P6. Molecules and Matter

Question 1

What is the density of a substance defined as?

Answer 1

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

Question 2

How do you calculate density?

Answer 2

density, p (kg/m³) = mass, m (kg) ÷ volume, V (m³)

Question 3

How do you measure density in an experiment?

Answer 3

Measure the mass of an empty beaker using a balance
Remove the beaker from the balance and pour the liquid from the measuring cylinder into the beaker
Use the balance again to measure the total mass of the beaker and the liquid
You can calculate the mass of the liquid by subtracting the mass of the empty beaker from the total mass of the beaker and the liquid

Question 4

What are the forces acting between particles in a solid?

Answer 4

The particles of a solid are held next to each other in fixed position. They vibrate about their fixed positions so the solid keeps its own shape. They are the least energetic of the states of matter

Question 5

What are the forces acting between particles in a liquid?

Answer 5

The particles of a substance in a liquid state are in contact with each other. They move about at random. So a liquid doesn’t have its own shape and can flow. They are more energetic than particles in a solid.

Question 6

What are the forces acting between particles in a gas?

Answer 6

The particles of a substance in its gas state move about at random much faster than they do in a liquid. They are much further apart from each other than the particles of a liquid. So the density of a gas is much less than that of a solid or liquid. They are the most energetic of the states of matter

Question 7

Why does the density of material change during a change of state?

Answer 7

There is little difference between the density of a liquid and its corresponding solid (eg water and ice). This is because the particles are tightly packed in both states. The same number of particles in a gas spread further apart than in the liquid or solid states. The same mass takes up a bigger volume - this means the gas is less dense.

Question 8

What is the melting point of a substance?

Answer 8

The temperature at which it melts (which is the same temperature at which it solidifies)

Question 9

What is the boiling point of a substance?

Answer 9

The temperature at which it boils (which is the same temperature at which it condenses)

Question 10

What is latent heat?

Answer 10

The energy transferred to a substance when it changes its state

Question 11

How can the melting and boiling points of a substance be changed?

Answer 11

Increases or decreases in pressure

Question 12

What is internal energy?

Answer 12

The energy stored by the particles of a substance

Question 13

What does internal energy of a substance sum of?

Answer 13

The kinetic energy they have due to their individual motions relative to each other and the potential energy they have due to their individual positions relative to each other

Question 14

What happens when the temperature of a substance increases or decreases?

Answer 14

The total kinetic energy of its particle increases/decreases.

Question 15

What happens when the physical state of a substance changes?

Answer 15

The total potential energy of its particles changes

Question 16

What are the structure of the particles in a solid?

Answer 16

There are strong forces of attraction between these particles. These forces bond the particles in fixed position
Each particle vibrates about an average position that is fixed
When a solid is heated, the particles’ energy stores increase and they vibrate more. If the solid is heated up enough, the solid melts because its particles haves gained enough energy to break away from the structure

Question 17

What are the structure of the particles in a liquid?

Answer 17

The forces of attraction are strong enough to stop the particles moving away from each other completely at the surface
When a liquid is heated, some of the particles gain enough energy to break away from the other particles. The molecules that escape from the liquid are in a gas state above the liquid

Question 18

What are the structure of the particles in a gas?

Answer 18

The particles move about at a high speed in random directions, colliding with each other and with the internal surface of their container.
When a gas is heated, its particles gain kinetic energy and on average move faster. This causes the pressure of the gas to increase because the particles collide with the container surface more often and with more force

Question 19

What is the specific latent heat of fusion of a substance?

Answer 19

The energy needed to change the state of 1kg of substance from solid to liquid at its melting piint

Question 20

What is the equation for specific latent heat of fusion?

Answer 20

Specific latent heat of fusion, Lf (J/kg) = energy, E (joules) ÷ mass, m (kg)

Question 21

What is the specific latent heat of vaporisation of a substance?

Answer 21

The energy needed to change the state of 1kg of the substance from liquid to vapour at its boiling point.

Question 22

What is the equation for specific latent heat of vaporisation?

Answer 22

Specific latent heat of vaporisation, Lv = energy, E (joules, J) ÷ mass, m (kilograms)

Question 23

What is the linear relationship between changes in temperatures and pressure for a gas?

Answer 23

If the temperature of a gas in a sealed container is increased, the pressure of the gas increases

Question 24

Why does the pressure of the gas increase if the temperature of a gas is increased?

Answer 24

The molecules moves faster so they hit the surfaces with more force
The number of impacts per second of gas molecules on the surfaces of a seal container increases, so the total force of the impacts increase

Question 25

How do you investigate the relationship between the pressure of a gas and its temperature?

Answer 25

Dry air is sealed in a sealed flask connected to a pressure gauge
The flask is in a big beaker of water which is heated to raise the temperature of the gas
The water is heated in stages to raise the temperature in states.
At each stage, the water is stirred to make sure that its temperature is the same throughout
The temperature of the water is measured using the thermometer. The pressure is read off the pressure gauge
If the measurements are plotted on a graph of pressure against temperature in °C, the results give you a straight line graph, which shows the increase of pressure is the same for equal increases of temperature

Question 26

What is the Brownian Motion?

Answer 26

Brownian motion is the random movement of particles in a liquid or a gas produced by large numbers of collisions with smaller particles which are often too small to see

Question 27

What is the calculation for Boyle’s Law?

Answer 27

pressure, p (Pascals) × volume, v (metres cubed, m³) = constant

Question 28

Why does the pressure of a gas increase when its volume decreases?

Answer 28

For a fixed mass of a gas at constant temperature
Its pressure is increased if its volume 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