The maximum density of water occurs at a temperature of

Analyze the question The question asks at what temperature water is densest. This is about the unusual behaviour of water when it is heated or cooled near its freezing point.

Explain the concept Most liquids contract when cooled and keep getting denser until they freeze. Water behaves differently, which is called the anomalous expansion of water:

• When water at 0°C is heated, it contracts instead of expanding, until it reaches 4°C.
• Above 4°C, it expands as normal.
• So the volume of a fixed mass of water is smallest at 4°C. Since density = mass/volume, the density is greatest at 4°C.

Step-by-step solution

1. Density = mass ÷ volume.
2. The mass stays constant, so density is greatest when volume is smallest.
3. Water's volume is smallest at about 4°C (more precisely 3.98°C).
4. At 0°C, water (and especially ice) is less dense. This is why ice floats on water.
5. At 37°C, water has expanded further and is less dense than at 4°C.
6. 273°C is not even a liquid state at normal pressure (it is 273 K that equals 0°C, so this option is a trap).

Checking the options:

• A. 0°C: water is less dense here than at 4°C. Wrong.
• B. 4°C: maximum density, about 1000 kg/m³. Correct.
• C. 37°C: less dense. Wrong.
• D. 273°C: water would be steam. Wrong.

Answer: B (4°C)

Exam tip Remember "4 for more": water at 4°C has the most density. This also explains why lakes freeze from the top down in cold countries. The 4°C water sinks to the bottom, so fish can survive under the ice. Do not confuse 273 K (= 0°C) with 273°C.