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P6: Molecules and matter (6.4: Internal energy (For a solid (There are…
P6: Molecules and matter
6.1: Density
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Objects with a density that is less than the density of a given liquid will float in that liquid (objects with a density less than 1000kg/m cubed will float in water)
6.2: States of matter
The 3 states of matter are solid, liquid and gas
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When a substance changes state, the number of particles in the substance stays unchanged, so the mass of the substance is the same before and after the change of state
In a solid, the particles vibrate about fixed positions so the solid has a fixed shape
In a liquid, the particles are in contact with each other but can move around at random - hence, a liquid doesn't have a fixed shape, and it can flow
In a gas, the particles are usually far apart and move about at random, much faster than particles in a liquid - so the density of gas is much less than that of a solid or liquid
The particles in a substance in its solid, liquid, or gas state have different amounts of energy depending on the state
For a given amount of a substance, its particles have the most energy in the gas state and the least energy in the solid state
6.3: Changes of state
The temperature at which a solid changes to a liquid is called the melting point - this is the same temperature at which the liquid changes to a solid, when it is called the freezing point
The temperature at which a liquid changes to a gas is called the boiling point - this is the same temperature at which a gas changes to a liquid (condenses)
Boiling takes place throughout a liquid at its boiling point - evaporation takes place from the surface of a liquid when its temperature is below its boiling point
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For a substance to change state, energy must be transferred to or from the substance - this energy is called latent heat
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6.4: Internal energy
The internal energy of a substance is the total energy in the kinetic and potential energy stores of the particles in the substance
The particles in a substance have kinetic energy due to their individual motions relative to each other
The particles in a substance have potential energy due to their individual positions relative to each other
When a substance is heated the energy of its particles increases, so its internal energy increases
If the total kinetic energy of the particles increases, the temperature of the substance increases
If the substance changes state, the potential energy of its particles increases
For a solid
There are strong forces of attraction between the particles and each particle vibrates about a fixed position
When a solid is heated, the particles' energy stores increase and they vibrate more
For a liquid
There are weaker forces of attraction between the particles - the forces stop the particles moving completely away from each other, but they're not strong enough to hold the particles in a fixed structure
When a liquid is heated, some particles gain enough energy to break away from the other particles and are in a gas state
For a gas
The forces of attraction between the particles are negligible, so the particles of a gas are completely separate from each other
When a gas is heated, its particles gain kinetic energy and move faster
The particles in a gas collide with each other and with the walls of their container - during these collisions the particles exert a force, and hence a pressure, on the walls of the container
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