Diffusion and density
By the end of this chapter you will be able to
- Describe diffusion using the random movement of particles and concentration
- Name the factors that change the rate of diffusion, including temperature and state
- Use density = mass / volume, with units of g/cm cubed
- Measure the volume of a regular and an irregular solid
- Predict whether an object floats or sinks by comparing its density with that of the liquid
You should already know
- Particles and the particle model (Chapter 4 of this book)
- Reading a measuring cylinder and using a table (Chapter 1 of this book)
Where this leads
- Atoms and elements, which use the particle idea further (Chapter 6)
National Curriculum for England, Key Stage 3 science, Year 7. Content: C2 diffusion is the random movement of particles, from a higher to a lower concentration, in gases and liquids, C3 density = mass / volume; measuring the density of regular and irregular solids and liquids; density and floating.
Exercise P Do you remember?
Method. Recall the particle model.
- Recall. The particles in a gas are far apart.far apart
- Write the answer. far apart
Check. Gas particles move quickly and randomly. ✓
Method. Recall the change.
- Recall. Liquid to gas is evaporation or boiling.evaporation
- Write the answer. evaporation
Check. Evaporation can happen at any temperature. ✓
Method. Multiply the three lengths.
- Multiply. Length x width x height = 2 x 2 x 2.2 x 2 x 2 = 8
- Write the answer. 8
Check. Eight 1 cm cubes fill it. ✓
Method. Divide.
- Divide. Sixty divided by ten is six.60 / 10 = 6
- Write the answer. 6
Check. Check: 6 x 10 = 60. ✓
Method. Recall the unit.
- Recall. Masses are measured in grams, written g.grams
- Write the answer. grams
Check. A kilogram is 1000 grams. ✓
Diffusion and the particle model
The particles in a gas or a liquid are always moving at random. Diffusion is the spreading of particles from a region of higher concentration to a region of lower concentration, as a result of this random movement. The concentration is the number of particles in a given space.
Diffusion happens in gases and in liquids, but not in solids, because the particles of a solid are held in fixed positions.
Key ideas
- Diffusion is the random movement of particles from higher to lower concentration.
- It happens in gases and liquids, not in solids.
- Particles move at random, so the net movement is from high to low concentration.
- Diffusion continues until the particles are evenly spread.
Method. Read the definition.
- Read the definition. Diffusion is the movement from a higher concentration to a lower concentration.higher to lower
- Apply it. So the particles move from the higher concentration to the lower one.higher to lower
- Write the answer. from higher to lower concentration
Check. The net movement is in this direction. ✓
Method. Think about the particles of a solid.
- Think about the particles. The particles of a solid are held in fixed positions, so they cannot spread through it.fixed positions
- Decide. So diffusion does not happen in a solid, and the answer is no.no
- Write the answer. no
Check. Diffusion needs particles that can move past each other. ✓
Method. Check the movement.
- Check the movement. The perfume particles are in the air, and they spread out from high to low concentration.spread out
- Decide. So this is diffusion, and the answer is yes.yes
- Write the answer. yes
Check. Gases diffuse in the air. ✓
Watch out
Watch out: diffusion is not caused by particles pushing each other. It is caused by the random movement of each particle, which means particles end up spreading out.
Exam tip
Exam tip. When you explain diffusion, use the words random movement, higher concentration and lower concentration.
Try it (with your teacher, or a grown-up at home). With a grown-up, place a drop of food colouring in a jar of still water, and do not stir it. Look at the jar every few minutes. Describe how the colour spreads, and say which way the particles of colour move.
Exercise 5A Diffusion and the particle model
Method. Recall the process.
- Recall. Spreading of particles from higher to lower concentration is diffusion.diffusion
- Write the answer. diffusion
Check. Diffusion is caused by random movement. ✓
Method. Use the definition.
- Use the definition. Diffusion is from higher to lower concentration.higher to lower
- Decide. So the answer is no.no
- Write the answer. no
Check. Random movement leads to spreading out. ✓
Method. Use the definition.
