6 Molecules and matter

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6.1 Density

- Mass per unit volume of a material

- Density(kg/m3)=mass(kg)/volume(m3)

Measuring density of a solid object

-Use an electronic balance

-solid object(eg. cube or cuboid)

- measure its dimension using ruler

-Use the measurements and equation (volume of a cuboid=a x b x c)

Density of a small irregular solid

-Lower the object on a thread into a measuring cylinder (partly filled with water)

-Volume=rise in water level

Measuring the density of a liquid

-Measuring cylinder

-Measure the mass of an empty beaker using a balance

-Remove beaker from balance and pour the liquid from the measuring cylinder into the beaker

-Using the balance measure the total mass of the beaker and the liquid.

-Calculate the mass of the liquid= subtract the mass of the empty beaker from the total mass of the beaker with the liquid

*Objects which have a lower density than water (<1000kg/m3) float in water

6.2 States of Matter

Mass- Amount of matter

Changes of state are examples of physical changes as no new substances are produced.

If a physical change is reversed, the substance recovers its original properties.

You can change the state of a substance by heating or cooling.

Conservation of mass

When a substance changes state, the number of particles remains unchanged-the mass of the substance after the change of state is the same as the mass of the substance before the change of state.

The Kinetic theory of matter

When the temperature of a substance increases, particles move faster

Solid

-molecules are held in fixed positions, solid keeps its own shape

-strong forces of attraction

-Regular lattice arrangement

-Very dense

-Expand with heat

-vibrate

-Cannot be compressed-close molecules

Liquid

-Particles are in contact with each other

-Move about at random

-No definite shape

-Flows

-cannot be compressed

-expand slightly with heat

-quite dense

Gas

-Move about at random

-much faster than a liquid

-much further apart-the density of a gas is much less than that of a liquid or solid.

-travel is straight lines and interact when they collide.

-do not keep a definite shape or volume

-the most energy (kinetic)

- the hotter a gas becomes, the faster they move.

-When heated , it will either expand or its pressure will increase.

*Different amount of energy- gas has more energy over a liquid and then a solid.

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6.3 Changes of State

Melting and boiling points:

-MELTING POINT-the temperature at which a solid changes to a liquid

-BOILING POINT- the temperature at which a a liquid turns to a gas.

-FREEZING POINT-the temperature at which a liquid changes to a solid.

*temperature time graphs-straight line is a change of state.

Latent heat is the energy transferred to a substance when it changes state.

6.4 Internal energy

Internal energy- the energy stored by the particles of a substance

The internal energy of the particles is due to:

-the kinetic energy -individual motions relative to each other

-the potential energy- individual positions relative to each other

solid:

-when a solid is heated, the particles' energy stores increase and vibrate more.

-If the solid is heated up enough, the solid melts(sublimates) as its particles have gained enough energy to break away from its structure.

Liquid:

-weaker forces of attraction are not strong enough to hold the particles in a rigid structure.

Gas:

- when heated its particles gain kinetic energy

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6.5 Specific latent heat

The specific latent heat is the amount of energy needed to change 1kg of a substance from one state to another without changing its temperature

Specific latent heat of fusion

-The energy supplied is called latent heat of fusion.

It is the energy needed by particles to break free

Energy needed to melt 1kg of a substance without changing its temperature

Specific latent heat of fusion,LF=energy,Joules/Mass(kg)

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Specific latent heat of vaporisation

-the energy needed to boil 1kg of a substance without changing its temperature

Specific latent heat of vaporisation,Lv=energy,Joules/Mass(kg)

Exothermic-energy is released, bonds are formed

Endothermic-energy is being put in, bonds are broken

6.6 Gas pressure and temperature

Never heat food in a sealed can:

-the pressure of the gas inside increases as the temperature increases

-the energy transferred to the gas when it is heated increases the kinetic energy of its molecules

-the average speed of the molecules increases and the molecules hit the container with more force and more often. The pressure of the gas increases

Brownian motion- the random motion of tiny particles in a fluid.