P6 - Molecules and Matter


P6.1 - Density

The Density of a subtance = mass per unit volume.

Denisty (kg/m³) = mass (kg) / volume (m³)

To measure the density of solids:

1. Weigh the object using a scale  >  For regular objects, measure the dimensions to find the volume  >  For irregular objects, place the object in a measuring cylinder partly filled with water and work out the volume from the rise in the water level, or use a eureka can.

2. Use the density equation ( d=m/v) to work out the density of the objects.

To measure the density of liquids:

1. Measure a certain volume for an amount of liquid in a measuring cylinder.

2. Measure the mass of an empty beaker.

3.Pour the liquid in the beaker, measure the total mass, then minus the mass of the beaker to work out the mass of the liquid. Use the density equation to work out the density of the liquid. 

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P6.2 - States of Matter

Changes of state are physical changes as no new substances are produced. The mass of the substance is conserved when it changes state, as the number of particles stays the same.

In solids:

The particles are in fixed positions next to eachother. They vibrate in their fixed positions, fixed volumes, don't change shape, so they are the least energetic of the states of matter.

In liquids:

The particles are in contact with eachother but move about at random. They don't stay in the same shape, they flow over eachother. They have more energy than particles in a solid.

In gases:

The particles move around randomly and faster than in a solid or liquid. They are also further apart, not touching eachother, so are the least dense of the states of matter, as well as the most energectic, have lots of kinetic energy, takes the shape of its container.

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P6.3 - Changes of State

solid > liquid = melting/fusion

liquid > solid = solidifying/freezing

liquid > gas = evaporation/boiling

gas > liquid = condensation

gas > solid = deposition (condensation)

solid > gas = sublimation

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P6.4 - Internal Energy

The Internal Energy of particles is the kinetic energy they have due to their individual motions and potential energy due to their positions relative to eachother.

When a substance is heated, the energy of its particles increases > kinetic energy^, temperature^.

If it changes state due to heating, then the total potential energy also increases as the particles get further away from each othe. The extra energy transferred to the subsrance is known as latent heat.

The strength of forces of attraction explain why a substance is a solid, liquid or gas.

The pressure of gas on a surface is caused by gas particles repeatingly colliding with the surface.

Specific Heat Equation = 

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P6.5 - Specific Latent Heat

Latent Heat:

  • "hidden energy"
  • energy needed to change ther state of 1kg or a substance at its melting (fusion) or boiling (vaporisation) point.
  • The specific latent heat of ice or water can be measured using a low-voltage heater connected to a joulemeter to melt the ice or boil the water.
  • Specific Latent Heat of Vaporisation (liquid to gas) = L (J/kg) = E/m > can be rearranged to E=mL
  • Specific Latent Heat of Fusion (solid to liquid) = L (J/kg) = E/m > can be rearranged to E=mL
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P6.6 - Gas Pressure and Temperature

The pressure of a gas is caused by random impacts of gas molecules on surfaces.

Pressure(Pa)(N/m ) = force/area

The effect on temperature on gas pressure can be measured by putting a flask of dry air in a water bath with a thermometer, and measuring the pressure with a pressure gauge at different temperatures.

If the temperature increases, the pressure of the gas increases. This is because the particles move faster with more kinetic energy. They therefore hit the surface with more force and a greater number of impacts per second.

The random motion of smoke particles is evidence of the random motion of gas molecules, as many gas particles collide with a much larger smoke particle, pushing it around.

The random motion of tiny particles in a fluid is called Brownian Motion.

The Brownian Motion is when particles in a gas(or a liquid) move in a random way. Their speeds and directions are constantly changing as the collide with each other and the container.

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P6.7 - Gas Pressure and Volume

Marshmallow in a jar experiment:

When the pressure was decreases in the jar, the marshmallows expanded because the pressure inside the marshmallows stayed the same, whilst the outside pressure decreased. Marshmallows are normally constricted by the pressure of the surroundings, so when the pressure decreased they can expand. When the pressure was then added back to the jar, the marshmallows got smaller than their original sizes because there was not any air left in the marshmallows as it had all been sucked out and sent to the surroundings.

Boyles Law:

Increase pressure of gas, decrease the volume (for a fixed mass of gas at a constant temperature)

= pV = constant (double p, half V)

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