At which temperature does liquid water show maximum density?
- (a)299 K
- (b)277 K
- (c)285 K
- (d)373 K
Correct — B, 277 K. Water reaches its maximum density at about 4 °C. Converting to the Kelvin scale, 4 °C = 4 + 273 = 277 K. Below 4 °C water expands again instead of contracting (the anomalous expansion of water), so ice at 0 °C is less dense than liquid water — which is why ice floats and lakes freeze from the surface downwards.
- (a)299 K — 299 K ≈ 26 °C — far above 4 °C; at this warm temperature water has already expanded and is less dense than at 4 °C.
- (c)285 K — 285 K ≈ 12 °C — above the density maximum; water is denser at 4 °C than at 12 °C.
- (d)373 K — 373 K = 100 °C is the boiling point of water at 1 atm, not the density maximum — water is least dense as vapour there. This is the classic trap.
Most liquids contract steadily as they cool. Water is anomalous: from 100 °C down to 4 °C it contracts and grows denser, but between 4 °C and 0 °C it expands. It is therefore densest at about 4 °C (≈ 277 K), and solid ice is less dense than liquid water, so ice floats.
The question is really a units check: know that water is densest at 4 °C, then convert to Kelvin by adding 273 → 277 K. The tempting wrong answer is 373 K, the boiling point.
- Water has maximum density (~1000 kg/m³) at about 4 °C = 277 K.
- Below 4 °C water expands (anomalous expansion); ice is less dense than water and floats.
- 0 °C = 273 K (freezing point); 100 °C = 373 K (boiling point at 1 atm).
- This anomaly lets fish and aquatic life survive under a surface layer of ice in winter.
Density peaks at 4 °C = 277 K, then falls again down to 0 °C — answer (b).
- Choosing 373 K — that is the boiling point, not the density maximum.
- Forgetting to add 273 when converting 4 °C to Kelvin.
'Temperature of maximum density of water' in Kelvin or Celsius, or 'why does ice float / lakes freeze from the top'.
No directly related past PYQ was found.
- practice — not a real PYQ
Ice floats on water because:
- (a)ice is warmer than water
- (b)ice is less dense than water
- (c)water is less dense than ice
- (d)of the surface tension of water
Answer(b) ice is less dense than water — water expands on freezing, so ice floats.
- practice — not a real PYQ
The temperature at which liquid water has its maximum density is about:
- (a)0 °C
- (b)4 °C
- (c)10 °C
- (d)100 °C
Answer(b) 4 °C — water is densest at roughly 4 °C (≈ 277 K).