Reaction of quick lime (CaO) with water to produce slaked lime (Ca(OH)2) is an example of
- A.Displacement reaction.
- B.Endothermic reaction.
- C.Decomposition reaction.
- D.Exothermic reaction.
Correct answer
Explanation
The correct answer is D, Exothermic reaction.. When quicklime reacts with water a large amount of heat is released, so the reaction is exothermic.
The reaction is CaO + H2O → Ca(OH)2 + heat. The mixture becomes so hot that the water may hiss and boil, which is why lime must be slaked with care. Two substances join to form a single product, so this is also a combination reaction. The slaked lime formed is used for whitewashing walls; it slowly reacts with carbon dioxide in the air to form a thin, shiny layer of calcium carbonate.
A is wrong because no element pushes another out of a compound; the two reactants simply combine.
B is wrong because an endothermic reaction absorbs heat, while slaking lime gives out heat.
C is wrong because decomposition breaks one compound into simpler ones; heating limestone into quicklime and carbon dioxide is decomposition, the reverse type.
Exam tip: CaO + H2O → Ca(OH)2 is a combination reaction and an exothermic one; CaCO3 → CaO + CO2 on heating is decomposition.
Practice Questions
View allWhich one of the following is not true for anodizing process?
- A.It makes aluminium corrosion resistant.
- B.Metals like aluminium, titanium and magnesium can be anodized.
- C.Clean aluminium article is the anode and oxygen gas is evolved at the cathode.
- D.It is used in aircraft industry.
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In anodizing the clean aluminium article is indeed the anode, but oxygen is released at the anode, not at the cathode; hydrogen is released at the cathode.
Read the full explanation →Which one of the following statements is not correct?
- A.The response of the ear to sound of increasing intensity is approximately logarithmic.
- B.The sensitivity of the human ear does not vary with the frequency of the sound.
- C.When two or more waves traverse the same medium, the displacement of any element of the medium is the algebraic sum of the displacements due to each wave.
- D.Longitudinal waves can travel in all media—solids, liquids and gases.
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The statement that the sensitivity of the human ear does not vary with frequency is false, because the ear is far more sensitive to some frequencies than to others.
Read the full explanation →Kwashiorkor disease in children is caused by
- A.sufficient carbohydrates but less fats in diet
- B.sufficient carbohydrates and fats but deficient proteins in diet
- C.sufficient vitamins but deficient fats in diet
- D.sufficient fats but deficient vitamins in diet
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Kwashiorkor is a disease of protein deficiency in a child who still gets enough calories. It usually appears when a child is weaned from breast milk onto a starchy diet with little protein.
Read the full explanation →Which one of the following is the correct order of pH for the given substances?
- A.Coffee < Lemon juice < Milk of magnesia < Blood
- B.Milk of magnesia < Blood < Coffee < Lemon juice
- C.Lemon juice < Blood < Coffee < Milk of magnesia
- D.Lemon juice < Coffee < Blood < Milk of magnesia
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Arranged from most acidic to most basic, the four substances fall in exactly this order. Lemon juice, rich in citric acid, has a pH of about 2 to 2.5.
Read the full explanation →Which one of the following diseases may be caused by the deficiency of vitamin C?
- A.Rickets
- B.Rabies
- C.Hepatitis
- D.Scurvy
Show answer
Scurvy is caused by a lack of vitamin C, or ascorbic acid, which the body needs to make collagen, the protein that holds skin, gums and blood vessels together.
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