When water is heated between 0°C and 4°C, it's density

Analyze the question: The question asks what happens to the density of water as it is heated from 0°C to 4°C only. This is about the anomalous expansion of water.

Concept: Most liquids expand when heated, so their density decreases. Water is unusual between 0°C and 4°C:

• When ice melts at 0°C, the water formed contracts as it is heated.
• Its volume keeps decreasing until it reaches a minimum at 4°C.
• Above 4°C, water expands normally.

Density = mass / volume. The mass stays the same, so when the volume decreases the density increases.

Solution:

1. From 0°C to 4°C, the volume of a fixed mass of water decreases.
2. Mass is constant, so density = mass/volume increases.
3. At 4°C, water reaches its maximum density, about 1000 kg/m³ (1 g/cm³).
4. Above 4°C, the density decreases again, but that range is not part of this question.

Checking the options:

• A: "increases for a while and then decreases" would describe heating through 0°C to above 4°C. The question only covers 0°C to 4°C, so it does not apply.
• B: Wrong, since the density does not decrease in this range.
• C: Correct. Density increases throughout the range.
• D: This is the normal behaviour of liquids, but not of water in this range.
• E: Wrong, since the density does change.

Tip: Remember "4°C = maximum density". Below 4°C, heating makes water denser. Above 4°C, heating makes it less dense. This is also why ponds freeze from the top down, and fish survive in the warmer, denser water at the bottom (about 4°C). Always read the temperature range in the question carefully, because it decides between options like A and C.