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KS3 Science · Year 8Chapter 1 · Working scientifically (Year 8)
Year 8 · Chapter 1

Working scientifically (Year 8)

By the end of this chapter you will be able to

  • Name the independent, dependent and control variables in a fair test
  • Assess the risks of an activity and say how to control them
  • Calculate the mean of repeat readings, and spot an anomaly
  • Choose and draw a suitable table or graph, and describe a pattern
  • Evaluate evidence, uncertainty and the limits of a model

You should already know

  • Working scientifically (Year 7): fair tests, tables and graphs (Chapter 1 of Year 7 Science)
  • Speed, distance and time (Year 7 Science)

Where this leads

  • Biology and chemistry practicals in the later chapters of this book, where every method uses these skills

National Curriculum for England, Key Stage 3 science, Year 8. Content: 8WS1 ask questions, plan enquiries, control variables, use rates and graphs to test ideas, 8WS2 assess risk and use apparatus, chemicals and living things safely, 8WS3 take repeat measurements, judge precision and calculate means, 8WS4 present data in tables, bar charts, line graphs and scatter graphs, including rates, 8WS5 evaluate methods, anomalies, uncertainty and limits of evidence, 8WS6 use scientific models, including particle, atomic and wave models, to explain and predict.

Exercise P  Do you remember?

1
FWhat is 36 divided by 4? Write the number.
2
FHow many centimetres are there in 2.5 metres? Write the number.
3
FWhat is the unit of mass in the SI system? Write the word.
4
FWhat is the word for a graph in which the points are joined by a line, showing change over time? Write the words.
5
FWhat is 7 + 8 + 9? Write the number.
1.1

Variables and fair tests

Year 8
8WS1: ask questions, plan enquiries, and control variables so that a fair test is possible
Video lesson: Variables and fair testsComing soon
Story. Ruby wants to find out whether the height of a ramp changes how far a toy car rolls. “I will change the height,” she says, “and measure the distance. But I must keep everything else the same.” Her teacher says that is exactly what a fair test means.

In an experiment, the independent variable is the one that you change on purpose. The dependent variable is the one you measure, to see whether it depends on the change. A control variable is any other variable that must be kept the same, so that it does not affect the result.

A test is fair when only the independent variable is changed. A good prediction is testable: it can be checked by measuring something.

heightdistance rolled along the benchstart point
A toy car rolls down a ramp. The height is changed; the distance rolled is measured.

Key ideas

  • The independent variable is the one you change on purpose.
  • The dependent variable is the one you measure.
  • Control variables are kept the same to make the test fair.
  • A testable prediction can be checked by measuring something.
Worked example 1.1.1
In Ruby’s test, which variable is the independent variable? Write the words.

Method. Find the variable that is changed on purpose.

  1. Find the change. Ruby changes the height of the ramp on purpose.
    changes on purpose
  2. Name it. The height of the ramp is the independent variable.
    height of the ramp
  3. Write the answer.
    the height of the ramp

Check. The height is set before each run. ✓

Worked example 1.1.2
In Ruby’s test, which variable is the dependent variable? Write the words.

Method. Find the variable that is measured.

  1. Find the measurement. Ruby measures how far the car rolls.
    measures
  2. Name it. The distance the car rolls is the dependent variable.
    distance the car rolls
  3. Write the answer.
    the distance the car rolls

Check. The distance depends on the height. ✓

Worked example 1.1.3
Name one control variable in Ruby’s test. Accept any one: the car, the surface of the ramp, or the starting point.

Method. Find a variable that must not change.

  1. Think. A control variable must stay the same for every run.
    stay the same
  2. Name one. The car, the surface and the starting point must not change.
    the car
  3. Write the answer.
    Accept any one: the car, the surface of the ramp, or the starting point.

Check. If the surface changed, the test would not be fair. ✓

Watch out

Watch out: if you change two things at once, you cannot say which one caused the change. Change only the independent variable.

Exam tip

Exam tip. Write the variable as a name, for example “the height of the ramp”, not just “height”.

Try it (with your teacher, or a grown-up at home). Roll a small toy car, or a book, down a ramp made from a ruler on a book, with a grown-up. Change only the height, keep the car and the surface the same, and measure how far it rolls. Say which variable is independent, dependent and controlled.

