# Molecules and Matter

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## Density

- density = mass / volume (in kg/m)

- to measure the density of a solid object or a liquid, measure its mass and its volume, then use the density equation

- rearranging the density equation gives:

mass = density x volume

volume = mass / density

- objects that have a lower density than water (ie. < 1000 kg/m ) float in water

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## States of Matter

- the particles of a solid are held next to each other in fixed positions, they are the least energetic of the states of matter

- the particles of a liquid move about at random and are in contact with each other, they are more energetic of the states of matter

- the particles of a gas move about randomly and are far apart (so gases are much less dense than solids and liquids) they are the most energetic of the states of matter

- when a substance changes state, its mass stays the same because the number of particles stays the same

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## Changes of State

- for a pure substance:

its melting point is the temperature at which it melts (which is the same temperature at which it solidifies)

its boiling point is the temperature at which it boils (which is the same temperature at which it condenses)

- energy is needed to melt a solid or to boil a liquid

- boiling occurs throughout a liquid at its boiling point, evaporation occurs from the surface of a liquid when its temperature is below its boiling point

- the flat section of a temperature- time graph gives the melting point or the boiling point of a substance

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## Internal Energy

- increasing the temperature of a substance increases its internal energy

- the strength of the forces of attraction between the particles of a substance explains why it is a solid, liquid or a gas

- when a substance is heated:

if its temperature rises, the kinetic energy of its particles increases

if it melts or it boils, the potential energy of its particles increases

- the pressure of a gas on a surface is caused by the particles of the gas repeatedly hitting the surface

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## Specific Latent Heat

- latent heat is the energy needed for a substance to change its state without chnaging its temperature

- specific latent heat of fusion (or of vaporisation) is the energy needed to melt (or to boil) 1kg of a substance without changing its temperature

- in latent heat calculations, use the equation:

E = m L

- the specific latent heat of ice (or of water) can be measured using a low-voltage heater to melt the ice (or to boil the water)

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## Gas Pressure and Volume

- for a fixed mass of gas at constant temperature:

its pressure is increased if its colume is decreased

reducing the volume of a gas increases the number of molecular impacts per second on the surfaces that are in contact with the gas

- use the equation

p V = constant if the mass and the temperature of the gas do not change

- the temperature of a gas can increase if it is compressed rapidly because work is done on it and the energy isnt transferred quickly enough to its surroundings

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## Gas Pressure and Temperature

- the pressure of a gas is caused by the random impacts of gas molecules on surface that are in contact with the gas

- if the temperature of a gas ina sealed container is increased, the pressure of the gas increases because;

the molecules move faster so they hit the surfaces with more force

the number of impacts per second of gas molecules on the surfaces of a sealed container increases, so the total force of the impacts increases

- the unpredictable motion of smoke particles is evidence of the random motion of gas molecules

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