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Q1. Calcium hydroxide is chemically represented as:
Calcium hydroxide has one calcium atom (Ca²⁺) and two hydroxide ions (OH⁻), so its formula is Ca(OH)₂. The parentheses around OH indicate two hydroxide groups. CaCO₃ is calcium carbonate, CaCl₂ is calcium chloride, and CaO is calcium oxide (quicklime).
Q2. Which of the following substances is classified as an acid?
Hydrochloric acid (HCl) is a strong acid that dissociates completely in water to produce H⁺ ions. NaOH is sodium hydroxide (a base), Ca(OH)₂ is calcium hydroxide (a base), and NH₄OH is ammonium hydroxide (a base). Acids are substances that produce hydrogen ions (H⁺) in aqueous solution.
Q3. Which indicator works by detecting a change in smell?
Onion is an olfactory indicator because its characteristic smell changes when exposed to acids or bases. In basic solutions, the smell of onion disappears, while in acidic solutions, it remains. Methyl orange and litmus work by colour change, and turmeric is a natural colour indicator that turns reddish-brown in bases.
Q4. Acids and bases neutralise each other because:
When an acid reacts with a base, the H⁺ ions from the acid combine with the OH⁻ ions from the base to form water (H₂O). This process neutralizes both the acidic and basic properties, resulting in a salt and water. The acid's sourness and the base's bitterness are both cancelled out, forming a neutral solution.
Q5. Magnesium hydroxide is correctly represented as:
Magnesium hydroxide contains magnesium (Mg²⁺) and two hydroxide ions (OH⁻), giving the formula Mg(OH)₂. MgO is magnesium oxide, MgCl₂ is magnesium chloride, and MgSO₄ is magnesium sulphate. Magnesium hydroxide is commonly known as milk of magnesia and is used as an antacid.
Q6. The process by which acids and bases cancel each other's effect is called:
Neutralisation is the chemical reaction between an acid and a base that produces salt and water. The H⁺ ions from the acid combine with the OH⁻ ions from the base to form water. This process neutralizes both substances. Reduction involves gain of electrons, oxidation involves loss of electrons, and displacement involves one element replacing another in a compound.
Q7. Which substance is used as an olfactory indicator?
Vanilla essence is an olfactory indicator because its smell disappears in basic solutions but remains in acidic solutions. Phenolphthalein and methyl orange are colour indicators, and litmus is a natural colour indicator extracted from lichens. Olfactory indicators are substances whose odour changes in acidic or basic media.
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