A quantity of water at 0 °C is heated to 30 °C. For each degree rise in temperature, its density will---------

Analyze the question: Water is heated from 0 °C to 30 °C. We need to know how its density changes over this range.

Concept: the anomalous expansion of water

Most liquids expand when heated, so their density falls steadily. Water behaves differently between 0 °C and 4 °C:

• From 0 °C to 4 °C, water contracts as it is heated, so its volume decreases and its density increases.
• At 4 °C, water has its minimum volume and therefore its maximum density (1000 kg/m³).
• Above 4 °C, water expands normally, so its volume increases and its density decreases.

Solution (step by step):

1. At 0 °C, the density of water is about 999.8 kg/m³.
2. Heating to 4 °C: the density rises to its maximum of about 1000 kg/m³.
3. Heating from 4 °C to 30 °C: the density falls steadily (about 995.7 kg/m³ at 30 °C).

So the density first rises and then falls.

Checking the options:

• A. Rise steadily: wrong, because it falls after 4 °C.
• B. Fall steadily: wrong, because it rises between 0 °C and 4 °C.
• C. Fall and then rises: this is the reverse of what happens. It describes the volume of water, not the density.
• D. Rise and then falls: correct.

Exam tip: Remember that 4 °C is the turning point. At 4 °C, water has its minimum volume and maximum density. Volume and density behave in opposite ways, so if a question asks about volume, flip the answer: volume falls and then rises, while density rises and then falls. This is also why ice floats and why fish survive in the water under frozen lakes.