Solids, Liquids and Gases

Double Science Award

a) Units

5.1: use the following units: degrees Celsius (°C), kelvin (K), joule (J), kilogram (kg), Newton (N), kilogram/metre³ (kg/m³), metre (m), metre² (m²), metre³ (m³), metre/second (m/s), metre/second² (m/s²), pascal, (Pa)

degrees Celsius (°C): used to measure temperature and is based on the freezing and boiling temperature of water.

kelvin (K): 0K= −273.15 °C = absolute zero

joule (J): used to measure energy

kilogram (kg): the common scale to measure mass

Newton (N): used to measure weight 10N=1kg (on earth)

kilogram/metre³ (kg/m³): can be used to measure density

metre (m): used to measure length or distance

metre² (m²): used to measure area

metre³ (m³): used to measure volume

metre/second (m/s): used to measure speed

metre/second² (m/s²): used to measure acceleration

pascal (Pa): used to measure pressure


b) Density and Pressure

5.2: recall and use the relationship between density, mass and volume:

density=mass/volume

ρ=m/V

As you can see from above that density is measured in kg/m³, however, this is not for all cases. For example: if the mass is measured in grams and the volume is in cm³, then the density is measured in g/cm³.

5.3: describe how to determine density using direct measurements of mass and volume

Use the equation above to find out the density when you have the mass and volume.
For example: if you have a cube block and it weighs 80g and its volume is 20cm³.
Then you do:

density=mass/volume

        =80/20

          =4g/cm³

Remember that if cut the block in half, the density DOES NOT change. Also, if you double the size of the block, the density also DOES NOT change. Be careful with questions like these, they might be tricking you!

5.4: recall and use the relationship pressure, force and area:

pressure=force/area

p=F/A

E.g. If you have a blunt knife, it is harder to cut things. If you have a sharp knife, it is easier, this is because there is the same amount of force over a SMALLER area, therefore the pressure is higher. Another example is when you push on a pin with the sharp side on the surface. It is easier to push the pin into the surface (depending on the surface of course) than on the flat side to go into the surface.

Let's try it:
If you have a force of 210N and an area of 30cm². Then in order to find the pressure you do:

210N/30cm²

=7N/cm²

Then the pressure would be 7N/cm²

5.5: understand that the pressure at a point in a gas or liquid which is at rest acts equally in all directions

If you have an object in the water, you have pressure acting on the object from all directions as there are molecules bouncing off everywhere of the surface of the object.

5.6: recall and use the relationship for pressure difference

pressure difference=height x density x gravitational pull

p= h x ρ x g

An example that shows there is a pressure difference depending on the height is when you take a bucket. If you fill it with water and make holes in the bucket, you will see that the water shoots out further from the bottom of the bucket in comparison to the water from the holes at the top of the bucket. Another simple example is when you go swimming, if you go to the bottom of the pool then you feel pressure on your ears.

Let's try it:

The height of a swimming pool is 2m, the density of water is 1g/cm³, and the gravitational pull on earth 10N/kg. To calculate the pressure difference between the bottom of the pool and the middle of the pool. So you do:

For the bottom of the pool:

                                                               2m x 1g/cm³ x 10N/kg
                                                                         = 20N/cm³

Then you take the middle of the pool: 

1m x 1g/cm³ x 10N/kg
= 10N/cm³

The pressure difference is therefore:

20N/cm³ - 10N/cm³

= 10N/cm³

c) Ideal Gas Molecules

5.7: understand the significance of Brownian motion

Brownian motion is when molecules in a gas or liquid move randomly due to collisions with other molecules. It is unpredictable therefore is described as random.
It is used in many important processes in life such as diffusion (for example in plants).

5.8: recall that molecules in a gas have random motion and that they exert a force and hence a pressure on the walls of the container

Molecules in gases move randomly due to other molecules, they exert a force on the surface of the container when the molecules hit the surface. Therefore there is pressure on the walls of the container.

5.9: understand that there is an absolute zero temperature which is -273°C

Absolute zero is the temperature at which molecules have no energy therefore they stop moving. Normally, molecules in gases move quickly, molecules in liquids are irregular and are in constant motion, and finally the molecules in solids vibrate. Either way they move about, however at -273°C they stop moving.