Exercise 1A  Variables and fair tests

1
FIn a fair test, which variable is changed on purpose? Write the words.
2
FWhich variable is measured to see the effect of the change? Write the words.
3
FWhat is the name of a variable that is kept the same? Write the words.
4
FIn the ramp test, what is the independent variable? Write the words.
5
FIn the ramp test, what is the dependent variable? Write the words.
6
FWhich of these is a testable prediction? Write a or b. (a) Ramps are fun. (b) The steeper the ramp, the further the car rolls.
7
FOmar changes the mass of the car and the height of the ramp at the same time. Is this a fair test? Write yes or no.
8
FA pupil tests whether salt speeds up the melting of ice. Name one control variable. Accept any one: the amount of ice, or the temperature of the room.
9
FAli says the car must be pushed the same way each time. Is this a control variable? Write yes or no.
10
PSPlant growth rate: a plant grows 12 cm in 4 weeks. What is the rate of growth, in cm per week? Write the number.
11
PSA gas syringe collects 30 cm3 of gas in 60 seconds. What is the rate, in cm3 per second? Write the number.
12
PSRuby says the height does not matter, because the car always rolls the same distance. Use the ramp table to give one reason to disagree. Accept any reason that quotes two different means, for example 42 cm at 10 cm and 153 cm at 40 cm.
13
RExplain why a fair test keeps the control variables the same. Write one sentence.
14
RChallenge: a pupil changes the height and also the car’s starting push, and gets a distance of 90 cm. Explain why the result cannot show that height alone causes the distance. Write one sentence.
1.2

Risk and safe apparatus

Year 8
8WS2: assess risk and use apparatus, chemicals and living things safely
Video lesson: Risk and safe apparatusComing soon
Story. Omar’s class is going to warm some water in a glass beaker for an investigation. “What could go wrong?” his teacher asks. Omar thinks about the hot water and the glass. “And how do we stop it?” she asks next.

A hazard is something that could cause harm. The risk is the chance that the hazard will cause harm, and how serious that harm could be. A control measure is an action that reduces the risk. A risk assessment lists the hazards, the risks and the control measures before any practical starts.

The table gives three examples of a risk assessment.

ActivityHazardRiskControl measure
Heating water in a beakerhot water and glassburns, or the beaker breaksuse a heat-resistant beaker, warm not boiling water, and a grown-up to supervise
Rolling a car down a rampa falling car or a heavy bookbruised feetstand clear of the ramp and keep the bench clear
Using a mirror in a light experimentbroken glasscut fingersuse a plastic mirror, or check the mirror for cracks first

Key ideas

  • A hazard is something that could cause harm.
  • A risk is the chance that a hazard causes harm.
  • A control measure reduces the risk; it may not remove the hazard.
  • Check apparatus and chemicals before you use them.
Worked example 1.2.1
In a beaker of hot water, what is the hazard? Write the words.

Method. Find the thing that could cause harm.

  1. Look. The hot water can cause harm if it touches the skin.
    can cause harm
  2. Name it. The hazard is hot water.
    hot water
  3. Write the answer.
    hot water

Check. Hot water and glass are both hazards. ✓

Worked example 1.2.2
What is the risk from the hot water in the beaker? Write the word.

Method. Ask what harm could happen.

  1. Think. Hot water can scald the skin if it spills.
    scald
  2. Name it. The risk is burns.
    burns
  3. Write the answer.
    burns

Check. Burns are the harm that could happen. ✓

Worked example 1.2.3
Write one control measure for warm water in a beaker. Accept any one: a heat-resistant beaker, warm not boiling water, or a grown-up to supervise.

Method. Think about how to reduce the risk.

  1. Think. A control measure must reduce the chance of harm.
    reduce the chance
  2. Name one. Using warm water, not boiling, reduces the risk of burns.
    warm, not boiling
  3. Write the answer.
    Accept any one: a heat-resistant beaker, warm not boiling water, or a grown-up to supervise.

Check. Each control measure reduces the risk. ✓

Watch out

Watch out: a control measure reduces a risk but does not always remove the hazard. The hot water is still hot.

