P4- Molecules And Matter

Question 1

What is density?

Answer 1

The mass per unit volume of a material.
Objects from low density materials typically have a low mass, whilst similar sized objects made from high density materials have a high mass.

Question 2

What is the equation for density?

Answer 2

Density= mass/ volume
p = m/ V

Question 3

What is the unit for density?

Answer 3

If the mass is measured in g and volume in cm³ then the density will be g/cm³.
However if the mass is measured in kg and volume in cm³ then the density will be in kg/m³.

Question 4

The density in solids and liquids are roughly the same, this is because…

Answer 4

The molecules are tightly packed together. This gives them a high density.

Question 5

Why do molecules in gas have a low density?

Answer 5

Because they are widely separated this makes gases have a significantly lower density than solids or liquids.

Question 6

How many times is the distance between molecules in a gas larger than molecules in a solid or liquid?

Answer 6

10 times larger.

Question 7

How much is the density of gas compared to a solid or liquid?

Answer 7

Around one thousandth.

Question 8

What change happens when a substance changes state?

Answer 8

The number of molecules stays the same, only thing that changes is the energy.

Question 9

What is it called when a gas turns into a liquid?

Answer 9

Condensing.

Question 10

Are changes of state reversible?

Answer 10

Yes unlike chemical changes.

Question 11

What is the time internal energy used to describe?

Answer 11

The thermal energy store of a substance.

Question 12

What two forms of energy do molecules possess?

Answer 12

Kinetic energy: due to the random motion/vibration.

Potential energy: due to the bonds between the molecules

Question 13

What is related to the average kinetic energy of a molecule?

Answer 13

The temperature of a material.

Question 14

What happens to the kinetic energy when a substance reaches a certain temperature?

Answer 14

The kinetic energy will stop increasing and the heat will go into potential energy, breaking the bonds between the molecules and changing the state of the material.

Question 15

What happens to the temperature of a material when a substance changes state (from solid to liquid or from liquid to gas)?

Answer 15

The temperature of the material remains constant.

Question 16

Why does the temperature remain constant when a substance is changing state even though it is still gaining (or losing) thermal energy?

Answer 16

This happens because the energy is being used to break the bonds between the molecules, instead of increasing the kinetic energy of the molecules (and hence the temperature) this energy is known as latent heat

Question 17

What does the latent heat of a substance depend on?

Answer 17

• what the substance is
• its state
• its mass

Question 18

What is specific latent heat?

Answer 18

The energy required to change the state of 1 kg of a substance.
(This energy is required to break bonds holding molecules together and is also given out again once the bonds reform)

Question 19

What is the equation for specific latent heat? And what are the units?

Answer 19

Change in energy = mass x specific latent heat
E = m x L
Units: specific latent heat (L) measured in joules per kilogram (J/kg)

Question 20

What is the specific latent heat of the material when there is a change from solid to liquid?

Answer 20

Specific latent heat of fusion is the valuefor a change from solid to liquid.

Question 21

What is the specific latent heat of the material when there is a change from liquid to gas?

Answer 21

Specific latent heat of vaporisation is the value for a change from liquid to gas.

Question 22

What is the temperature of the gas related to of the molecules in that gas?

Answer 22

Kinetic energy

Question 23

What happens if the temperature of the gas increases?

Answer 23

The average speed of the molecules will also increase.

Question 24

What happens if the state of the molecules increase?

Answer 24

The molecules will collide harder and more often against any walls.

Question 25

What happens when a gas is heated with the constant volume?

Answer 25

The pressure of fat gas will increase.