Density, ρ, is the mass per unit volume of a material. 𝜌= 𝑚 𝑉 3 Particle model of matter Topic overview Density Density, ρ, is the mass per unit volume of a material. 𝜌= 𝑚 𝑉 m is the mass, measured in kg, V is the volume, measured in m3 and ρ is the density measured in kg/m3. © Hodder & Stoughton 2016

Density 3 Particle model of matter Topic overview Different states of matter: Solid – atoms/molecules close together in a regular structure, fixed positions because of strong forces between atoms/molecules. Liquid – atoms /molecules close together, forces between them less strong than for solids so able to move and allow liquids to flow. Gas – atoms/ molecules far apart, little force between them. Gases will expand to fill their container. © Hodder & Stoughton 2016

Density of solids and liquids is greater than gas. 3 Particle model of matter Topic overview Density Density of solids and liquids is greater than gas. Different materials have different densities because the atoms/molecules have different masses. Most solid materials are more dense than their liquid; water is one exception. © Hodder & Stoughton 2016

Changes of state 3 Particle model of matter Topic overview A change of state is when a material changes from solid to liquid, liquid to gas or solid to gas. In these changes the total mass stays constant. The changes are physical changes and are reversible. Melting – change from solid to liquid state. Freezing – change from liquid to sold state. Boiling/evaporation – change from liquid to gas state. Condensation – change from gas to liquid state. Sublimation – change from solid to gas state. A change of state is a result of a change in internal energy of the material. © Hodder & Stoughton 2016

Internal energy 3 Particle model of matter Topic overview Internal energy is the total of the kinetic energy and potential energy of the atoms or molecules in the substance. When a substance is heated, the internal energy is increased. When a substance is cooled, the internal energy is decreased. A change of internal energy can result in either: A change of temperature because of changed kinetic energy A change of state because of changed potential energy © Hodder & Stoughton 2016

Specific heat capacity 3 Particle model of matter Topic overview Specific heat capacity The energy change, ΔE, needed to change the temperature, ∆𝜃, of a mass, m, of a material is called the heat capacity. The specific heat capacity, c, of a material is the energy required to change the temperature of 1 kg of the substance by 1°C. ∆𝐸=𝑚×𝑐×∆𝜃 Specific heat capacity is measured in J/kg °C if ΔE is measured in Joules, m in kg and ∆𝜃 in °C. © Hodder & Stoughton 2016

Specific latent heat 3 Particle model of matter Topic overview The energy, E, needed to change the state, of a mass, m, of a material without changing the temperature is called the latent heat. The specific latent heat, L, is the energy required to change the state of 1 kg of a material without changing the temperature. 𝐸=𝑚×𝐿 Specific latent heat is measured in J/kg if E is measured in Joules and m in kg. Specific latent heat of fusion is the energy required when 1 kg of material changes from solid to liquid with no temperature change. Specific latent heat of vaporisation is the energy required when 1 kg of material changes from liquid to gas with no temperature change. © Hodder & Stoughton 2016

Cooling curves 3 Particle model of matter Topic overview When a substance cools, it will cool more quickly at first since the temperature difference between it and the surroundings is greater at first. When a substance cools and changes state the temperature will remain constant as the material changes state. © Hodder & Stoughton 2016

3 Particle model of matter Topic overview Particle motion The particle model or kinetic theory of gases assumes that atoms/molecules in a gas: are in constant random motion. collide with each other and the walls of the container without loss of kinetic energy. cause a force at right angles to the surface of the container at each collision. Temperature is related to the average kinetic energy of atoms/molecules. An increase in the average kinetic energy is an increase in temperature. © Hodder & Stoughton 2016

3 Particle model of matter Topic overview Pressure in a gas The atoms/molecules of a gas at a higher temperature have more kinetic energy and so are moving faster. The faster and the more frequent the collisions of atoms/molecules with the walls of their container, the greater the force on the walls. For a constant volume, a greater force on the walls results in an increase in pressure. © Hodder & Stoughton 2016

Pressure and volume 3 Particle model of matter Topic overview Changing the volume, V, of a fixed mass of gas at constant temperature causes a change in the number of collisions with the walls of the container per unit area of wall. A decrease in volume causes more collisions per unit area so an increase in pressure, p. An increase in volume causes fewer collisions per unit area so a decrease in pressure, p. For a fixed mass of gas at constant temperature: 𝑝×𝑉=𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡 p is measured in pascal (Pa) V is measured in m3 © Hodder & Stoughton 2016

Work done on a gas 3 Particle model of matter Topic overview When work is done on a gas, e.g. compressing a gas by using a pump, the average k.e. of the atoms/molecules increases. Work done = force x distance moved in direction of force When the average kinetic energy of the atoms/molecules increases, the temperature of the gas rises. © Hodder & Stoughton 2016