Which one of the following regarding density of water at atmospheric pressure is correct?
- (a)Density of water at 4°C is 1000 kg/m³.
- (b)Density of water at 0°C is 1000 kg/m³.
- (c)Density of water at 0°C is 100 kg/m³.
- (d)Density of water at 4°C is 10 kg/m³.
Correct — A, the density of water at 4 °C is 1000 kg/m³. Water reaches its maximum density at 4 °C, where one cubic metre has a mass of 1000 kg (equal to 1 g/cm³). At 0 °C water is slightly less dense (about 999.8 kg/m³) owing to its anomalous expansion, so option (b) is wrong, and the values 100 kg/m³ and 10 kg/m³ are far too small — off by factors of ten and a hundred.
- (b)Density of water at 0°C is 1000 kg/m³. — Water is densest at 4 °C, not at 0 °C; at 0 °C it is a shade lighter (about 999.8 kg/m³), which is why cold water rises and freezes at the top.
- (c)Density of water at 0°C is 100 kg/m³. — This is off by a factor of ten — the density of water is about 1000 kg/m³, not 100 kg/m³.
- (d)Density of water at 4°C is 10 kg/m³. — Far too low — 10 kg/m³ is closer to a gas; liquid water is about 1000 kg/m³.
Water shows anomalous expansion: as it cools it contracts and grows denser, but below 4 °C it starts to expand again, so its maximum density (1000 kg/m³ = 1 g/cm³) occurs at 4 °C rather than at the freezing point.
The litre and kilogram were historically tied to water near 4 °C, so 1000 kg/m³ is the value to remember. The trap options either shift the peak density to 0 °C or scale the number wrongly.
- Water has its maximum density at 4 °C, equal to 1000 kg/m³ (1 g/cm³).
- Below 4 °C water expands on cooling (anomalous expansion), so ice is less dense than water and floats.
- At 0 °C liquid water is about 999.8 kg/m³ and ice about 917 kg/m³.
- This anomaly lets aquatic life survive beneath a frozen surface, as the denser 4 °C water sinks to the bottom.
- Assuming water is densest at 0 °C (its freezing point) instead of 4 °C.
- Mixing up the numerical value — 1000 kg/m³ is the same as 1 g/cm³.
Given as value-matching options — anchor on 'maximum density 1000 kg/m³ at 4 °C' and reject the wrong temperatures or scaled-down numbers.
UPSC_2011_GS1_Q502011The 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 — water's maximum density at 4 °C. That UPSC item explains why a lake stays liquid at the bottom while its surface freezes (the densest 4 °C water sinks); this NDA item simply states that density value, 1000 kg/m³ at 4 °C.
- practice — not a real PYQ
Water has its maximum density at
- (a)0 °C
- (b)4 °C
- (c)100 °C
- (d)−4 °C
Answer(b) 4 °C — water is densest at this temperature.
- practice — not a real PYQ
The density of water at 4 °C is
- (a)1 kg/m³
- (b)100 kg/m³
- (c)1000 kg/m³
- (d)10000 kg/m³
Answer(c) 1000 kg/m³ — equal to 1 g/cm³.