Roll the Play-Doh into a solid ball to make it sink. The ball shape will make it heavier, or denser, than water. Sculpt the Play-Doh into a boat and it will float. Why? Its new shape displaces (pushes aside) more water than the ball shape.

The water level will stay about the same after the ice cube melts because water from the ice takes up less space than the ice itself. Remember: when water freezes and changes to ice, the ice takes up about one-tenth more space than liquid water.

Dissociation (ionisation), pH and buffers

There is a slight tendency for water molecules to dissociate into ions according to the equation: 2H2O = H₃O⁺ + OH^- water oxonium hydroxide molecule ion (hydroxyl) ion It is simpler, however, to consider the dissociation as: H₂O = H⁺ + OH^- water oxonium hydroxide molecule ion (hydroxyl) ion In a litre of water this dissociation produces I /10 000 000 (10^-7) mole of hydrogen ions. This is equivalent to a pH of 7, which is neutral. If the concentration of hydrogen ions was greater, say 1/1000 (10^-3) mole hydrogen ions per litre, the pH would be 3 and the solution would be acidic. Any pH below 7 is acidic, any above is basic. An acid is therefore a substance that donates hydrogen ions and a base is a hydrogen ion acceptor. Note that the pH scale is not linear but logarithmic.

Water has very powerful solvent properties and it dissolves more substances than any other common liquid.

Water as a solvent

Many substances dissolve in water. A substance that dissolves in a liquid is said to be soluble. Sugar, for example, is soluble in water. Water is the solvent and sugar is known as the solute. The solvent and the solute together form a solution. When water is the solvent, the solution is said to be an aqueous solution. Since there is so much water in living organisms, water is a very important solvent. Substances such as glucose dissolve in the water of the blood, which allows it to be carried around the human body. In plants, the most commonly transported substance is sucrose, which is also soluble in water.

There is a limit to the amount of solute that can be dissolved in a solvent. For example, if increasing amounts of sugar are added to water there will come a point at which no more sugar will dissolve in the water. The solution is then said to be saturated. The point at which saturation is reached depends on the temperature of the solvent, because warm water will dissolve more sugar than cold water. There is an important exception to this general rule. Gases can dissolve in liquids. However, gases actually become less soluble as the temperature rises. Water will hold more dissolved oxygen at a low temperature than it will at a high temperature. This has an important effect on the living organisms found in water. Fish such as salmon and trout need a lot of oxygen and are very sensitive to the amount of oxygen dissolve in the water. One particular type of water pollution, thermal pollution, can have a dramatic effect on these fish. Power stations use water from rivers and seas as a coolant and return it to the source as warmer water. However, the warmer water contains less oxygen, so salmon and trout often e when hot water is pumped into their river.

Sometimes it is possible to make more of a substance dissolve in water without raising the temperature. One method of achieving this is to increase the pressure being applied to the liquid. If a force is applied to a liquid, more gas molecules will dissolve in the liquid. Fizzy drinks are an example of this technique in action. The bubbles in drinks are made by the gas carbon dioxide, which is dissolved into the drink under pressure. When the can or bottle is opened, the pressure is released and the excess gas is able to escape in the form of bubbles. As the gas bubbles to the surface, the drink loses its fizz. It will eventually become flat. In some places of the world water comes out of the ground already fizzy! The water in the ground is under pressure and contains dissolved gases, such as carbon dioxide. As the water leaves the ground the pressure is reduced, allowing the bubbles of gas to escape into the atmosphere.