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
2
What changes the density of ocean water, and ocean currents
Integrated Science Sec 1 Term 1, Unit 1: pp. 16–19
GoalName the factors that change the density of sea water and explain how density differences move water in the ocean.
New words
salinity · the amount of dissolved salts in waterocean current · a steady flow of water in the oceanincompressible · hardly changing in volume when pressed; liquids are nearly incompressible
Explanation
Three things change the density of sea water. Salinity: more dissolved salt makes water denser. Temperature: colder water is denser (for fresh water this is true down to 4 °C). Pressure: water is almost incompressible, so its density rises only a little, and only at great depth. Differences in density drive vertical ocean currents. Cold, salty surface water is dense, so it sinks, and other water moves in to replace it; far away, deep water rises again. These currents carry heat and salt from the tropics towards the poles, bring nutrients from deep water to the surface, and spread fresh water from rivers and melting glaciers around the world. Sea water is about 1.03 g/cm³ (extra fact), so its relative density is about 1.03.
polar surface water cools and gets salty→denser→sinks→flows deep towards the tropics→rises, bringing nutrients up
Worked examples
Compare 250 mL of fresh water (250 g) with 250 mL of sea water. Sea water has density about 1.03 g/cm³, so 250 cm³ has a mass of 250 × 1.03 = 257.5 g. Same volume, more mass: sea water is denser.
Two water samples have the same temperature but A is saltier. Which sinks if they meet? A, because more dissolved salt makes it denser. Now suppose they have the same salinity but B is colder: B is denser and sinks.
Why can’t ocean currents be explained by wind alone? Some move in the deep ocean, far from the wind, where density differences move the water. Vertical currents link the surface and the deep ocean.
Class activity
Egg test, with a grown-up. Fill a tall glass with tap water and gently lower a raw egg in a spoon. Describe where it goes. An adult then stirs in salt, spoon by spoon, until the egg rises or floats. Describe what happened and say what the salt did to the density of the water. Wash your hands afterwards. The egg is not eaten.
Practice
1
Name the three factors that change the density of sea water.
Salinity, temperature and (slightly, at great depth) pressure.
2
Sea water at the surface near a pole gets colder and saltier. What does it do, and why?
It sinks, because it becomes denser.
3
What is the mass of 400 cm³ of sea water of density 1.03 g/cm³?
400 × 1.03 = 412 g.
4
Give one job of vertical ocean currents.
For example: they bring nutrients from deep water to the surface, or carry heat and salt towards the poles.