- Recall. Diffusion happens in gases and liquids.gases and liquids
- Write the answer. gases and liquids
Check. Diffusion does not happen in solids. ✓
Method. Think about the particles of a solid.
- Think. The particles of a solid are held in fixed positions.fixed positions
- Decide. So the answer is no.no
- Write the answer. no
Check. Particles must be able to move past each other. ✓
Method. Recall the word.
- Recall. The concentration is the number of particles in a given space.concentration
- Write the answer. concentration
Check. A higher concentration has more particles in the same space. ✓
Method. Think about where the perfume starts.
- Think. The perfume is sprayed at one end, so the particles start there.starts there
- Decide. So the end with the spray has the higher concentration.the end with the spray
- Write the answer. the end with the spray
Check. The particles spread to the other end. ✓
Method. Think about the particles.
- Think. Each particle moves at random, and the spreading comes from this.at random
- Decide. So diffusion is random.random
- Write the answer. random
Check. The net movement is from high to low concentration. ✓
Method. Think about even spreading.
- Think. When the particles are evenly spread, there is no difference in concentration.evenly spread
- Decide. So the net movement stops, and the answer is yes.yes
- Write the answer. yes
Check. Particles still move, but there is no net movement. ✓
Method. Think about the cause.
- Think. The particles move at random, which spreads them out. They are not pushed in one direction.at random
- Decide. So the pupil is not right.no
- Write the answer. no
Check. Diffusion is random movement, not a push. ✓
Method. Think about the fair test.
- Think. Only the variable being tested should change, so the other things must stay the same.stay the same
- Name one. For example, the same amount of water.same amount of water
- Write the answer. Accept any one: the same jar, the same amount of water, the same drop size.
Check. A fair test keeps all other variables the same. ✓
Method. Think about the movement.
- Think about the movement. Liquid particles move at random and slide past each other.slide past
- Think about the result. So they spread out from higher to lower concentration, over time.over time
- Write the answer. Accept any sentence that says particles in a liquid move at random and slide past each other, so they spread out over time.
Check. Even close particles can move. ✓
Method. Think about net movement.
- Think about movement. The particles keep moving at random.keep moving
- Think about the net result. With the same concentration everywhere, there is no net movement from one place to another.no net movement
- Write the answer. Accept any sentence that says the particles keep moving at random, but there is no net movement because the concentration is the same everywhere.
Check. Net movement, not random movement, is what we observe. ✓
Factors that affect diffusion
The rate of diffusion depends on how fast the particles are moving. Particles that have more energy move faster, so they spread more quickly. Diffusion is faster in warm substances than in cold ones.
Diffusion is much faster in a gas than in a liquid, because gas particles move more quickly and are further apart, so they travel further between collisions.
The class recorded how long one drop of food colouring took to spread evenly through the same volume of water at three temperatures.
| Water temperature | Time to spread evenly (minutes) |
|---|---|
| cold water | 30 |
| room temperature water | 15 |
| warm water | 5 |
Key ideas
- Diffusion is faster at a higher temperature, because particles move faster.
- Diffusion is much faster in a gas than in a liquid.
- In a fair test, keep the drop size and the volume of water the same.
- Use the times in a table to compare the rates.
Method. Find the shortest time.
- Read the table. The shortest time is 5 minutes, for the warm water.5 minutes
- Decide. So warm water gave the fastest diffusion.warm water
- Write the answer. warm water
Check. The warm water spread the colour in 5 minutes. ✓
Method. Divide the cold time by the warm time.
- Read the times. The cold water took 30 minutes and the warm water took 5 minutes.30 and 5
- Divide. Divide 30 by 5 to find how many times longer.30 / 5 = 6
- Write the answer. 6
Check. 5 x 6 = 30. ✓
Method. Use the fact about gas particles.
- Read the fact. Gas particles move more quickly and are further apart than liquid particles.more quickly
- Decide. So diffusion is faster in a gas.gas
- Write the answer. gas
Check. Perfume spreads in air much faster than it spreads through water. ✓
Watch out
Watch out: a faster rate does not mean the particles diffuse to a different place; it means they reach an even spread in less time.
