Week 4 · 1-3 Physical Properties of Water (B): heat and temperature, internal energy, the Kelvin scale, specific heat and Q = mcΔt with worked examples, sea breeze, latent heat of vaporisation, effects on marine organisms · 3 lessons
3
Sea breeze, latent heat and life in warm and cold water
Integrated Science Sec 1 Term 1, Unit 1: pp. 20–27
GoalUse specific heat and latent heat to explain the sea breeze, oceans as heat stores, sweat cooling and life at different temperatures.
New words
sea breeze · a wind blowing from the cool sea towards the warmer land during the daylatent heat of vaporisation · the energy absorbed by 1 kg of a liquid turning into vapour at constant temperaturecold-blooded and warm-blooded · animals whose body temperature follows the surroundings, and animals (birds, mammals) that keep it constant
Explanation
In sunlight the land and sand warm up faster than the sea, because water has a high specific heat and also mixes. The air over the land warms, expands and rises, and cooler air moves in from over the sea: this is the sea breeze. At night the wind reverses. Water also needs a lot of energy to change from liquid to vapour: its latent heat of vaporisation is about 2 260 000 J/kg at 100 °C (extra fact), about 5 times the 420 000 J needed to heat 1 kg from 0 °C to 100 °C, because hydrogen bonds must be broken. During the change the temperature stays constant. When sweat evaporates, or a leaf transpires, this energy is taken from the skin or the leaf, which cools. Oceans and lakes store heat by day and release it slowly, so places near them have steadier temperatures. Most organisms live in a narrow temperature range. Cold-blooded animals such as fish follow the temperature around them; birds and mammals keep a steady body temperature. Corals can die in water that is too warm.
Worked examples
land warms faster→air over land rises→cool air moves in from the sea→sea breeze by day
Put the sea breeze in order. (1) Sunlight warms the land faster than the sea. (2) The air over the land warms, expands and rises. (3) Cooler air flows in from the sea. (4) A wind blows from sea to land during the day.
Compare two kinds of heating for 1 kg of water. Warming it from 0 °C to 100 °C takes 1 × 4200 × 100 = 420 000 J. Turning it into vapour at 100 °C takes about 2 260 000 J, which is 2 260 000 ÷ 420 000 ≈ 5.4 times more. The graph is a sketch: the flat part is the change into vapour at constant temperature.
How much heat is needed to vaporise 0.2 kg of water at 100 °C? Q = m × L = 0.2 × 2 260 000 = 452 000 J. During this the temperature stays at 100 °C.
Class activity
Wet cloth in a breeze, with a grown-up. Wrap a damp cloth round one cup of tepid tap water and a dry cloth round another, and stand both outdoors in a breeze or by an open window (or fan them with a piece of card) for twenty minutes. Touch the cups and describe which feels cooler. Explain with the idea of evaporation taking heat away. Wash your hands afterwards.
Practice
1
Why does the sea breeze blow from the sea to the land during the day?
The land warms faster, its air rises, and cooler air from the sea moves in.
2
What happens to the temperature of water while it is changing into vapour at 100 °C?
It stays constant; the energy is used to break hydrogen bonds.
3
How does sweat help the body cool down?
When it evaporates it takes latent heat from the skin.
4
Give one example of a cold-blooded and one of a warm-blooded animal.
For example: a fish or a frog is cold-blooded; a bird or a cat is warm-blooded.
