P3 - Particle model of matter

Question 1

What is Density and its equation?

What is the relationship between mass and density?

Answer 1

• Density is defined as:

The mass per unit volume of a material

• Objects made from low density materials typically have a low mass
• Similarly sized objects made from high density materials have a high mass
  
  • For example, a bag full of feathers is far lighter compared to a similar bag full of metal
  • Or another example, a balloon is less dense than a small bar of lead despite occupying a larger volume
• Density is related to mass and volume by the following equation:

Question 2

Why are gases less dense than solids?

Answer 2

• Gases, for examples, are less dense than solids because the molecules are more spread out (same mass, over a larger volume)

Question 3

What are the units for density?

Answer 3

• The units of density depend on what units are used for mass and volume:
  
  • If the mass is measured in g and volume in cm³, then the density will be in g/cm³
  • If the mass is measured in kg and volume in m³, then the density will be in kg/m³
• This table gives some examples of densities on common materials
  
  • If a material is more dense than water (1000 kg/m³), then it will sink

Question 4

What is the formula for volume in a:

Sphere
Cube
Cylinder

Answer 4

Question 5

How do you convert between g/cm^3 and kg/m^3

Answer 5

1 g/cm³ is equal to 1,000 kg/m³

• To convert from kg/m³ to g/cm³, divide by 1,000.
• To convert from g/cm³ to kg/m³, multiply by 1,000.

Aluminium has a density of 2.7 g/cm³, or 2,700 kg/m³. Lead has a density of 11.6 g/cm³, or 11,600 kg/m³.

Iron has a density of 7.9 g/cm³. What is this in kg/m³?

7.9 multiplied by 1,000 gives 7,900 kg/m³.

Question 6

How do you measure the volume of an irregular object?

Answer 6

If the object has an irregular shape, the volume can be measured using a displacement can.

The displacement can is filled with water above a narrow spout and allowed to drain until the water is level with the bottom of the spout.

As the irregular object is lowered into the displacement can, the water level rises. All the displaced water comes out of the spout and is collected in a measuring cylinder.

The displaced water in the cylinder occupies the same amount of space as the object in the can, which means that their volumes are the same.

Question 7

What is matter made up of?

What does the particle model allow us to do?

Answer 7

• All matter is made up of very small particles, or atoms
• The particle model is a model that describes the arrangement and movement of particles in a substance
• The particle model can be used to explain
  
  • The different states of matter e.g. solids, liquids and gases
  • Physical properties e.g. differences in density

Question 8

What are particles like in solids?

Answer 8

Solids

• In a solid:
  
  • The particles are closely packed
  • The particles vibrate about fixed positions
  • regular arrangement
• Solids have:
  
  • A definite shape (they are rigid)
  • A definite volume

Question 9

What are particles like in Liquids?

Answer 9

• In a liquid:
  
  • The particles are closely packed
  • The particles can flow over one another
  • are randomly arranged
• Liquids have:
  
  • No definite shape – they are able to flow and will take the shape of a container
  • A definite volume

Question 10

What are particles like in Gases?

Why are gases highly compressible?

Answer 10

• In a gas:
  
  • The particles are far apart
  • The particles move randomly
  • move quickly in all directions
• Gases have:
  
  • No definite shape – they will take the shape of their container
  • No fixed volume – if placed in an evacuated container they will expand to fill the container
• Gases are highly compressible, this is because:
  
  • There are large gaps between the particles
  • It is easier to push the particles closer together than in solids or liquids

Question 11

Compare solids, liquids and gases

Answer 11

Question 12

What happens when a substance changes state?

Answer 12

• When a substance changes state, the number of molecules in that substance doesn’t change and so neither does its mass
  
  • The only thing that changes is its energy

Question 13

What are all the different changes of state?

Answer 13

They are physical changes as no new substance is formed and the reaction is reversible

Question 14

What is Internal Energy

Answer 14

• Internal energy is defined as:

Energy is stored inside a system by the particles (atoms and molecules) that make up the system. This is called internal energy

• The molecules within a substance possess two forms of energy:
  
  • Kinetic energy (due to their random motion / vibration)
  • Potential energy (due to their position relative to each other)

This is called internal energy. Internal energy is the total kinetic energy and potential energy of all the particles (atoms and molecules) that make up a system.

Heating changes the energy stored within the system by increasing the energy of the particles that make up the system. This either raises the temperature of the system or produces a change of state.

Question 15

What happens to an object when you heat it?