Exam tip
Exam tip. When you compare rates, use the temperature as the reason: higher temperature, particles faster, diffusion faster.
Try it (with your teacher, or a grown-up at home). With a grown-up, put one drop of food colouring into a glass of cold tap water, and a second drop into a glass of warm tap water (not hot). Time how long each glass takes to become an even colour. Use the same drop size and the same volume of water.
Exercise 5B Factors that affect diffusion
Method. Read the table.
- Read the table. The cold water took 30 minutes.30
- Write the answer. 30
Check. The cold water is the slowest. ✓
Method. Find the longest time.
- Read the table. The longest time is 30 minutes, for the cold water.30 minutes
- Write the answer. cold water
Check. Cold water slows the particles. ✓
Method. Read the table.
- Read the table. The times go down as the temperature goes up: 30, then 15, then 5 minutes.go down
- Decide. So the answer is yes.yes
- Write the answer. yes
Check. Higher temperature means faster particles. ✓
Method. Subtract.
- Read the times. Cold water took 30 minutes and room-temperature water took 15 minutes.30 and 15
- Subtract. The difference is 30 minus 15.30 - 15 = 15
- Write the answer. 15
Check. 15 + 15 = 30. ✓
Method. Use the fact.
- Use the fact. Gas particles move faster and are further apart, so diffusion is faster.faster
- Write the answer. gas
Check. Perfume spreads quickly through the air. ✓
Method. Read the table.
- Read the table. The warm water took 5 minutes.5
- Write the answer. 5
Check. Warm water is the fastest. ✓
Method. Divide the room-temperature time by the warm time.
- Read the times. Room-temperature water took 15 minutes and warm water took 5 minutes.15 and 5
- Divide. 15 divided by 5 is 3.15 / 5 = 3
- Write the answer. 3
Check. 5 x 3 = 15. ✓
Method. Think about the energy of the particles.
- Think. Cold particles have less energy, so they move more slowly and diffuse more slowly.more slowly
- Decide. So the pupil is not right.no
- Write the answer. no
Check. Temperature changes how fast particles move. ✓
Method. Check the data and the reason.
- Check the data. The warm water took 5 minutes, and the cold water took 30 minutes.5 and 30
- Decide. Warm particles move faster, so Ali is right.yes
- Write the answer. yes
Check. The table supports Ali's reason. ✓
Method. Think about the fair test.
- Think. A different drop size changes the amount of colour, which affects the time.amount of colour
- Decide. So the criticism is reasonable, and the answer is yes.yes
- Write the answer. yes
Check. Keep the drop size the same in a fair test. ✓
Method. Think about the energy.
- Think about the energy. A higher temperature gives particles more energy.more energy
- Think about the movement. So they move faster, and spread out more quickly.more quickly
- Write the answer. Accept any sentence that says higher temperature gives the particles more energy, so they move faster and spread more quickly.
Check. Temperature and particle speed are linked. ✓
Method. Think about the particles.
- Think about the gas. Gas particles are further apart and move more quickly.further apart
- Think about the result. So they travel further before colliding, and spread out faster.faster
- Write the answer. Accept any sentence that says gas particles are further apart and move more quickly, so they travel further and diffuse faster.
Check. Particle spacing and speed both matter. ✓
Density: mass, volume and density
Density is the mass of a substance in a given volume. It is found using:
density = mass / volume
Mass is measured in grams (g) and volume in cubic centimetres (cm³), so density is measured in grams per cubic centimetre (g/cm³). Pure water has a density of 1 g/cm³.
The table gives the mass and volume of four samples, with their densities.
| Sample | Mass (g) | Volume (cm³) | Density (g/cm³) |
|---|---|---|---|
| wooden block | 40 | 50 | 0.8 |
| stone | 90 | 30 | 3.0 |
| metal cube | 80 | 10 | 8.0 |
| plastic cube | 25 | 50 | 0.5 |
Key ideas
- density = mass / volume
- Density is measured in g/cm³.
- A heavy object of small volume can have a high density.
