Water, Solutions and Hardness – NDA Chemistry PYQs

Q1. Which one of the following is considered to be the purest source of water?
(a) River water (b) Sea water (c) Well water (d) Rain water

Answer: (d) Rain water
Rain water forms by natural evaporation and condensation — essentially a natural distillation process. It carries no dissolved minerals from the ground. River, sea, and well water all contain dissolved salts and minerals from soil and rocks.
[NDA 2025-II]

---

Q2. Which one of the following salts is responsible for temporary hardness of water?
(a) Sulphates of calcium (b) Chlorides of magnesium (c) Bicarbonates of magnesium (d) Sulphates of magnesium

Answer: (c) Bicarbonates of magnesium
Temporary hardness is caused only by bicarbonates (hydrogencarbonates) of calcium and magnesium — Ca(HCO₃)₂ and Mg(HCO₃)₂. Sulphates and chlorides of calcium and magnesium cause permanent hardness.
[NDA 2025-II]

---

Q3. Temporary hardness in water is due to which one of the following of Calcium and Magnesium?
(a) Hydrogencarbonates (b) Carbonates (c) Chlorides (d) Sulphates

Answer: (a) Hydrogencarbonates
Ca(HCO₃)₂ and Mg(HCO₃)₂ dissolve in water and cause temporary hardness. These bicarbonates decompose on boiling — releasing CO₂ and forming insoluble CaCO₃ which precipitates. This removes the hardness.
[NDA 2017-I]

---

Q4. Which of the following statements regarding properties of water is/are correct?

1. Water has higher specific heat in comparison with other liquids.
2. Water has no dipole moment.
3. Water has low heat of vaporisation.
   (a) 1 only (b) 2 and 3 (c) 1 and 3 (d) 3 only

Answer: (a) 1 only
Statement 1 ✓ — water has very high specific heat (4.18 J/g°C). Statement 2 ✗ — water IS polar and HAS a dipole moment. The bent structure of H₂O means the bond dipoles add up, not cancel. Statement 3 ✗ — water has HIGH heat of vaporisation due to strong hydrogen bonds between molecules.
[NDA 2013-I]

Q5. Which of the following statements about hard water is/are correct?

1. Temporary hardness of water is due to the presence of soluble magnesium and calcium hydrogencarbonate.
2. Temporary hardness of water can be removed by boiling.
3. Calgon’s method is applied to remove temporary hardness of water.
4. Permanent hardness of water is removed by Clark’s method.
   (a) 1 and 2 only (b) 1, 2 and 3 (c) 3 and 4 (d) 1, 2, 3 and 4

Answer: (a) 1 and 2 only
Statement 1 ✓ — temporary hardness = bicarbonates of Ca and Mg. Statement 2 ✓ — boiling decomposes bicarbonates → CaCO₃ precipitates → water softens. Statement 3 ✗ — Calgon’s method removes PERMANENT hardness (not temporary). Statement 4 ✗ — Clark’s method removes TEMPORARY hardness (not permanent).
[NDA 2013-I]

---

Q6. Permanent hardness of water cannot be removed by which one of the following methods?
(a) Treatment with washing soda (b) Calgon’s method (c) Boiling (d) Ion exchange method

Answer: (c) Boiling
Boiling only removes temporary hardness by decomposing bicarbonates. Sulphates and chlorides of calcium and magnesium (which cause permanent hardness) are thermally stable — they survive boiling and remain dissolved. Washing soda, Calgon, and ion exchange all remove permanent hardness.
[NDA 2018-II]

---

Q7. Permanent hardness of water is due to the presence of:
(a) Sulphates of sodium and potassium
(b) Sulphates of magnesium and calcium
(c) Carbonates of sodium and magnesium
(d) Bicarbonates of magnesium and calcium

Answer: (b) Sulphates of magnesium and calcium
Permanent hardness is caused by sulphates (CaSO₄, MgSO₄) and chlorides (CaCl₂, MgCl₂) of calcium and magnesium. These cannot be removed by boiling. Bicarbonates cause temporary hardness. Sodium and potassium salts do not cause hardness.
[NDA 2015-I]

---

Q8. Permanent hardness of water is due to the presence of soluble:

1. Chloride of calcium 2. Bicarbonate of calcium 3. Sulphate of magnesium 4. Bicarbonate of magnesium
   (a) 1 only (b) 1 and 3 only (c) 2 and 4 (d) 1, 2 and 3

