Which one among the following statements with reference to the properties of water is not correct?
- (a)The specific heat of water is abnormally high.
- (b)Latent heat of fusion of water is very low.
- (c)Density of water is higher than ice.
- (d)Pure water is a non-conductor of electricity.
Correct — B, 'latent heat of fusion of water is very low'. This is the statement that is NOT correct, and since the question asks for the incorrect one, it is the answer. The latent heat of fusion of ice, the energy needed to melt it at 0 °C, is actually very high, about 334 joules per gram (334 kJ per kg). Calling it 'very low' is false. The other three statements are all true properties of water, so the odd one out is (b).
- (a)The specific heat of water is abnormally high. — This is true, so it is not the answer. Water's specific heat, about 4.18 J per gram per °C, is unusually high, which is why water heats and cools slowly and moderates climate.
- (c)Density of water is higher than ice. — This is true, so it is not the answer. Liquid water is denser than ice (water is densest at 4 °C), which is exactly why ice floats on water.
- (d)Pure water is a non-conductor of electricity. — This is true, so it is not the answer. Absolutely pure water has very few ions and conducts electricity very poorly; ordinary water conducts only because of dissolved salts.
Water has several anomalous properties that arise from hydrogen bonding between its molecules. It has an abnormally high specific heat and a high latent heat of fusion, it reaches maximum density at 4 °C so that ice is less dense than liquid water, and when pure it is a very poor conductor of electricity. The question hinges on the one statement that misstates one of these.
The technique for a 'which is not correct' question is to test each statement against known facts and find the single false one. Three of the four here are standard true properties of water. The trap statement inverts a real property — it calls the latent heat of fusion 'very low' when in fact it is high (about 334 J/g). Because the question asks for the incorrect statement, that inverted one is exactly what you must pick.
- The latent heat of fusion of ice is high, about 334 J/g (334 kJ/kg) — so statement (b) is false.
- Water's specific heat is abnormally high, about 4.18 J/g/°C, letting it store and moderate large amounts of heat.
- Liquid water is denser than ice, being densest at 4 °C, which is why ice floats.
- Absolutely pure water is a very poor conductor of electricity; conduction in ordinary water comes from dissolved ions.
- Reading past the word 'not' and picking a true statement instead of the false one.
- Believing the latent heat of fusion of water is low; it is in fact high (about 334 J/g).
Asked as a 'which statement is not correct' item on the special properties of water, or as single true or false facts about specific heat, latent heat and density.
Consider the following statements: 1. Steam at 100 °C and boiling water at 100 °C contain the same amount of heat. 2. Latent heat of fusion of ice is equal to the latent heat of vaporization of water. 3. In an air-conditioner, heat is extracted from the room air at the evaporator coils and is rejected out at the condenser coils. Which of these statements is/are correct?
- (a) 1 and 2
- (b) 2 and 3
- (c) Only 2
- (d) Only 3
Answer(d) Only 3
Same concept — the latent heat of water. Its key rests on the fact that the latent heat of fusion of ice (~334 kJ/kg) is far smaller than the latent heat of vaporization of water (~2260 kJ/kg), the very quantity misdescribed as 'very low' in the NDA statement (b).
The surface of a lake is frozen in severe winter, but the water at its bottom is still liquid. What is the reason?
- (a) Ice is a bad conductor of heat
- (b) Since the surface of the lake is at the same temperature as the air, no heat is lost
- (c) The density of water is maximum at 4°C
- (d) None of the statements (a), (b) and (c) given above is correct
Answer(c) The density of water is maximum at 4°C
Same concept as statement (c) of the NDA item — liquid water is denser than ice. Because water is densest at 4 °C, the colder, lighter water and ice stay on top while denser 4 °C water sinks, keeping a lake's bottom liquid even when its surface freezes.
- practice — not a real PYQ
Ice floats on water because:
- (a)Ice is warmer than water
- (b)Ice is denser than water
- (c)Ice is less dense than water
- (d)Ice has a higher specific heat
Answer(c) Ice is less dense than water — liquid water is densest at 4 °C, so ice floats.
- practice — not a real PYQ
The approximate latent heat of fusion of ice is:
- (a)34 J/g
- (b)334 J/g
- (c)2260 J/g
- (d)4.18 J/g
Answer(b) 334 J/g — the (high) energy needed to melt 1 g of ice at 0 °C.