Exam tip

Exam tip. Name the hazard first, then the risk, then a control measure that matches the risk.

Try it (with your teacher, or a grown-up at home). Look at the risk table above and talk through one activity from your class. Name its hazard, its risk and two control measures. Do not heat anything yourself; a grown-up or your teacher does any heating.

Exercise 1B  Risk and safe apparatus

1
FWhat is the word for something that could cause harm? Write the word.
2
FWhat is the word for the chance that a hazard causes harm? Write the word.
3
FWhat is the name for an action taken to reduce a risk? Write the words.
4
FIn a risk assessment, what is the hazard of a broken glass beaker? Write the words.
5
FIs a plastic beaker safer than a glass beaker for warm water? Write yes or no.
6
FIn the car ramp activity, what is the risk from a heavy book falling off the bench? Write the word.
7
FShould pupils boil water in a glass beaker in class without supervision? Write yes or no.
8
FWhich is a safer way to reduce the risk of the ramp car falling on feet: standing clear of the ramp, or standing in front of it? Write the words.
9
FA pupil wants to use a chemical that has no safety information. What should they do first? Write the word: check, ignore, or taste.
10
FWhat is the name of the written plan that lists hazards, risks and control measures? Write the words.
11
PSRuby says a hazard and a risk are the same thing. Is Ruby right? Write yes or no.
12
PSName one hazard when using a hot plate in a demonstration. Accept any one: a hot surface, or steam.
13
RExplain why a control measure reduces a risk but does not remove the hazard. Write one sentence.
1.3

Repeats, means and precision

Year 8
8WS3: take repeat measurements, judge precision, identify anomalies and calculate means
Video lesson: Repeats, means and precisionComing soon
Story. Ruby measures the distance her car rolls three times at each height. One reading at 30 cm is much bigger than the others. “Is that a mistake?” she asks. Her teacher says, “Work out the mean, and then decide what to do with that reading.”

Repeating a measurement makes a result more reliable. The mean is the total of the repeat readings divided by how many readings there are. An anomaly is a reading that does not fit the pattern of the others. It must be checked and explained, not just hidden.

Precision is how finely a measurement is made, for example to the nearest millimetre rather than the nearest centimetre.

The table shows three repeat distances at each ramp height, with the mean worked out.

Ramp height (cm)Three distances (cm)Mean (cm)
1042, 39, 4542
2080, 84, 8282
30118, 121, 115118
40150, 156, 153153

Key ideas

  • Mean = total of the readings divided by the number of readings.
  • An anomaly does not fit the pattern and must be explained.
  • A more precise measuring tool gives finer readings.
  • Repeats show whether a result can be trusted.
Worked example 1.3.1
Find the mean of the three distances at 20 cm: 80 cm, 84 cm and 82 cm. Write the number of cm.

Method. Add the readings, then divide by 3.

  1. Add. The total is 80 + 84 + 82 = 246 cm.
    246
  2. Divide. Divide the total by the 3 readings: 246 / 3.
    246 / 3 = 82
  3. Write the answer.
    82 cm

Check. Check: 82 x 3 = 246. ✓

Worked example 1.3.2
At 30 cm the readings are 118 cm, 121 cm and 175 cm. Which reading is an anomaly? Write the number of cm.

Method. Find the reading far from the others.

  1. Compare. The first two readings are close to each other, at 118 cm and 121 cm.
    close together
  2. Name it. The reading of 175 cm does not fit the pattern, so it is the anomaly.
    175
  3. Write the answer.
    175 cm

Check. Check the test for a mistake before removing it. ✓

Worked example 1.3.3
Without the anomaly, what is the mean of the two readings 118 cm and 121 cm? Write the number of cm.

Method. Find the mean of the two remaining readings.

  1. Add. The total is 118 + 121 = 239 cm.
    239
  2. Divide. Divide by 2: 239 / 2.
    239 / 2 = 119.5
  3. Write the answer.
    119.5 cm

Check. Check: 119.5 x 2 = 239. ✓

Watch out

Watch out: do not include an anomaly in a mean without saying so. State clearly any reading that was left out, and why.

