1. 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.
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2. 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.
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3. 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.
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4. 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.
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5. 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
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6. 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.
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7. 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.
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8. 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.
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9. 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.
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10. 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.
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11. 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
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12. 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