Physical properties of water and animals in aquatic habitats

The big idea: An animal in water is held up, slowed down, chilled quickly and kept at a steady temperature. An animal in air is none of these.

Four physical properties of water explain it: buoyancy, viscosity, thermal conductivity and specific heat capacity.

Remember it as: Water holds you up, holds you back, steals your heat and keeps its temperature. Air does none of these.

Real example: the black-throated loon (Gavia arctica) lives in both media. It hunts fish under the water of northern lakes and flies between lakes through the air, so every difference between water and air shows on this one bird.

A litre of water has a mass of about 1 kg. A litre of air has a mass of about 1.2 g, roughly 800 times less.

A dense fluid pushes up hard on a body in it. That push is buoyancy, and it is why floating in water takes no effort, while nothing floats in air without flying.

Real example: a loon floats at rest, and most birds float high, with air in their bones and feathers. The loon is different: its bones are solid, not air-filled, so it rides low and can dive quickly after fish.

Floating is buoyancy, not surface tension: A sea slug or a seal floating near the surface is held up by buoyancy, the push of dense water. Surface tension is a different property: the skin-like film at the surface that a pond skater's feet rest on. Name buoyancy for anything that floats in the water.

Move your hand through air and you feel nothing. Move it through water and the water pushes back. That resistance is viscosity, and water is about 50 times more viscous than air.

The push a moving body feels is called drag. In water, drag is large, so shape matters.

Real example: the loon's body is a torpedo, and its feet are set so far back that they work as paddles behind the body. The same feet make it clumsy on land: a loon can barely walk, and it needs a long run across the water to take off into the thin air.

Put one hand in water at 20 degrees and the other in air at 20 degrees. The water feels colder. It is not colder: it takes heat out of your hand faster, about 25 times faster.

That is thermal conductivity, and it is higher in water than in air.

Real example: a loon's feathers are waterproof and lie tight, trapping a thin layer of air next to the skin. Air is a poor conductor, so the trapped layer is the bird's insulation against the lake water that would otherwise pull heat out of it all day.

Blubber insulates: Blubber is a layer that slows heat loss. It does not make heat, and in this topic it is not an energy store: say insulation.

Heat a pan of water and a pan of air with the same flame: the air warms in seconds, the water takes minutes.

Water needs about 4,200 joules to warm one kilogram by one degree; air needs about 1,000. That is specific heat capacity, and water's is higher.

Real example: a small pond in summer. The air above it swings by 17 degrees in a day; the water moves by about 2. The fish, insect larvae and the loon's prey live in a habitat with no sudden temperature changes.

A wet hand feels cold as it dries. The water is not cold: as it evaporates it takes heat with it.

Sweating uses this. Water is a good coolant; air is not, because air does not evaporate from the skin.

A classroom experiment shows the same thing without a body. Two cups of hot water cool in a room at 18 degrees; one is wrapped in tissue paper soaked in water. The wrapped cup cools faster, because the water evaporating from the paper takes heat from the cup, the way sweat takes heat from the skin.

Real example: a marathon runner can lose about a litre of sweat an hour. Evaporating that litre takes about 2.3 million joules of heat out of the body, which is why a runner in dry air stays cool and a runner in humid air, where sweat cannot evaporate, overheats.

Name the right property: Cooling by sweat is water's high heat of vaporisation: the heat needed to turn liquid into vapour. Specific heat capacity is a different property (the heat needed to change temperature), and naming it here does not score.

Methane is the gas in natural gas. Its molecule, CH4, is about the same size and mass as a water molecule, H2O. Yet at room temperature methane is a gas and water is a liquid.

The difference is the hydrogen bonds. Water has them, methane does not.

Remember it as: Same size, different manners: water holds hands (hydrogen bonds), methane does not.

Real example: on Titan, Saturn's largest moon, the surface is about minus 180 degrees, and there methane is the liquid: it fills lakes and falls as rain. On Earth, always far warmer than minus 161, methane is a gas and water is the liquid that fills lakes and seas.

Always give the cause: 'Water has a high boiling point' on its own does not score. Say why: water forms hydrogen bonds and methane does not, and breaking those bonds takes energy.