- Pure water has a density of 1 g/cm³.
Method. Use density = mass / volume.
- Write the values. The mass is 40 g and the volume is 50 cm³.40 and 50
- Divide. Density = 40 divided by 50.40 / 50 = 0.8
- Write the answer. 0.8 g/cm³
Check. Check: 0.8 x 50 = 40. ✓
Method. Use density = mass / volume.
- Write the values. The mass is 90 g and the volume is 30 cm³.90 and 30
- Divide. Density = 90 divided by 30.90 / 30 = 3
- Write the answer. 3 g/cm³
Check. Check: 3 x 30 = 90. ✓
Method. Rearrange: mass = density x volume.
- Rearrange. Mass = density x volume.mass = density x volume
- Multiply. Mass = 2 x 15.2 x 15 = 30
- Write the answer. 30 g
Check. Check: 30 divided by 15 is 2. ✓
Watch out
Watch out: density is mass divided by volume, not volume divided by mass. Check the units: g and cm³ give g/cm³.
Exam tip
Exam tip. Write the formula first, then put in the numbers, then give the unit g/cm³.
Try it (with your teacher, or a grown-up at home). With a teacher, weigh a small wooden block and a small metal object on balance scales. Measure the length, width and height of the block with a ruler, and work out its volume. Then find the density of the block using density = mass / volume.
Exercise 5C Density: mass, volume and density
Method. Read the table.
- Read the table. The metal cube has a density of 8.0 g/cm³.8.0
- Write the answer. 8 g/cm³
Check. Mass 80 g divided by volume 10 cm³ gives 8. ✓
Method. Recall the fact.
- Recall. Pure water has a density of 1 g/cm³.1
- Write the answer. 1 g/cm³
Check. The units are grams per cubic centimetre. ✓
Method. Read the table.
- Read the table. The plastic cube has a mass of 25 g and a volume of 50 cm³.25 and 50
- Divide. Density = 25 divided by 50.25 / 50 = 0.5
- Write the answer. 0.5 g/cm³
Check. 0.5 x 50 = 25. ✓
Method. Multiply density by volume.
- Multiply. Mass = 3 x 10.3 x 10 = 30
- Write the answer. 30 g
Check. 30 divided by 10 is 3. ✓
Method. Divide the mass by the density.
- Divide. Volume = mass / density = 60 / 3.60 / 3 = 20
- Write the answer. 20 cm³
Check. 3 x 20 = 60. ✓
Method. Divide the mass by the volume.
- Divide. Density = 40 / 50.40 / 50 = 0.8
- Write the answer. 0.8 g/cm³
Check. 0.8 x 50 = 40. ✓
Method. Divide the mass by the volume.
- Divide. Density = 16 / 8.16 / 8 = 2
- Write the answer. 2 g/cm³
Check. 2 x 8 = 16. ✓
Method. Compare the densities.
- Compare. The stone has a density of 3 g/cm³, and the metal cube has 8 g/cm³.3 and 8
- Decide. So the metal cube has the greater density.metal cube
- Write the answer. metal cube
Check. The metal cube is much denser. ✓
Method. Compare the densities.
- Compare. The wooden block has a density of 0.8 g/cm³, and the stone has 3 g/cm³.0.8 and 3
- Decide. So the stone is denser, and the pupil is not right.no
- Write the answer. no
Check. Bigger does not mean denser. ✓
Method. Divide the mass by the density.
- Divide. Volume = 30 / 0.5.30 / 0.5 = 60
- Write the answer. 60 cm³
Check. 0.5 x 60 = 30. ✓
Method. Use density = mass / volume.
- Work out. The block with 10 cm³ has density 20 / 10 = 2 g/cm³.20 / 10 = 2
- Compare. The block with 20 cm³ has density 20 / 20 = 1 g/cm³, so the smaller volume is denser.20 / 20 = 1
- Write the answer. 10 cm³
Check. With the same mass, a smaller volume gives a higher density. ✓
Method. Think about both quantities.
