Particles on the move
Mixing gases
The particles in a gas move randomly. Both the speed of the particles and the direction of the particles are random.
If we mix two gases in one container, each one will have particles that move randomly.
That means each gas will spread to fill the container.
The movement of the particles of each gas is called diffusion.
Diffusion means the overall random movement of particles from an area where they are in higher concentration to an area where they are in lower concentration. Concentration is the number of particles in a particular volume.
Look at the diagrams. They show how two gases diffuse.
1. At the start, the tap is closed.
The blue gas particles are at high concentration on the left, and zero concentration on the right.
The red gas particles are at high concentration on the right, and zero concentration on the left.
2. The tap is opened and the gases start to spread out (diffuse). At random, some particles of each gas will pass through the space where the tap opens.
The blue gas particles are now at quite high concentration on the left, and low concentration on the right.
The red gas particles are at quite high concentration on the right, and low concentration on the left.
3. After some time, the gases have completely diffused.
There is equal concentration of both gases on both sides.
Diffusion stops when the concentrations are equal. However, the movement of individual particles does not stop when diffusion stops.
Diffusion explains how you can smell food when it is cooking.
When food is heated, some particles in the food change state and become gas.
The gas particles from the food move randomly and so spread out through the air by diffusion.
The strength of the smell will get stronger as you move closer to the food. This is because the concentration of the particles is higher, the closer you get to the source of the particles.
Many animals use this change in strength of smell to find food.
Mixing liquids
Diffusion also happens in liquids. Particles in a liquid also move at random.
The picture shows what happens when blue ink is added to water.
The blue ink moves from the area of high concentration to the areas of low concentration, until it is completely diffused throughout the water.
Speed of diffusion
The speed of diffusion depends on:
- the difference in concentration of the particles
- temperature
The bigger the difference in the concentrations of the particles, the faster the diffusion.
The higher the temperature, the faster the diffusion. Higher temperature makes particles move faster, so the particles can spread out faster.
For example, when you make a cup of tea, the tea diffuses through the water. It diffuses faster in hot water than it does in cold water.
Questions
1 Zara’s mother opens a bottle of perfume. Zara is at the other side of the room.
After a few minutes, Zara can smell the perfume.
a Which of these explains why Zara smells the perfume?
Write one letter.
- A All the gas particles of the perfume move in one direction.
- B All the gas particles of the perfume move randomly.
- C All the particles in the air move in one direction.
- D All the particles in the air are stationary (not moving).
Show Answer
B – Because gas particles move randomly and spread throughout the room.
b Which of these changes would result in Zara smelling the perfume in a shorter time?
In each case, no other variable changes.
Write the letters for all the correct statements.
- A The perfume in the bottle is at a higher temperature.
- B Zara moves further away from the bottle of perfume.
- C Zara’s mother puts the lid on the perfume bottle.
- D The air in the room is at a lower temperature.
Show Answer
A – Higher temperature increases particle speed.
D is incorrect – lower temperature slows diffusion.
B and C would reduce diffusion speed.
2 Describe what is meant by the term ‘diffusion’.
Show Answer
Diffusion is the movement of particles from an area of high concentration to an area of low concentration.
3 The diagram shows two types of particle in containers, A and B. The containers have the same volume.
a Explain whether the particles in the containers are in a solid, liquid or gas state.
b Explain how the concentrations of the blue particles compare in containers A and B.
c Explain how the concentrations of the red particles compare in containers A and B.
Show Answer
a. They are in a gas state – particles are spread out and moving freely.
b. Blue particle concentration is higher in container A.
c. Red particle concentration is higher in container B.
4 Sofia is making an orange flavour drink. She pours a small volume of concentrated orange juice into a glass. She then adds water until the glass is full.
a Explain how the orange colour from the juice spreads into the water.
b Sofia says: ‘When the orange colour has stopped spreading, the particles in the liquid have stopped moving.’
Explain whether Sofia is correct.
Show Answer
a. The orange colour spreads by diffusion – from high to low concentration.
b. Sofia is incorrect – even when colour stops spreading, particles continue to move randomly.
5 Which of these will result in diffusion?
Write the letters for all the correct statements.
- A Adding milk to coffee.
- B Adding sand to water.
- C Adding salty water to pure water.
Show Answer
A and C – both involve particles mixing due to diffusion. Sand does not dissolve or diffuse.
Think Like a Scientist
In this investigation, you will investigate how temperature affects the speed of diffusion in liquids.
You will carry out the investigation by yourself.
- three or more identical beakers
- water
- a way to heat the water
- a way to cool the water (optional)
- water-soluble ink or food dye
- measuring cylinder
- thermometer
- dropper pipette
- stopwatch
- Add equal volumes of water at different temperatures to each of your beakers.
- Measure and record the temperature of the water in each beaker.
- Use the dropper pipette to carefully add a small volume of ink or food dye to the bottom of each beaker. Squeeze the top of the pipette gently so it does not squirt out into the water and start mixing. Add the ink or food dye in order from the lowest to the highest temperature.
- Use a stopwatch to time the diffusion process in each beaker.
- Stop the stopwatch when the ink or food dye has fully spread out through the water of the beaker being tested.
Show Answer
The higher the temperature of the water, the faster the ink or dye spreads out. This means diffusion happens more quickly at higher temperatures.
Show Answer
At higher temperatures, water molecules have more kinetic energy and move faster, causing particles of dye to spread out more quickly through diffusion.
Show Answer
Using the same volume of water ensures that the diffusion time is only affected by temperature, not the amount of water. It makes the test fair.
Show Answer
(1) Use the same volume of dye in each test. (2) Drop the dye from the same height and location in each beaker. (3) Repeat each test and take an average time.