Chapter 3: Particle Model of Matter

3.1-Density of Materials

The Particle Model can explain Density and the three states of matter

• Density is a measure of the compactness of a substance. It related the mass of a substance to how much space it takes up
  • Density(kg/m3) = mass(kg) / volume(m3)
• The density of an object depends on what it is made of and how its particles are arranged
• A dense material has its particles packed tightly together.
• The particles in a less dense material are more spread out-if you compressed the material, its particles would move closer together, and it would become more dense

The three states of matter are solid, liquid and gas

• The states of matter all have different qualities
• Solids-
  • strong forces of attraction hold the particles close together in a fixed, regular arrangement.
  • The particles don’t have much energy so they can only vibrate about their fixed positions.
  • The density is generally highest in this state as the particles are closest together
• Liquids-
  • there are weaker forces of attraction between the particles.
  • The particles are close together, but can move past each other, and form irregular arrangements
  • They have more energy than the particles in a solid, and less dense than solids
• Gases-
  • there are almost no forces of attraction between the particles.
  • Particles have more energy than in liquids and gases-
  • they’re free to move and at higher speeds.
  • Generally less dense

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Need to measure density in different ways

Density of solid object

• Use a balance to measure its mass
• If it’s a regular solid, start by measuring its length, width and height with a piece of equipment(ruler).
• Then calculate volume using relevant formula for the shape
• For an irregular object, you can find volume by submerging it in a eureka can filled with water.
  • The water displaced will be transferred into a measuring cylinder
• Record the volume of water in measuring cylinder
• Plug the objects mass and volume into formula to find its density

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Density of a liquid:

• Place a measuring cylinder on a balance and zero the balance
• Pour 10ml of the liquid into the measuring cylinder and record the liquids mass
• Pour another 10ml into the measuring cylinder , repeating the process until the cylinder is full and recording the total volume and mass each time
• For each measurement , use the formula to find the density
• Finally, take an average of your calculated densities.
• This gives you a value for the density of the liquid.

3.2-Internal Energy and Change of State

Internal energy is the energy stored by the particles that make up a system

• The particles in a system vibrate or move around-they have energy in their kinetic energy store
• They also have energy in their potential energy stores due to their positions
• The energy stored in a system is stored by its particles.
  • The internal energy of a system is the total energy that its particles have in their kinetic and potential energy stored
• Heating the system transfers energy to its particles(they gain energy in their kinetic energy stores and move faster), increasing the internal energy
• This leads to a change in temperature or a change in state.
• If the temperature changes, the size of the change depends on the mass of the substance, what it’s made of and the energy input.
• A change in state occurs if the substance is heated enough-the particles will have enough energy in their kinetic energy stores to break the bonds holding them together

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A change of state conserves mass

• When you heat a liquid, it boils and becomes a gas.
• When you heat a solid, it melts and becomes a liquid.
  • These are both changes of state.
• The state can also change due to cooling.
• The particles lose energy and form bonds.
• The changes of states are:
  • A change of state is a physical change.
  • This means you don’t end up with a new substance-it’s the same substance as you started with, just in a different form
  • If you reverse a change of state, the substance will return to its original form and get back its original properties
  • The number of particles doesn’t change-they’re just arranged differently. This means mass is conserved-none of it is lost when the substances change state.

3.3-Specific Heat Latent

A change of state requires energy

• When a substance is melting or boiling, you’re still putting in energy and so increasing the internal energy, but the energy’s used for breaking intermolecular bonds rather than raising the temperature.
• There are flats spots on the heating graph where energy is being transferred but not being used to change the temperature.
• When a substance is condensing or freezing, bonds are forming between particles which releases energy.
• This means the internal energy decreases, but the temperature doesn’t go down until all the substances has turned to liquid or a solid.
• The flat parts of the graph show this energy transfer.
• The energy needed to change the state of a substance is called latent heat

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Specific latent heat is the energy needed to change the state of a 1kg matter

• The specific latent heat of a substance is the amount of energy needed to change 1kg of it from one state to another without changing its temperature
• For cooling, specific latent heat is the energy released by a change in state
• Specific latent heat is different for different materials, and for changing between different states
• The specific latent heat for changing between a solid and liquid is called the specific latent heat of fusion.
• The specific latent heat for changing between a liquid and a gas is called the specific latent heat of vaporization

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There’s a formula for specific latent heat

• You can work out the energy needed when a substance of mass m changes state using this formula

Energy = Mass x Specific latent heat

• Energy is given in joules, mass in kg and SLH in J/kg

3.4-Particles Motion in Gases

Average energy in kinetic stores is related to temperature

• The particles in a gas are constantly moving with random directions and speeds.
• If you increase the temperature of a gas, you transfer energy into the kinetic energy stores of its particles
  • The temperature of a gas is related to the average energy in the kinetic energy stores of the particles in the gas.
  • The higher the temperature, the higher the average energy
  • So as you increase the temperature of a gas, the average speed of its particles increases.
  • This is because the energy in the particles’ kinetic energy stores is 1/2mv2

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Colliding gas particles create pressure

• As gas particles move about at high speeds, they bang into each other and whatever else happens to get in the way.
• When they collide with something, they exert a force on it.
  • In a sealed container, the outward gas pressure is the total force exerted by all of the particles in the gas on a unit area of the container walls
• Faster particles and more frequent collisions both lead to an increase in net force, and so gas pressure. Increasing temperature will increase the speed, and also the pressure
• Alternatively, if temperature is constant, increasing the volume of a gas means the particles get more spread out and hit the walls of the container less often.
  • The gas pressure decreases
• Pressure and volume are inversely proportional-when volume goes up, pressure goes down.
• For a gas of fixed mass at a constant temperature, the relationship is:
  • pV=constant, p=pressure, v=volume

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A change in pressure can cause a change in volume

• The pressure of a gas causes a net outwards force at right angles to the surface of its container
• There is also a force on the outside of the container due to the pressure of the gas around it
• If a container can easily change its size, then any change in these pressures will cause the container to compress or expand, due to the overall force

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Doing work on a gas can increase its temperature

• If you transfer energy by applying a force, then you do work.
• Doing work on a gas increases its internal energy, which can increase its temperature
• You can do work on a gas mechanically.
• The gas applies pressure to the plunger of the pump, and so exerts a force on it.
  • Work has to be done against this force to push down the plunger
• This transfers energy to the kinetic energy stores of the gas particles, increasing the temperature.
• If the pump is connected to a tyre, you should feel it getting warmer

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