- Think about mass. A large block with a much larger mass can be denser.larger mass
- Think about density. Density depends on both mass and volume.both
- Write the answer. Accept any sentence that says density depends on mass and volume together, so a bigger block can be denser if its mass is much larger.
Check. Density is a ratio of two things. ✓
Method. Divide the mass by each density.
- First cube. Volume = 45 / 3 = 15 cm³.45 / 3 = 15
- Second cube. Volume = 45 / 1 = 45 cm³, which is bigger.45 / 1 = 45
- Decide. The denser cube is the first one, with the smaller volume of 15 cm³.first cube
- Write the answer. 15 cm³
Check. The denser cube is smaller for the same mass. ✓
Measuring volume and density
A regular solid has a simple shape, so its volume can be found from its length, width and height: volume = length x width x height, in cm³.
An irregular solid has no simple shape. Put it into water in a measuring cylinder. The water level rises by the same volume as the solid, because the solid pushes the water out of its way. This is called displacement. One millilitre (mL) is the same as one cubic centimetre (cm³).
Key ideas
- Regular solid: volume = length x width x height.
- Irregular solid: volume = the rise in water level, in a measuring cylinder.
- 1 mL = 1 cm³.
- Then density = mass / volume.
Method. Multiply the three lengths.
- Write the values. Length 5 cm, width 4 cm, height 2 cm.5, 4, 2
- Multiply. Volume = 5 x 4 x 2.5 x 4 x 2 = 40
- Write the answer. 40 cm³
Check. Forty 1 cm cubes fill the block. ✓
Method. Find the rise in the water level.
- Read the levels. The level was 40 cm³ before, and 55 cm³ after.40 and 55
- Subtract. The rise, which is the volume of the stone, is 55 minus 40.55 - 40 = 15
- Write the answer. 15 cm³
Check. 40 + 15 = 55. ✓
Method. Use density = mass / volume, with the volume from the first example.
- Write the values. Mass 32 g, volume 40 cm³.32 and 40
- Divide. Density = 32 / 40.32 / 40 = 0.8
- Write the answer. 0.8 g/cm³
Check. 0.8 x 40 = 32. ✓
Watch out
Watch out: read the bottom of the water surface at eye level. Use the scale on the measuring cylinder, not the top of the cylinder.
Exam tip
Exam tip. For an irregular solid, always subtract the starting level from the final level.
Try it (with your teacher, or a grown-up at home). With a teacher, put a stone into a measuring cylinder half full of water. Read the level before and after. Subtract to find the volume of the stone, in cm³. Then weigh the stone and calculate its density.
Exercise 5D Measuring volume and density
Method. Recall the formula.
- Recall. Volume of a regular block = length x width x height.length x width x height
- Write the answer. length x width x height
Check. Use the same units for all three. ✓
Method. Multiply the three lengths.
- Multiply. Volume = 3 x 2 x 4.3 x 2 x 4 = 24
- Write the answer. 24 cm³
Check. 24 is 3 x 8. ✓
Method. Multiply the three lengths.
- Multiply. Volume = 5 x 4 x 2.5 x 4 x 2 = 40
- Write the answer. 40 cm³
Check. This is the block in the example. ✓
Method. Recall the fact.
- Recall. One millilitre is the same as one cubic centimetre.1 mL = 1 cm³
- Write the answer. 1
Check. So 15 mL is 15 cm³. ✓
Method. Subtract the starting level from the final level.
- Subtract. The volume is 55 minus 40.55 - 40 = 15
- Write the answer. 15 cm³
Check. 40 + 15 = 55. ✓
Method. Divide the mass by the volume.
- Divide. Density = 45 / 15.45 / 15 = 3
- Write the answer. 3 g/cm³
Check. 3 x 15 = 45. ✓
Method. Recall the tool.
- Recall. A measuring cylinder is used to measure the volume of water displaced.measuring cylinder
- Write the answer. measuring cylinder
Check. The rise in water level gives the volume. ✓
Method. Recall the formula.
- Recall. Density = mass / volume, so the mass is needed too.mass
- Write the answer. mass
Check. Both mass and volume are needed. ✓
Method. Divide the mass by the volume.
