Edexcel IGCSE Chemistry
Ø States of matter:
· Boiling happens when the liquid is heated so strongly that the particles are moving fast enough to break all of the forces of attraction in the liquid. The liquid boils. Bubbles of gaseous particles are formed throughout the whole liquid and rise to the surface, forming a gas.
· Evaporation, however, is different. Some particles on the surface of the liquid, which have more energy than the average, have enough energy to break away and form a gas. In a closed container, gaseous particles will also be re-joining the liquid, and the number of particles of gas over the top of a liquid will be roughly constant.
· If the gas is cooled, the particles lose kinetic energy and this allows the forces of attraction to bring the particles closer together. The particles eventually clump together to form a liquid.
· When a substance melts from a solid to a liquid, the particles gain kinetic energy, and vibrate faster and faster. This allows the particles to overcome the forces of attraction that hold them together in the solid. The regular pattern is broken down, and the particles can now slide past one another. However, when a substance freezes from a liquid to a solid, the particles lose kinetic energy, which allows the forces of attraction between the particles to hold them together. The particles arrange themselves into a regular pattern and are no longer able to slide past one another.
· Some substances, such as carbon dioxide and ammonium chloride, can sublime directly from a solid to a gas. The particles gain kinetic energy and vibrate faster and faster. Eventually, the forces of attraction between the particles are completely broken, and they are able to escape from the solid.
· Particles will move to fill the space available to them. They can do this in liquids and gases, although it takes longer in liquids, as the particles are more closely packed and move more slowly than in a gas.
· An example is the diffusion of bromine from one container to another. After five minutes, both the gas jars are uniformly brown. This happens because both air and bromine particles are moving randomly, and there are large gaps between the particles, so they can easily mix together.
· When potassium permanganate crystals are dissolved in water, a purple solution is formed. A few tiny crystals can produce a highly intense colour. This indicates that there are a large number of particles of potassium permanganate in a very small amount of solid.
· Both these experiments suggest that matter is made up of tiny, moving particles.
· The properties of a compound are likely to be different from the elements that have been used to make it, whereas a mixture will have the properties of each substance that has been used to make it.