S
Temperature
Boiling: the temperature stays constant. The energy put in makes the particles move faster and overcomes the forces holding the liquid together. Melting: the temperature stays constant. The energy put in is used to overcome the forces holding the lattice together.
C gas B liquid
In regions A, B and C the temperature rises with heating. The energy of the particles increases and they vibrate or move faster. In region B, the rate of evaporation increases with temperature.
A solid
Time Reversing the experiment gives a cooling curve. The temperature stays constant during condensation and freezing – energy is given out. Condensation and freezing are exothermic processes. Melting, evaporation and boiling are endothermic processes.
Figure 2.24
The energy changes taking place during heating and cooling.
The way the particles in the three states are arranged also helps to explain the temperature changes when a substance is heated or cooled. Figure 2.24 (overleaf) summarises the energy changes taking place at the different stages of a heating-curve or cooling-curve experiment.
Figure 2.25 The diffusion of potassium manganate(vii) in water as it dissolves.
movement of oxygen from the lungs to the blood, and of carbon dioxide from the blood to the lungs. The diffusion of gases A few drops of liquid bromine are put into a gas jar and the lid is replaced. After a short time the jar becomes full of brown gas. Bromine vaporises easily and its gas will completely fill the container (Figure 2.26). Gases diffuse to fill all the space available to them. Diffusion is important for our ‘sensing’ of the world around us. It is the way smells reach us, whether they are pleasant or harmful.
Diffusion in fluids The idea that fluids are made up of moving particles helps us to explain processes involving diffusion. Dissolving A potassium manganate(vii) crystal is placed at the bottom of a dish of water. It is then left to stand. At first the water around the crystal becomes purple as the solid dissolves (Figure 2.25). Particles move off the surface of the crystal into the water. Eventually the crystal dissolves completely and the whole solution becomes purple. The particles from the solid become evenly spread through the water. Whether a solid begins to break up like this in a liquid depends on the particular solid and liquid involved. But the spreading of the solute particles throughout the liquid is an example of diffusion. Diffusion in solution is also important when the solute is a gas. This is especially important in breathing! Diffusion contributes to the 38
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Figure 2.26 Bromine vapour diffuses (spreads) throughout the container to fill all the space.
Original material © Cambridge University Press 2014