- Divide. Density = 32 / 40 = 0.8.32 / 40 = 0.8
- Decide. So the answer is yes.yes
- Write the answer. yes
Check. 0.8 x 40 = 32. ✓
Method. Subtract the starting level from the final level.
- Subtract. The volume is 72 minus 60.72 - 60 = 12
- Write the answer. 12 cm³
Check. 60 + 12 = 72. ✓
Method. Divide the mass by the volume.
- Divide. Density = 36 / 12.36 / 12 = 3
- Write the answer. 3 g/cm³
Check. 3 x 12 = 36. ✓
Method. Think about the reading.
- Think about the reading. A wrong reading gives the wrong water level.wrong level
- Think about the result. So the volume, and then the density, would be wrong.wrong
- Write the answer. Accept any sentence that says reading at the wrong place gives a wrong level, so the calculated volume and density would be wrong.
Check. Read at eye level, at the bottom of the curve. ✓
Method. Calculate the density, then compare it with water.
- Calculate. Density = 45 / 15 = 3 g/cm³.45 / 15 = 3
- Compare. Water is 1 g/cm³, so 3 is more than water.more
- Write the answer. 3 g/cm³; more
Check. A density above 1 g/cm³ sinks in water. ✓
Density, floating and sinking
An object floats in a liquid if its density is less than the density of the liquid. It sinks if its density is more than the density of the liquid.
Pure water has a density of 1 g/cm³. So an object with a density below 1 g/cm³ floats in water, and one with a density above 1 g/cm³ sinks.
The four samples from section 5.3 are used here.
| Sample | Mass (g) | Volume (cm³) | Density (g/cm³) |
|---|---|---|---|
| wooden block | 40 | 50 | 0.8 |
| stone | 90 | 30 | 3.0 |
| metal cube | 80 | 10 | 8.0 |
| plastic cube | 25 | 50 | 0.5 |
Key ideas
- An object floats if it is less dense than the liquid.
- An object sinks if it is denser than the liquid.
- Pure water has a density of 1 g/cm³.
- The shape of the object does not decide whether it floats; the density does.
Method. Compare the density with water.
- Read the density. The plastic cube has a density of 0.5 g/cm³.0.5
- Compare. 0.5 is less than 1, the density of water, so the cube floats.floats
- Write the answer. float
Check. It is less dense than water. ✓
Method. Compare the density with water.
- Read the density. The stone has a density of 3 g/cm³.3
- Compare. 3 is more than 1, so the stone is denser than water.more than 1
- Write the answer. sink
Check. A denser object sinks. ✓
Method. Compare the four densities.
- Read the densities. The densities are 0.8, 3, 8 and 0.5 g/cm³.0.8, 3, 8, 0.5
- Decide. The smallest is 0.5, for the plastic cube.plastic cube
- Write the answer. the plastic cube
Check. The plastic cube is the least dense sample. ✓
Watch out
Watch out: a heavy object does not always sink. The plastic cube is light for its size, so it floats. Compare density, not mass.
Exam tip
Exam tip. To predict floating or sinking, compare the density of the object with the density of the liquid. Say the numbers.
Try it (with your teacher, or a grown-up at home). With a grown-up, drop a small wooden or plastic object and a small metal object into a bowl of water. Predict which will float and which will sink, using density. Then test your predictions, and write down whether they were correct.
Exercise 5E Density, floating and sinking
Method. Compare with water.
- Compare. 0.8 is less than the density of water, which is 1 g/cm³.less than 1
- Decide. So it floats.float
- Write the answer. float
Check. The wooden block floats. ✓
Method. Compare with water.
- Compare. 3 is more than 1, so it is denser than water.more than 1
- Decide. So it sinks.sink
- Write the answer. sink
Check. The stone sinks. ✓
Method. Recall the fact.
- Recall. Pure water has a density of 1 g/cm³.1
- Write the answer. 1 g/cm³
Check. It is the reference for floating. ✓
Method. Compare with water.
