Science Focus Topic 5 Notes: The Particle Model and Changes of State | Print |
 Heat Capacity and Specific Heat Capacity The amount of of temperature change, when thermal energy is added to the particles is another property that particles in different materials have. Different materials will increase or decrease their average energy depending on how much thermal energy is provided. Heat Capacity is the amount of thermal energy that warms or cools an object by 1oC (it depends on the mass and the type of particle the object is made of). Specific Heat Capacity is the amount of thermal energy that warms or cools 1 gram, of a specific type of particle, by 1oC. Changes of State Some substances like water (or wax) can undergo observable changes through all three states of matter - solid liquid and gas. Some substances, like hydrogen, require high pressures and low temperatures (-253oC) to make the particles slow down enough for them to change their state from a gas to a liquid. Any pure substance can exist in all three states of matter. Melting and Boiling Points When heat is transferred in a space the average energy of the particles - the temperature of the substance - is affected, by increasing or decreasing. A substance will change it's state when it reaches certain temperatures - called boiling and melting points. Table 3 (p. 221) At everyday temperatures on Earth, most substances are either gases or solids. What Happens When A Liquid Evaporates? In a liquid, the particles are moving very quickly. At the surface, some of the particles are able to escape into the air, while others do not have enough energy to escape and remain in the liquid. As high energy particles escape, the average energy of the remaining particles is less and so the liquid cools. The cool liquid then cools the surface on which it is resting. This is called evaporative cooling. It is common and useful in many situations: Joggers cooling down as their sweaty clothes dry out Water cools down a roof on hot summer day A wet cloth is placed on your forehead when you have a fever Why The Temperature Stays The Same During a phase change, the average energy of the particles remains the same, but, the particles are rearranging themselves. Particles become less organized as their energy increases, so the substance changes from a solid to a liquid to a gas. As the energy of the particles becomes less, the particles rearrange themselves more orderly, so a gas changes to a liquid and then to a solid. The total energy of the particles changes - by increasing or decreasing, because the particles are not increasing or decreasing their speed, just their arrangement. The average energy doesn't change. The energy change is hidden from a thermometer and is called 'hidden heat' or 'latent heat'. Topic Review p. 225