There are three states of matter: Which state something is in depends on... ...temperature, pressure and type of material. This is because temperature, pressure and type of material affect how strong the forces of attraction between particles are in the material. In a solid the forces of attraction between particles are strong. This holds them close together in fixed positions. The particles do not move from these fixed positions so they keep a definite shape and volume. If we drop our solid into this jar it doesn’t change. It stays the same shape and it stays the same volume. In a solid, the particles vibrate in their positions. How much they vibrate depends on temperature. The hotter they get the more they vibrate, which makes solids expand slightly when heated. In a liquid the forces of attraction between particles are weak. This holds them close together but they are in random positions and they move. The particles keep a definite volume but not a definite shape. If we drop our liquid into this jar it will flow to fill the bottom. It has changed its shape but not its volume. The particles in a liquid are moving about with random motion. The hotter they get the faster they move, so a liquid will expand when heated. In a gas the forces of attraction between particles are very weak. The particles in a gas travel in straight lines until something changes their direction. This means they will spread out. If we pump our gas into this They will expand to fill jar it will expand to fill it. whatever container they are in. Gases do not keep definite shape or volume. To fit into a container a gas will change... shape... ...and volume. Gases in a container are under pressure. The smaller the container the higher the pressure. When they get hotter, the particles in a gas move faster. This causes them to expand until they have filled all available space. Then heating increases the pressure. Heating a solid makes the particles vibrate more. This weakens the forces holding the solid together. At a certain temperature the particles have enough energy to escape the solid. This is called the melting point. Above the melting point the solid becomes a liquid. Heating further makes the particles move faster, weakening the forces between them. At a certain temperature the particles have enough energy to escape the liquid. This is called the boiling point. As the particles cool they no longer have enough energy to overcome the attraction between them. Bonds form between the particles and they condense from gas to liquid and freeze from liquid to solid. It is important to know the proper names for transition between the states of matter; melting, freezing, boiling and condensing. The amount of energy needed for something to change state depends on how strong the forces between the particles are. Stronger forces need more Weaker forces need less energy energy to break them so the to break them so the melting melting and boiling points are and boiling points are low. high. Sometimes chemical equations will have a state symbol next to them. Aqueous means dissolved in water. (s) = solid (l) = liquid The difference between aqueous (aq) and liquid (l) can be shown using table salt. Dissolved salt has not melted, it is aqueous. (g) = gas (aq) = aqueous Here is an example of an equation showing state symbols: Fe(s) + CuSO4(aq) → FeSO4(aq)+ Cu(s) The copper has precipitated from an aqueous state to a solid. You may be asked what state something is in at a certain temperature. Example: Water has a melting point of 0oC and a boiling point of 100oC. What is the state of water at 60oC? Above the boiling point (100oC) water is a gas. Below melting point (0oC) water is a solid. Any temperature in between and water is a liquid. So at 60oC water is a liquid. Extension • A fourth state of matter which rarely occurs on Earth is plasma. Plasma is electrically conductive gas. It contains free electrons because some of the electrons have dissociated from the gas atoms. Plasma has different properties from the other three states of matter. • Some substances can go straight from a solid to a gas without becoming a liquid in between. This is called sublimation. An example is solid carbon-dioxide (also known as dry ice) and carbon dioxide gas.