Answer 15

• Heating a system changes a substance’s internal energy by increasing the kinetic energy of its particles
  
  • The temperature of the material, therefore, is related to the average kinetic energy of the molecules
• The higher the temperature, the higher the kinetic energy of the molecules and vice versa
  
  • This means they move around faster
• This increase in kinetic energy (and therefore internal energy) can:
  
  • Cause the temperature of the system to increase
  • Or, produce a change of state (solid to liquid or liquid to gas)

Question 16

What happens to a substance when it changes state?

Answer 16

• When a substance reaches a certain temperature, the kinetic energy of the molecules will stop increasing and the energy will go into increasing its potential energy instead
• This breaks the bonds between the molecules, causing them to move further apart and leads to a change of state
• The kinetic energy remains the same, meaning that the temperature will remain the same, even though the substance is still being heated

Question 17

What is the increase in temperature of a system dependent on?

Answer 17

• If the temperature of the system increases, the increase in temperature of this system depends on:
  
  • The mass of the substance heated
  • The type of material
  • The energy input to the system

Question 18

What is specific heat capacity

Answer 18

The amount of energy required to raise the temperature of 1 kg of the substance by 1 °C

• Different substances have different specific heat capacities
  
  • If a substance has a low specific heat capacity, it heats up and cools down quickly (ie. it takes less energy to change its temperature)
  • If a substance has a high specific heat capacity, it heats up and cools down slowly (ie. it takes more energy to change its temperature)

Question 19

How do calculate change in thermal energy?

Answer 19

ΔE=mcΔθ

• Where:
  
  • ΔE = change in thermal energy, in joules (J)
  • m = mass, in kilograms (kg)
  • c = specific heat capacity, in joules per kilogram per degree Celsius (J/kg °C)
  • Δθ = change in temperature, in degrees Celsius (°C)

Question 20

Why do measure specific heat capacity?

Answer 20

• Specific heat capacity is mainly used for liquids and solids
• The specific heat capacity of different substances determines how useful they would be for a specific purpose eg. choosing the best material for kitchen appliances
  
  • Good electrical conductors, such as copper and lead, are excellent conductors of heat due to their low specific heat capacity
  • On the other hand, water has a very high specific heat capacity, making it ideal for heating homes as the water remains hot in a radiator for a long time

Question 21

Water of mass 0.48 kg is increased in temperature by 0.7 °C. The specific heat capacity of water is 4200 J / kg °. Calculate the amount of energy transferred to the water.

Answer 21

Step 1: Write down the known quantities

• • Mass, m = 0.48 kg
    
    • Change in temperature, Δθ = 0.7 °C
    • Specific heat capacity, c = 4200 J / kg °C

Step 2: Write down the relevant equation

ΔE = mcΔθ

Step 3: Calculate the energy transferred by substituting in the values

ΔE = (0.48) × (4200) × (0.7) = 1411.2

Step 4: Round the answer to 2 significant figures

ΔE = 1400 J

Question 22

What happens to the temperature of an object during a change in state?

Answer 22

• The temperature remains constant when melting and boiling, despite energy being added, as shown in the graph below:

Question 23

What is the specific Latent Heat and its unit?

Answer 23

• Energy is required to change the state of a substance
  
  • This energy is known as latent heat
• The specific latent heat is defined as:

The amount of thermal energy required to change the state of 1 kg of a substance with no change in temperature

• There are two types of specific latent heat:
  
  • Specific latent heat of fusion (solid to liquid and vice versa)
  • Specific latent heat of vaporisation (liquid to gas and vice versa)
• Latent heat is represented by the symbol L with units joules per kilogram (J/kg)

Question 24

What is the specific Latent heat of Fusion?

Answer 24

• More specifically, the specific latent heat of fusion is defined as:

The thermal energy required to convert 1 kg of solid to liquid with no change in temperature

• This is used when melting
• a solid or freezing a liquid
• When a solid substance is melted, its temperature stays constant until all of the substance has melted
• The latent heat of fusion is the energy needed to break the bonds between the molecules
• If the substance in its liquid state is cooled, it will solidify at the same temperature as its melting point
  
  • When this happens, the particles bond together into a rigid structure
  • Latent heat is transferred to the surroundings as the substance solidifies and the particles form stronger bonds

Question 25

What is the specific Latent heat of Vaporisation?