- Compare. The metal cube has a density of 8 g/cm³, which is more than 1.8 is more than 1
- Decide. So it sinks.sink
- Write the answer. sink
Check. Metal is usually denser than water. ✓
Method. Compare with water.
- Compare. The plastic cube has a density of 0.5 g/cm³, which is less than 1.0.5 is less than 1
- Decide. So it floats.float
- Write the answer. float
Check. It is less dense than water. ✓
Method. Compare each density with 1 g/cm³.
- Compare. The wooden block (0.8) and the plastic cube (0.5) are both less than 1.0.8 and 0.5
- Decide. So these two float.wooden block and plastic cube
- Write the answer. wooden block and plastic cube
Check. Stone and metal cube sink. ✓
Method. Compare each density with 1 g/cm³.
- Compare. The stone (3) and the metal cube (8) are both more than 1.3 and 8
- Decide. So these two sink.stone and metal cube
- Write the answer. stone and metal cube
Check. They are denser than water. ✓
Method. Use the density.
- Think. It is the density, not the mass, that decides floating.density
- Decide. A large object of low density floats, so the pupil is not right.no
- Write the answer. no
Check. Compare density, not mass. ✓
Method. Compare with water.
- Compare. 1.2 is more than 1, the density of water.1.2 is more than 1
- Decide. So it sinks.sink
- Write the answer. sink
Check. Even slightly more dense than water, it sinks. ✓
Method. Compare with water.
- Compare. 0.9 is less than 1, the density of water.0.9 is less than 1
- Decide. So it floats.float
- Write the answer. float
Check. Even slightly less dense than water, it floats. ✓
Method. Compare the densities.
- Compare. The plastic cube has density 0.5, which is more than 0.3.0.5 is more than 0.3
- Decide. So the cube is denser than this liquid, and it would sink. Ali is right.sink
- Write the answer. yes
Check. Check each density before deciding. ✓
Method. Compare the densities.
- Compare. The stone has density 3, which is less than the liquid, 4.3 is less than 4
- Decide. So the stone floats in this liquid, and the prediction is correct.yes
- Write the answer. yes
Check. The liquid must be denser than the object. ✓
Method. Compare the densities.
- Compare with water. The cube (0.5) is less dense than water (1), so it floats.0.5 is less than 1
- Compare with the liquid. The cube is denser than the 0.4 liquid, so it sinks in that liquid.0.5 is more than 0.4
- Write the answer. Accept any sentence that says the cube is less dense than water, but denser than a liquid of 0.4 g/cm³, so it sinks in that liquid.
Check. A floating object depends on the liquid it is in. ✓
Chapter summary
| Skill | Method |
|---|---|
| Diffusion | Random movement of particles from higher to lower concentration, in gases and liquids. |
| Rate of diffusion | Faster at a higher temperature. Much faster in a gas than in a liquid. |
| Density | density = mass / volume, in g/cm³. Pure water is 1 g/cm³. |
| Volume | Regular solid: length x width x height. Irregular solid: displacement in a measuring cylinder (1 mL = 1 cm³). |
| Floating | Floats if less dense than the liquid; sinks if denser. |
Can you…?
- I can describe diffusion, using random movement and concentration
- I can explain how temperature and state change the rate of diffusion
- I can use density = mass / volume, with the units g/cm³
- I can find the volume of a regular and an irregular solid
- I can predict whether an object floats or sinks, using density
Key terms
- diffusion
- the spreading of particles from higher to lower concentration, due to random movement
- concentration
- the number of particles in a given space
- density
- the mass of a substance in a given volume; density = mass / volume
- displacement
- the rise in water level when a solid is put into the water
- float
- stay on the surface, because the object is less dense than the liquid
- sink
- move to the bottom, because the object is denser than the liquid
- g/cm³
- the unit of density: grams per cubic centimetre
Exercise 5Z Check-up
Method. Recall the process.
- Recall. Spreading from higher to lower concentration is diffusion.diffusion
- Write the answer. diffusion
Check. Diffusion is random movement. ✓
Method. Recall the state.
