Dissolving gases

When water flows down a stream, a very important process takes place. Oxygen from the air dissolves in the water. This means that a whole range of living things can survive in the stream and the river below.

Oxygen is a gas in the air. Like all gases it has particles which can move about freely. When they hit each other or the particles of other air gases, they simply bounce off and move on. When oxygen particles hit the surface of the water some of them do not bounce off. They slide between the water particles and dissolve.

Streams and the upper parts of rivers are good places for oxygen to dissolve in water. One reason for this is that the water splashes over cascades and waterfalls. This makes the water surface larger as drops and bubbles are formed. The larger water surface gives more oxygen particles a chance to dissolve. Another reason why these are good places for oxygen to dissolve is that the water is cold. Gases dissolve better in cold water than warm.

As the water flows through the upper part of the river it allows special kinds of river life to survive. These animals, such as stonefly nymphs and trout, are adapted to living in water with a large amount of oxygen in it. When the water flows further down the river, it warms up a little and loses some of its oxygen. In this part of the river are fish, such as carp, which can survive on smaller amounts of oxygen.

Oxygen is not the only gas to dissolve in water. All the gases in the air dissolve in it too but they are less important for water life.

If you ask an adult to heat up some water for you, you can watch dissolved air escaping. As the temperature of the water rises you will see bubbles form. First there will be a few small bubbles then more will develop and increase in size. The small ones will hold onto the sides of the pan but as they increase in size, they break away and float to the surface.
When a dissolved solid is separated from water, the water is allowed to evaporate. In evaporation, water particles burst through the liquid surface and escape into the air. They form water vapour, a gas, which mixes with other air gases. When a dissolved gas separates from water, the gas particles pick up speed and burst out of the water surface. This may occur at the surface of a lake, pond or pan. It can also occur at any tiny projection on the inner wall of the pan. When this happens the air particles meet together and push back on the water and make a bubble. They cannot re-enter the water because it has become increasingly warm so they bounce around and are joined by more air particles until a large bubble is made which floats to the surface and pops.

Next time you have a drink of water remember you are not just drinking a liquid but are drinking a solution which contains dissolved air.

How do water animals breathe in oxygen?

They have gills. We tend to think that only fish have gills but this is not true. Insects such as stonefly nymphs have gills and so do many snails and some kinds of worms. All water life needs oxygen - not just animals that live in streams and rivers. Oxygen enters the surface of the sea and is breathed by sea creatures.

When the sea crashes on a shore, will the amount of oxygen in the water increase?

Yes, it will. At any place where the surface of the water is increased there will be an increase in the amount of air dissolving in the water.

How is the surface of a liquid increased when it flows and splashes?

Think of a beaker of water. The only surface in contact with the air is the small area at the top of the liquid. Now think of pouring the water into a bowl from a height of about a metre. As the water leaves the beaker it forms a long falling column with a much larger surface in contact with the air. When it hits the bowl it breaks up into drops which bounce back up into the air. The surface area of all the droplets is greater than the falling water column. When the water settles in the bowl it has much more oxygen in it than if the beaker had been placed close to the bowl and the water had just been slowly tipped in.

What is the gas in fizzy drinks?

It is carbon dioxide.

How is carbon dioxide put into a fizzy drink?

Gases have a major difference from solids and liquids. They can be squashed. Solids and liquids cannot be squashed because their particles are close together. Gas particles can have large spaces between them. This means that the gas can be squashed because the particles can be forced closer together. You force air particles together when you pump up a tyre on a bicycle. When air is squashed we say that it is put under pressure. In the making of fizzy drinks carbon dioxide is put under pressure and squashed into a small space. When the gas is released into water, large numbers of its particles are pushed between the water particles. This means that a large amount of carbon dioxide is dissolved in the drink.

Why does the drink fizz when you open a can or a bottle?

When the drink is put into the can or bottle some of the carbon dioxide escapes and fills the space at the top. The carbon dioxide particles are still under pressure and push so hard on the water that no more carbon dioxide particles can escape. When you open the bottle or can you release these particles. This reduces the pressure on the carbon dioxide in the drink and most of its particles shoot out and form bubbles.