Exam tip

Exam tip. Show the total and the number of readings, then the division. Give the units with the mean.

Try it (with your teacher, or a grown-up at home). Time how long it takes you to count slowly from 1 to 10 with a stopwatch, three times. Write down the three times, find the mean, and say what the spread of your results tells you about precision.

Exercise 1C  Repeats, means and precision

1
FWhat is the total of the three readings 42, 39 and 45? Write the number.
2
FWhat is the mean of 42, 39 and 45? Write the number.
3
FWhat is the mean of 80, 84 and 82? Write the number.
4
FWhat is the mean of 118, 121 and 115? Write the number.
5
FWhat is the mean of 150, 156 and 153? Write the number.
6
FWhat is the mean of 0.40, 0.42 and 0.38 seconds? Write the number of seconds.
7
FWhat is the word for a reading that does not fit the pattern of the others? Write the word.
8
FShould a mean be worked out with an anomaly included, without saying so? Write yes or no.
9
FWhich measuring tool is more precise: a ruler read to the nearest cm, or a ruler read to the nearest mm? Write mm or cm.
10
FA stopwatch reading is 0.4 seconds. How many decimal places does it give? Write the number.
11
PSOmar says three repeat readings are always enough. Is Omar right? Write yes or no.
12
PSWorking scientifically: a pupil gets readings of 20, 22 and 45 cm. Name the anomaly and say what the pupil should do. Accept: the 45 cm reading; check the method and say why it is excluded if it is.
13
RExplain why repeating a measurement makes the result more reliable. Write one sentence.
14
RChallenge: a stopwatch is read to the nearest 0.1 s and gives 2.3 s, 2.5 s and 2.4 s. Work out the mean. Write the number of seconds.
1.4

Presenting data

Year 8
8WS4: present data in tables, bar charts, line graphs and scatter graphs, including rates
Video lesson: Presenting dataComing soon
Story. Ruby plots her ramp results so that the class can see the pattern. “Should I use a bar chart or a scatter graph?” she asks. Her teacher says, “It depends on the kind of data. Ask whether the variables are categories or numbers.”

A bar chart compares categories or groups, such as the mean distance at each ramp height. A scatter graph shows two sets of numbers, one for each point, and is used to look for a pattern, or correlation. When y increases as x increases, the correlation is positive. When y decreases as x increases, it is negative.

On any graph, the independent variable goes on the x-axis and the dependent variable on the y-axis.

Ramp height against distance rolled (every repeat)010203040050100150200Ramp height (cm)Distance rolled (cm)
A scatter graph of every repeat, with the points rising from left to right.
Mean distance at each ramp height4210 cm8220 cm11830 cm15340 cmRamp height (cm)
A bar chart of the mean distance for each ramp height.

Key ideas

  • Bar charts compare categories or groups.
  • Scatter graphs show two numerical variables and any pattern between them.
  • Positive correlation: y rises as x rises. Negative correlation: y falls as x rises.
  • Independent variable on the x-axis; dependent variable on the y-axis.
Worked example 1.4.1
Which graph suits the mean distance at each ramp height, as four separate groups? Write the words.

Method. Decide whether the data are groups or numbers.

  1. Think. Each ramp height is a separate group, so the data are grouped.
    groups
  2. Name it. Groups are best shown in a bar chart.
    bar chart
  3. Write the answer.
    bar chart

Check. A bar chart compares groups side by side. ✓

Worked example 1.4.2
On the scatter graph the points rise from left to right. Is the correlation positive or negative? Write the word.

Method. Look at the direction of the points.

  1. Look. As the ramp height rises, the distance rolled also rises.
    rises
  2. Name it. When y increases as x increases, the correlation is positive.
    positive
  3. Write the answer.
    positive

Check. Negative would mean the points fall from left to right. ✓

Worked example 1.4.3
In the bar chart, what is the difference between the mean at 40 cm and the mean at 30 cm? Write the number of cm.

Method. Subtract the smaller mean from the larger.