- Recall. Diffusion does not happen in a solid, whose particles are in fixed positions.solid
- Write the answer. solid
Check. Diffusion happens in gases and liquids. ✓
Method. Find the shortest time.
- Read the table. The shortest time, 5 minutes, is for warm water.5 minutes
- Write the answer. warm water
Check. Warm particles move faster. ✓
Method. Recall the formula.
- Recall. Density = mass / volume.mass / volume
- Write the answer. mass divided by volume
Check. The units are g/cm³. ✓
Method. Divide the mass by the volume.
- Divide. 40 / 50 = 0.8.40 / 50 = 0.8
- Write the answer. 0.8 g/cm³
Check. 0.8 x 50 = 40. ✓
Method. Multiply density by volume.
- Multiply. Mass = 2 x 15 = 30.2 x 15 = 30
- Write the answer. 30 g
Check. 30 divided by 15 is 2. ✓
Method. Multiply the three lengths.
- Multiply. 3 x 2 x 4 = 24.3 x 2 x 4 = 24
- Write the answer. 24 cm³
Check. 24 is 3 x 8. ✓
Method. Subtract the starting level from the final level.
- Subtract. 55 minus 40 is 15.55 - 40 = 15
- Write the answer. 15 cm³
Check. 40 + 15 = 55. ✓
Method. Compare with water.
- Compare. 0.5 is less than 1, the density of water.0.5 is less than 1
- Decide. So it floats.float
- Write the answer. float
Check. Less dense than water means it floats. ✓
Method. Think about the energy.
- Think about the energy. Warm particles have more energy.more energy
- Think about the movement. So they move faster and spread more quickly.faster
- Write the answer. Accept any sentence that says warm particles have more energy, so they move faster and spread out more quickly.
Check. Temperature changes particle speed. ✓
Total 16 marks. Allow about 16 minutes.
Exercise 5X Practice test questions
Method. Recall the process.
- Recall. Diffusion is the random spreading from higher to lower concentration.diffusion
- Write the answer. diffusion
Check. Diffusion happens in gases and liquids. ✓
Method. Think about the particles.
- Think about the particles. Gas particles are further apart and move more quickly.further apart
- Think about the result. So they spread out faster than liquid particles.spread out
- Write the answer. Accept any sentence that says gas particles are further apart and move more quickly, so they spread faster.
Check. Particle spacing and speed both matter. ✓
Method. Divide the mass by the volume.
- Divide. Density = 60 / 20 = 3.60 / 20 = 3
- Write the answer. 3 g/cm³
Check. 3 x 20 = 60. ✓
Method. Subtract the starting level from the final level.
- Subtract. 40 minus 25 is 15.40 - 25 = 15
- Write the answer. 15 cm³
Check. 25 + 15 = 40. ✓
Method. Compare with water.
- Compare. 2 is more than 1, the density of water.2 is more than 1
- Write the answer. sink
Check. A denser object sinks. ✓
Method. Compare the densities.
- Compare. 3 is more than 0.8.3 is more than 0.8
- Write the answer. stone
Check. The stone has the higher density. ✓
Method. Recall the fact.
- Recall. Pure water has a density of 1 g/cm³.1
- Write the answer. 1
Check. It is the reference for floating. ✓
Method. Compare the densities.
- Compare. The plastic cube has a density of 0.5 g/cm³.0.5
- Decide. Less than 1, the density of water, so it floats.less than 1
- Write the answer. Accept any sentence that says its density is less than the density of water, so it floats.
Check. Density decides floating, not size. ✓
Method. Divide the mass by the volume.
- Divide. Density = 16 / 8 = 2.16 / 8 = 2
- Write the answer. 2 g/cm³
Check. 2 x 8 = 16. ✓
Method. Think about the fair test.
- Think about the fair test. Only the temperature should change.only the temperature
- Think about the result. So the difference in times is caused by the temperature.caused by
- Write the answer. Accept any sentence that says these must stay the same, so that only the temperature changes the rate of diffusion.
Check. Keep other variables fixed. ✓