5.10: describe the Kelvin scale of temperature and be able to convert between the Kelvin and Celsius scales

The Kelvin scale is based on the movement of molecules. -273°C = 0K. At this temperature, the molecules stop moving. To convert from celsius to Kelvin, just take the celsius and add 273 to get the kelvin. To convert Kelvin to celsius, you take the kelvin and subtract 273.

For example:

0°C = 273K

0K = -273°C

Remember that 1K DOES NOT equal 273°C, it doesn't work that way. So 10°C would equal 283K, the Kelvin scale of temperature goes up one at a time. 

For example: 

20°C = 293K

20K = -253°C

5.11: understand that an increase in temperature results in an increase in the speed of gas molecules

When you increase the temperature, it means you are increasing the heat energy that you are giving the gas. Therefore, the molecules move faster as there is more kinetic energy.

5.12: describe the qualitative relationship between pressure and Kelvin temperature for a gas in a sealed container

As the temperature increases, the pressure increases because there is more heat energy hence more kinetic energy. Due to the rise in kinetic energy, the molecules move faster and collide with the surface of the container more. This results in more force thus increasing the pressure inside the sealed container.

5.13: use the relationship between the pressure and volume of a fixed mass of gas at constant temperature

p1V1=p2V2

Basically this equation shows that if a fixed amount of gas at the same temperature as another of the same gas, the relationship between the pressure and volume would be the same.

For example: 
If the pressure of 100cm³ of air is 20N/cm², then the relationship would be the same as pressure of 200cm³ of air at 10N/cm²

Triple Award Science

Note that triple award includes the double award material as well as the additional triple award material

5.7: understand that a substance can change state from solid to liquid by the process of melting

Let's take an ice cube for example, when it melts it becomes a liquid which is water. The bonds between the molecules weaken and eventually the regular structure of the molecules breaks and it becomes irregular. The bonds are weakened due to the input of heat energy.

5.8: understand that a substance can change state from liquid to gas by the process of evaporation or boiling

When you boil water you see steam coming out of the spout of the kettle. The water changed from a liquid to a gas. When you increase the heat energy, the bonds between the molecules break and the molecules are released into the air as a gas.
The difference between boiling and evaporating is that evaporating happens all the time when the substance is a liquid. However, boiling happens at a higher temperature and needs external heat to bring the liquid to a boil.

5.9: recall that particles in a liquid have a random motion within a close-packed irregular structure

The molecules of a liquid are not fixed like a solid therefore are free to move however unlike a gas, the molecules are still close-packed. It does not form a regular structure like a solid as the bonds between the molecules are not strong enough to hold the substance together, that's why it takes the shape of it's container.
5.10: recall that particles in a solid vibrate about fixed positions within a close-packed regular structure

The molecules or particles in a solid vibrate because there is not enough room for them to move therefore they vibrate. Also, the bonds between the molecules in a solid are much stronger than in a liquid therefore it creates a close-packed regular structure.

5.16: understand that the kelvin temperature of the gas is proportional to the average kinetic energy of its molecules

As I have told you above, the Kelvin scale of temperature is based on the amount of kinetic energy in the molecules. You would understand that if you increase the heat energy given to the gas, the temperature would rise and also the amount of average kinetic energy the molecules have. On the other hand, if you decrease the amount of heat energy, then the temperature drops and the average amount of kinetic energy in its molecules is also lower.
Temperature is actually the average amount of kinetic energy in the molecules however, the energy is converted to heat energy so it can also be referred to as the measure of heat energy.

5.18: use the relationship between the pressure and Kelvin temperature of a fixed mass of gas at constant volume:

(P1/T1)=(P2/T2)

When you have a fixed amount of gas, the relationship between the pressure and Kelvin temperature is the same even if the values are different.

For example: You have 100cm³ of air, with the pressure at 10N/cm² and at 1K temperature. The relationship would be the same if the pressure was 20N/cm² and at 2K temperature.

(10N/cm² / 1K ) = ( 20N/cm² / 2K )

10 = 10

As you can see the equation shows that the relationship between the pressure and kelvin temperature of a fixed volume of gas is always the same.