Answer: (b) 1 and 3 only
Chloride of calcium (CaCl₂) = permanent hardness ✓. Sulphate of magnesium (MgSO₄) = permanent hardness ✓. Bicarbonate of calcium = temporary hardness ✗. Bicarbonate of magnesium = temporary hardness ✗. Only options 1 and 3 are causes of permanent hardness.
[NDA 2011-I]

Q9. Alum is used as:
(a) An analgesic (b) A fertiliser (c) A disinfectant (d) A purifier for water

Answer: (d) A purifier for water
Alum (AlK(SO₄)₂·12H₂O) is a coagulant used in water treatment. Al³⁺ ions released by alum cause fine colloidal particles of clay and silt to clump together (coagulate) into larger particles that settle or are filtered out. This clears turbid water. Alum does NOT kill bacteria.
[NDA 2009-I]

---

Q10. Heavy water implies:
(a) Water used in heavy industries
(b) Water which contains SO₄²⁻ and Cl⁻ of calcium and magnesium
(c) Deuterated water
(d) Water which has maximum density

Answer: (c) Deuterated water
Heavy water (D₂O) is water where normal hydrogen (H-1, protium) is replaced by deuterium (H-2, which has one proton and one neutron). Its molecular mass is 20 u — heavier than ordinary water (18 u). It is used as a moderator in nuclear reactors.
[NDA tested via CDS]

---

Q11. Which one of the following statements about water properties is NOT correct?
(a) Water starts boiling when its vapour pressure becomes equal to atmospheric pressure
(b) Water is known as universal solvent
(c) Permanent hardness of water is due to presence of MgCl₂, CaCl₂, MgSO₄ and CaSO₄
(d) Density of ice is greater than that of water

Answer: (d) Density of ice is greater than that of water
Ice is LESS dense than liquid water — this is why ice floats. Water reaches maximum density at 4°C. Below 4°C, hydrogen bonds form an open hexagonal lattice in ice that occupies more space than liquid water. Options (a), (b), and (c) are all correct.
[NDA tested via CDS]

---

Q12. Consider the following statements about water as a solvent:

1. Water is a good solvent for ionic compounds but poor solvent for covalent compounds.
2. Water is a good solvent for covalent compounds but poor solvent for ionic compounds.
3. Water has maximum density at 277 K.
   (a) 1 and 3 only (b) 2 and 3 only (c) 1 and 2 only (d) 1, 2 and 3

Answer: (a) 1 and 3 only
Statement 1 ✓ — water is polar, so it dissolves ionic and polar compounds well but not non-polar covalent compounds (oils, fats). Statement 3 ✓ — water has maximum density at 4°C = 277 K. Statement 2 ✗ — this is the reverse of statement 1 and is false.
[NDA 2010-II]

Q13. Statement I: Soaps do not form lather with water containing salts of calcium and magnesium.
Statement II: Calcium and magnesium salts of long chain fatty acids are insoluble in water.
(a) Both true, II correctly explains I
(b) Both true, II does not explain I
(c) I true, II false
(d) I false, II true

Answer: (a) Both true, Statement II correctly explains Statement I
Ca²⁺ and Mg²⁺ ions in hard water react with soap to form insoluble calcium and magnesium fatty acid salts — scum. This scum cannot form micelles. Soap is consumed in forming scum before any lather or cleaning occurs.
[NDA tested via CDS]

---

Q14. Washing soda is used for softening hard water. What type of hardness does it remove?
(a) Temporary hardness only
(b) Permanent hardness only
(c) Both temporary and permanent hardness
(d) It does not soften hard water

Answer: (c) Both temporary and permanent hardness
Washing soda (Na₂CO₃) reacts with Ca²⁺ and Mg²⁺ ions in water to form insoluble carbonates — CaCO₃ and MgCO₃ — which precipitate out. This works regardless of whether the original anion was bicarbonate, sulphate, or chloride. So washing soda removes both types.
[NDA 2025-II, NDA tested via CDS]

---

Q15. Which one of the following methods can be used to remove arsenic from contaminated ground water?
(a) Boiling (b) Reverse osmosis (c) Ion exchange (d) Coagulation-adsorption

Answer: (d) Coagulation-adsorption
Arsenic in groundwater is typically in dissolved ionic form — boiling does not remove dissolved ions. Ion exchange can remove it but coagulation-adsorption (using alum or ferric salts) is the most effective and commonly used method for arsenic removal at community level.
[NDA 2011-I]