Week 3 · 1-3 Physical Properties of Water (A): density, relative density, the hydrometer, what changes the density of ocean water, ocean currents, the 4 °C maximum density and why lakes freeze from the top · 3 lessons
3
The 4 °C turning point: why lakes freeze from the top
Integrated Science Sec 1 Term 1, Unit 1: pp. 16–19
GoalExplain why fresh water is densest at 4 °C and why a lake freezes from the top while life goes on below.
New words
maximum density · the greatest density of water, reached at 4 °C, where it is 1000 kg/m³contract · to take up less volumeinsulate · to slow down the loss of heat
Explanation
Most liquids keep contracting as they get colder. Water is an exception. When pure water cools from higher temperatures it contracts and becomes denser down to 4 °C, where its density is greatest, 1000 kg/m³. From 4 °C down to 0 °C it expands a little and becomes less dense again, and above 4 °C it expands as it warms. In autumn the surface water of a lake cools and sinks when it reaches 4 °C. After that the surface water keeps cooling below 4 °C, becomes less dense, and stays on top, so the surface freezes first. Ice is less dense than liquid water, about 917 kg/m³, because its molecules are held farther apart in an open arrangement. So ice floats. The floating layer of ice insulates the water below, so fish and other living things survive under the ice. Keep this rule for fresh water: salt lowers both the freezing point and the temperature of maximum density.
Worked examples
Read the density of fresh water (real values): 0 °C: 999.8, 4 °C: 1000.0, 8 °C: 999.9, 10 °C: 999.7, 20 °C: 998.2 kg/m³. The differences are tiny, but the pattern matters: the highest value is at 4 °C. The graph shows how far below 1000.0 each value is, so the lowest point of the line is the densest water. The temperatures are not evenly spaced.
How much of a floating ice block is under water? The share is 917 ÷ 1000 = 0.917, so about 91.7%, nearly 92%. Only about 8% sticks out. Also, 1000 kg of water becomes ice of volume 1000 ÷ 917 ≈ 1.09 m³ instead of 1 m³, about 9% more volume.
air below 0 °C→ice on top, 0 °C→water near 4 °C below→life continues
A pond has water at about 4 °C near the bottom and 0 °C ice on top. Is the water at the bottom warmer than the water just under the ice? Yes, and that is why the pond bottom stays liquid: the densest water lies low and the ice insulates it.
Class activity
Ice cube test, with a grown-up. Float an ice cube in a clear glass of tap water. Draw the glass, mark the water level, and show the part of the cube above the water. Estimate the share under water. Then say, in a cause-and-effect chain, why ice floats and why this matters for a lake in winter.
Practice
1
At which temperature is fresh water densest, and what is its density?
At 4 °C; 1000 kg/m³.
2
Why does the top of a lake freeze first?
Surface water cooling below 4 °C becomes less dense, so it stays on top and freezes.
3
Why does ice float?
Its molecules are farther apart in an open arrangement, so ice (about 917 kg/m³) is less dense than water.
4
How does a layer of ice help the fish below?
It insulates the water, so the water below stays liquid and warmer.
