P6: Molecules and matter

States of matter

• Solid

• Liquid

• Gas

Arrangement of particles in a solid

1. V close tog

2. Arranged in regular pattern

3. Vibrate, but don’t move from place to place

Arrangement of particles in a liquid

1. Close tog

2. Not arranged in regular pattern

3. Move around each other

Arrangement of particles in a gas

1. V far apart

2. Not arranged in any pattern

3. Moving v rapidly

Density

Mass per unit volume of a material

Density equation

Mass / volume

• kg / m3

Density of solid, liquid, gas

1. Solid- v high

2. Liquid- high

3. Gas- low

Why do solids have a v high density?

Particles packed close tog → lot of mass per unit vol

Why do liquids have a high density?

Particles close tog → lot of mass per unit vol

Why do gases have a low density?

Particles v far apart → small mass per unit vol

What is unusual about polystyrene and why?

Solid, but has low density

• Full of air spaces → small mass for vol

When will objects float in a liquid?

If object has density less than density of liquid

How to measure density of a solid object or liquid?

1. Measure its mass

2. Measure its vol

3. Use density equation

What type of change is changes of state?

Physical changes

Why are changes of state physical changes, not chemical?

Material recovers its original properties if the change is reversed

• + no new substances produced

When do substances change state?

• Put in / remove energy

• Heat / cool

Law of conservation of mass for change of state

When substances change state mass is conserved

Changes of state

1. Melt- S→L

2. Freeze- L→S

3. Boil- L→G

4. Evaporate- L→G

5. Condense- G→L

6. Sublimate- S→G

In which state do particles have most energy?

Gas

How do you know all particles have kinetic energy?

Particles in:

1. S: vibrate

2. L: move around each other

3. G: move v rapidly

How are particles held tog?

Intermolecular forces

How are molecules held tog?

Chemical bonds

How do you know particles have potential energy?

They have:

• IF or chemical bonds betw them

Internal energy

Energy is stored in a system by the particles (atoms and molecules) that make up the system

What makes up internal energy?

Total kinetic + potential energy of all the particles that make up a system

What happens to IE if you heat a substance?

Increase IE

What happens to IE if you cool a substance?

Decrease IE

Explain changes to state in terms of increasing IE

1. Heat solid, increase IE

2. At MP, solid melts into liquid

3. Heat liquid, increase IE

4. At BP, liquid boils to gas

Explain changes to state in terms of decreasing IE

1. Cool gas, reduce IE

2. At BP, gas condenses to liquid

3. Cool liquid, reduce IE

4. At MP, liquid freezes to solid

Sublimation

When solid turns directly to a gas

Evaporation

When liquid turns to gas, but only on the surface of the liquid

Why does evaporation only occur on the surface of the liquid?

Only particles on the surface have enough energy to turn into a gas

How does heating affect the energy stored within the system?

Changes energy stored by increasing the energy of the particles that make up the system

What does heating a substance do?

1. Raises temp of the system

2. Produces a change of state

Specific heat capacity

Amt of energy needed to raise the temp of 1kg of the substance by 1°C

Specific heat capacity equation

∆ E  = m c  ∆ θ

If the temp of the system increases, what does the increase in temp depend on?

1. Mass of substance heated

2. Type of material

3. The energy input to the system

What is the internal energy of a substance due to?

1. KE- due to particles movement

2. PE- due to forces betw particles + bonds betw atoms in molecules

During a change of state, why does is there a horizontal line?

Energy put in weakens / breaks FoA betw particles

• IE of system increases

• NOT changing temp

Heating graph

Cooling graph

On a heating graph, what happens when you heat a substance?

Increase temp, as we r increasing energy of particles

Boiling vs evaporation

• B: occurs thru out a liquid at BP

• L: occurs only on surface of liquid, below BP

Latent heat

Energy needed for a substance to change state

Increasing the temp of a substance…

Increases its IE

What varies betw SLG?

Strength of FoA

What affects the MP + BP of a substance?

Impurities

What happens when a change of state occurs?

The energy supplied changes the energy stored (internal energy) but not the temp

What happens to temp during a change of state?

• Stops increasing

• Stays constant

Why is temp constant during a change of state?

Energy put in weakens / breaks FoA betw particles

Specific latent heat

Amt of energy needed to change the state of 1kg of the substance with no change in temp

Specific latent heat of fusion

Amt of energy needed to change 1kg of substance from solid to liquid with no change in temp

Specific latent heat of vaporisation

Amt of energy needed to change 1kg of substance from liquid to vapour with no change in temp

Energy for a change of state equation

E  = m L

Particles in a gas are in?

Constant random motion

• v far apart

What is the pressure of a gas caused by?

Particles colliding w the walls of the container that the gas is held in

How to increase the pressure of a gas?

Increase the:

1. No. of collisions per second

2. Energy of each collision

What does increasing the no. of collisions per s + energy of each collision do in a gas?

Increase temp

How to change pressure exerted by a gas?

Change temp of gas

Temperature of a gas

Average KE of particles

Pressure of gases at low temp + why?

Low pressure

1. Low temp → particles have lower KE

2. So fewer collisions per s

3. + lower energy collisions

Pressure of gases at high temp + why?

High pressure

1. High temp → particles have higher KE

2. So more collisions per s

3. + higher energy collisions

Relation between the temp of a gas + its pressure at constant volume

Increasing temp → increases pressure, bc”

1. Increase temp → particles have more KE

2. → more collisions per s

3. + higher energy collisions

What do particle collisions cause?

A force that acts at right angles to the walls of the gas container → causes gas pressure

How does this force that acts at right angles to the walls of the container cause gas pressure?

When the force acts over an area of the contained

What does increasing vol of a container (temp constant) do to gas pressure?

Decrease pressure

Why does increasing vol cause a decrease in gas pressure?

• Increased space betw particles

• So particles travel much further b4 colliding w walls of the container

• So less no. of collisions per s betw particles + walls of the container

Relationship of pressure of gas to volume?

Pressure of a gas inversely proportional to volume

What does it mean if pressure of a gas is inversely proportional to volume?

Vol increases → pressure decreases

For a fixed mass of gas held at a constant temp:

pressure × volume = constant

• p1V1=p2V2

What do pressure changes cause?

A gas to be compressed / expanded

What does pressure produce?

A net force at right angles to the wall of the gas container (or any surface)

Work

Transfer of energy by a force

What does doing work on an enclosed gas do?

Increases the internal energy of the gas

What does increases IE of a gas by doing work on it cause + why?

Increase in temp of the gas

• Bc KE of gas particles increased

• + temp= avg KE of particles

To compress a gas, what must be done?

Work

Why does compressing a gas increase its IE?

Carried out work

• Applied a force to the gas

• So ET to gas particles

Why do bicycle pumps get warm when inflating a tyre?

Work done on the gas → we increase its IE