Physics : P6 - Molecules and Matter

The density of a subtance is defined as its mass per unit volume.

Therefore, denisty (kg/m³) = mass (kg) / volume (m³)

Measure the density of solids:

Weigh the object using a scale. For regular objects, meeasure 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. Use the density equation to work out the density of the objects.

Measure the density of liquids:

Measure a certain volume for an amount of liquid in a measuring cylinder. Measure the mass of an empty beaker. 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.

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, not changing 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.

Melting point: Change of state of a substance between solid and liquid

Boiling point: Change of state of a substance between liquid and gas

Boiling occurs throughout a liquid at its boiling point.

Evaporation occurs from the surface of a liquid at any temperature.

The flat section of a temperature - time graph shows the melting or boiling point of a substance, as the energy transferred to the substance when it changes state is called latent heat, where the substance doesn't change temperature.

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 particles have more kinetic energy. If it changes state, the particles have more potential energy.

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 repeatedlt hitting the surface.

In latent heat calculations, use the equation L (J/kg) = E / m

Latent heat is the energy needed for a substance to change state without changing temperature. 

The specific latent heat of fusion is the energy needed to melt 1kg of a solid. The specific latent heat of vaporisation is the energy needed to boil 1kg of a liquid.

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.

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

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.