
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Read the full explanation →Which one of the following is primarily responsible for conduction of current in a metal?
- A.Bound electrons
- B.Free electrons
- C.Both bound and free electrons
- D.Ions
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In a metal, the outer electrons of the atoms are not tied to any one atom and drift through the lattice when a voltage is applied, carrying the current.
Read the full explanation →Which one of the following statements is correct about the magnification of an optical microscope?
- A.Magnification increases with the increase in focal length of eyepiece
- B.Magnification increases with the increase in focal length of objective
- C.Magnification does not depend upon the focal length of eyepiece
- D.Magnification decreases with the increase in focal length of eyepiece
Show answer
A compound microscope magnifies more when both its lenses have short focal lengths. In a compound microscope the objective forms a real, enlarged image, and the eyepiece magnifies it further like a simple magnifier.
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