Answer 25

• The specific latent heat of vaporisation is defined as:

The thermal energy required to convert 1 kg of liquid to gas with no change in temperature

• This is used when vaporising a liquid or condensing a gas
• When a liquid substance is heated, at its boiling point, the substance boils and turns into vapour
• The latent heat of vaporisation is the energy needed by the particles to break away from their neighbouring particles in the liquid
• If the substance in its gas state is cooled, it will condense at the same temperature as its boiling point
  
  • Therefore, latent heat is transferred to the surroundings as the substance condenses into a liquid and its particles form new molecular bonds

Question 26

How do you calculate specific latent heat?

Answer 26

• he amount of energy E required to melt or vaporise a mass of m with latent heat L is:

E = mL

• Where:
  
  • E = thermal energy required for a change in state, in joules (J)
  • m = mass, in kilograms (kg)
  • L = specific latent heat, in joules per kilogram (J/kg)

Question 27

What are Heating and Cooling Graphs?

Answer 27

• Heating and cooling graphs are used to summarise:
  
  • How the temperature of a substance changes with increased energy added to it (or removed from it)
  • Where changes of state occur
• Heating and cooling graphs tend to be the same
  
  • Heating is when the heat is added and the kinetic energy of the molecules increases (red arrows to the right)
  • Cooling is when heat is removed (or goes to the surroundings) and the kinetic energy of the molecules decreases (blue arrows to the left)

Question 28

Heating Graphs

Answer 28

• When heat is added to a solid, the temperature starts to increase as the particles vibrate and gain kinetic energy
• At a substance’s melting point, the heat energy added goes into breaking the intermolecular bonds and there no more increase in kinetic energy or temperature
  
  • This is melting and the substance is now a liquid
• As heat is continually added, the temperature of the liquid continues to increase as the particles gain more kinetic energy
• At a substance’s boiling point, the heat energy added goes into breaking the intermolecular bonds further and there is no more increase in kinetic energy or temperature
  
  • This is evaporation or vaporisation and the substance is now a gas

Question 29

Explain this Cooling Graphs

Answer 29

• The process is repeated backwards for cooling as heat is taken away
  
  • A gas turns back into liquid through condensation
  • A liquid turns back into a solid through freezing

The different sections of the graph show:

• ORIGIN to A: Added heat energy is being used to increase the kinetic energy of the particles while it is a solid
• A to B: Added heat energy is being used to break the bonds between the solid molecules, increasing the potential energy and melting the substance
• B to C: Added heat energy is being used to further increase the kinetic energy of the particles while the substance is a liquid
• C to D: Added heat energy is being used to break the bonds between the liquid molecules, further increasing the potential energy and boiling the substance
• D to E: Added heat energy is being used to further increase the kinetic energy of the particles while the substance is a gas

Question 30

What is the difference between specific heat capacity and specific latent heat?

Answer 30

• The specific heat capacity and specific latent heat are slightly different, and it is important not to confuse one for the other
• Specific heat capacity is the amount of heat energy required to raise the temperature of a substance by a certain amount
  
  • The substance will still be in the same state, just raised to a different temperature
  • E.g. A liquid heated from 5 °C to 20 °C
• Specific latent heat is the amount of heat energy needed to cause a change of state, i.e. the boiling of water or the melting of ice
  
  • The substance will be changing states, but still at the same temperature
  • E.g. A liquid evaporating into a gas

Question 31

What does random motion mean?

Answer 31

• Random motion means that the molecules are travelling in no specific path and undergo sudden changes in their motion if they collide:
  
  • With the walls of its container
  • With other molecules
• The random motion of tiny particles in a fluid is known as Brownian motion

Question 32

What is fluid?

Answer 32

Liquid or gas

Question 33

What is Pressure?
How do we calculate pressure?

How do we increase the amount of pressure acting on the wall of a container?

Answer 33

• Molecules of gas in a container will collide with the container walls
• Pressure is defined as the force exerted per unit area of the container

Pressure = Force/Area

F= Force(Newtons, N)

P = Pressure(pascals, Pa)

A = Area(m^2)

• Therefore a gas at high pressure has more frequent collisions on the container walls and therefore overall more force per unit area
• If the gas is heated up, the molecules will travel at a higher speed
  
  • This means they will collide with the walls more often
  • This creates an increase in pressure
• Therefore, at a constant volume, an increase in temperature increases the pressure of a gas and vice versa
• Diagram A shows molecules in the same volume collide with the walls of the container more with an increase in temperature
• Diagram B shows that since the temperature is proportional to the pressure, the graph against each is a straight line