Tag: Salt

1. A solution turns red litmus blue, its pH is likely to be

(a) 1  (b) 4  (c) 5  (d) 10

Answer: (d) 10

Explanation:
A solution turning red litmus blue is a base, and basic solutions have pH greater than 7. Among the given options, pH 10 is strongly basic.

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2. A solution reacts with crushed egg-shells to give a gas that turns lime-water milky. The solution contains

(a) NaCl (b) HCl (c) LiCl (d) KCl

Answer: (b) HCl

Explanation:
Egg-shells contain calcium carbonate (CaCO₃). Acids like HCl react with carbonates to produce CO₂ gas, which turns lime water milky.

$${\text{CaCO}}_3+2\text{HCl}\to {\text{CaCl}}_2+{\text{H}}_2\text{O}+{\text{CO}}_2\uparrow$$

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3. 10 mL NaOH solution is neutralised by 8 mL HCl solution. How much HCl will be needed for 20 mL of NaOH?

(a) 4 mL (b) 8 mL (c) 12 mL (d) 16 mL

Answer: (d) 16 mL

Explanation:
Neutralisation is directly proportional:

$$10\text{ mL NaOH}\to 8\text{ mL HCl}$$

So,

$$20\text{ mL NaOH}\to 8\times 2=16\text{ mL HCl}$$

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4. Which type of medicine is used for treating indigestion?

(a) Antibiotic (b) Analgesic (c) Antacid (d) Antiseptic

Answer: (c) Antacid

Explanation:
Indigestion is caused by excess acid in the stomach. Antacids like magnesium hydroxide or baking soda neutralise the acid.

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5. Write word equations and balanced equations for the following reactions:

(a) Dilute sulphuric acid reacts with zinc granules

Word Equation:
Sulphuric acid + Zinc → Zinc sulphate + Hydrogen gas

Balanced Equation:

$${\text{H}}_2{\text{SO}}_4+\text{Zn}\to {\text{ZnSO}}_4+{\text{H}}_2\uparrow$$

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(b) Dilute hydrochloric acid reacts with magnesium ribbon

Word Equation:
Hydrochloric acid + Magnesium → Magnesium chloride + Hydrogen gas

Balanced Equation:

$$2\text{HCl}+\text{Mg}\to {\text{MgCl}}_2+{\text{H}}_2\uparrow$$

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(c) Dilute sulphuric acid reacts with aluminium powder

Word Equation:
Sulphuric acid + Aluminium → Aluminium sulphate + Hydrogen gas

Balanced Equation:

$$3{\text{H}}_2{\text{SO}}_4+2\text{Al}\to {\text{Al}}_2{\text{(SO}}_4{\text{)}}_3+3{\text{H}}_2\uparrow$$

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(d) Dilute hydrochloric acid reacts with iron filings

Word Equation:
Hydrochloric acid + Iron → Iron chloride + Hydrogen gas

Balanced Equation:

$$2\text{HCl}+\text{Fe}\to {\text{FeCl}}_2+{\text{H}}_2\uparrow$$

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6. Compounds such as alcohol and glucose contain hydrogen but are not acids. Describe an activity to prove it.

Activity:

• Take solutions of alcohol and glucose in two beakers.

• Fix two carbon/platinum electrodes and connect them to a bulb and battery.

• Switch on the circuit.

Observation:

The bulb does not glow in either solution.

Conclusion:

Although alcohol and glucose contain hydrogen, they do not produce H⁺ ions in water, so they do not conduct electricity and are not acids.

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7. Why does distilled water not conduct electricity, whereas rain water does?

Answer:

• Distilled water is completely pure and contains no ions, so it cannot conduct electricity.

• Rain water contains dissolved acids and salts (like carbonic acid formed by dissolving CO₂), which provide ions, so it conducts electricity.

8. Why do acids not show acidic behaviour in the absence of water?

Answer:
Acids do not show acidic behaviour in the absence of water because they do not produce hydrogen ions (H⁺) without dissolving in water.
Acids ionise only in aqueous solution to release H⁺ or H₃O⁺ ions, which are responsible for acidic properties.

$$\text{HCl}+{\text{H}}_2\text{O}\to {\text{H}}_3{\text{O}}^{+}+{\text{Cl}}^{-}$$

Without water → no ionisation → no acidic behaviour.

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9. Five solutions A,B,C,D,E show pH values 4, 1, 11, 7 and 9

(a) Neutral: pH 7 → Solution D

(b) Strongly alkaline: pH 11 → Solution C

(c) Strongly acidic: pH 1 → Solution B

(d) Weakly acidic: pH 4 → Solution A

(e) Weakly alkaline: pH 9 → Solution E

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Arrange pH in increasing order of hydrogen-ion concentration

Greater H⁺ concentration = stronger acidity = lower pH
Order:

$$\text{pH 11}<\text{pH 9}<\text{pH 7}<\text{pH 4}<\text{pH 1}$$

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10. Equal lengths of magnesium ribbons placed in test tubes A & B with equal concentrations of acids HCl and CH₃COOH. Which shows more fizzing and why?

Answer:
Fizzing will be more vigorous in test tube A (HCl).

Reason:
Hydrochloric acid is a strong acid and ionises completely in water to release more H⁺ ions.
Acetic acid is a weak acid and ionises partially.

$$\text{Mg}+2\text{HCl}\to {\text{MgCl}}_2+{\text{H}}_2\uparrow$$

More H⁺ ions → faster reaction → more hydrogen gas bubbles (fizzing).

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11. Fresh milk has pH 6. How will pH change as it turns into curd? Explain.

Answer:
As milk turns into curd, its pH decreases below 6.

Reason:
Bacteria convert lactose (milk sugar) into lactic acid, increasing acidity and lowering pH.

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12. A milkman adds a very small amount of baking soda to fresh milk.

(a) Why does he shift the pH from 6 to slightly alkaline?

To prevent milk from turning sour quickly, as acids formed will first neutralise the baking soda.

(b) Why does the milk take a longer time to set as curd?

Because curd formation requires acidic conditions, and the added baking soda neutralises acids initially, slowing down the process.

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13. Plaster of Paris should be stored in a moisture-proof container. Explain why.

Answer:
Plaster of Paris reacts with moisture to convert back into gypsum, forming a hard solid mass.

$${\text{CaSO}}_4\cdot \frac{1}{2}{\text{H}}_2\text{O}+\frac{3}{2}{\text{H}}_2\text{O}\to {\text{CaSO}}_4\cdot 2{\text{H}}_2\text{O}$$

So, it becomes useless if exposed to moisture.

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14. What is a neutralisation reaction? Give two examples.

Answer:
A neutralisation reaction is a reaction in which an acid reacts with a base to form salt and water.

$$\text{Acid}+\text{Base}\to \text{Salt}+\text{Water}$$

Examples:

1. HCl + NaOH → NaCl + H₂O

2. ${\text{H}}_2{\text{SO}}_4+{\text{Ca(OH)}}_2\to {\text{CaSO}}_4+2{\text{H}}_2\text{O}$

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15. Give two important uses of washing soda and baking soda.

Washing Soda (Na₂CO₃·10H₂O)

• Used in softening hard water

• Used in the manufacture of glass, soap and paper

Baking Soda (NaHCO₃)

• Used in baking to make cakes and bread soft and fluffy

• Used as an antacid to remove acidity in the stomach