  1. Read the means. The mean at 40 cm is 153 cm and at 30 cm is 118 cm.
    153 and 118
  2. Subtract. The difference is 153 minus 118.
    153 - 118 = 35
  3. Write the answer.
    35 cm

Check. 118 + 35 = 153. ✓

Watch out

Watch out: a line graph joins points that are measured over time. Do not join points that are not in a sequence, unless a line of best fit shows the trend.

Exam tip

Exam tip. Label both axes with the quantity and the unit, and give each graph a title.

Try it (with your teacher, or a grown-up at home). Use a sheet of squared paper and plot the ramp means as a bar chart, with the ramp height on the x-axis and the mean distance on the y-axis. Then describe the pattern in one sentence.

Exercise 1D  Presenting data

1
FWhich type of graph shows the mean distance for each ramp height as separate bars? Write the words.
2
FWhich graph is best for two sets of numerical data, with each point plotted? Write the words.
3
FWhat is the word for a pattern in which y increases as x increases? Write the words.
4
FWhat is the word for a pattern in which y decreases as x increases? Write the words.
5
FOn a graph, which axis is the independent variable on? Write the words: x-axis or y-axis.
6
FUsing the ramp means, what is the mean distance at 30 cm? Write the number of cm.
7
FUsing the ramp means, what is the difference between the mean at 40 cm and the mean at 30 cm? Write the number of cm.
8
FUsing the ramp means, which ramp height gave the greatest mean distance? Write the height in cm.
9
PSMagnets picked up 4, 9 and 15 paperclips. What is the difference between the strong magnet and the weak magnet? Write the number.
10
PSAli says the scatter graph of the ramp results shows a negative correlation. Is Ali right? Write yes or no.
11
PSWorking scientifically: the scatter graph uses 4 heights with 3 repeats each. How many points are plotted? Write the number.
12
RExplain whether a line of best fit is sensible for the ramp points. Write one sentence.
13
RChallenge: a pupil says the graph of distance against height shows that the car always rolls further when the ramp is steeper, but a single point at 30 cm is above the line. Explain what the pupil should check. Write one sentence.
1.5

Evaluating evidence and models

Year 8
8WS5: evaluate methods, anomalies, uncertainty and the limits of evidence, 8WS6: use particle, atomic and wave models to explain and predict
Video lesson: Evaluating evidence and modelsComing soon
Story. After the ramp test, the class writes an evaluation. “We had three repeats,” says Sam, “but the ruler only goes to the nearest millimetre.” The teacher says, “Good. Say what limits your evidence, and say how far a reading could be from the true value.”

Uncertainty is the range within which the true value is likely to lie. A reading made to the nearest millimetre has an uncertainty of about half a millimetre either side. Accuracy is how close a result is to the true value. Precision is how close repeated results are to each other.

A model is a simple picture that explains and predicts. The particle model explains gases, liquids and solids. Models are changed when new evidence shows they are incomplete. A wave model explains how vibrations carry energy, for example in sound.

Key ideas

  • Uncertainty is the range in which the true value is likely to be.
  • Accuracy is closeness to the true value; precision is closeness of repeats.
  • A good evaluation names limits of the evidence and suggests improvements.
  • A model explains and predicts, and is changed when new evidence arrives.
Worked example 1.5.1
A ruler reads to the nearest millimetre, and gives 12 mm. The uncertainty is 0.5 mm either side. What is the smallest the true value could be? Write the number of mm.

Method. Take the uncertainty away from the reading.

  1. Subtract. The reading is 12 mm and the uncertainty is 0.5 mm.
    12 - 0.5
  2. Calculate. The smallest possible value is 11.5 mm.
    12 - 0.5 = 11.5
  3. Write the answer.
    11.5 mm

Check. 11.5 + 0.5 = 12. ✓

Worked example 1.5.2
Which model explains why a gas spreads out to fill its container? Write the name of the model.

Method. Match the explanation to a model.

  1. Think. A gas is made of moving particles that spread out and fill the space.
    moving particles
  2. Name it. This is explained by the particle model.
    particle model
  3. Write the answer.
    particle model

Check. The particle model explains states of matter. ✓

Worked example 1.5.3
Give one limit of the ramp evidence: say what the test did not control. Accept any one: the surface, the starting push, or only three repeats.

Method. Think about what could change the result.

  1. Think. Anything not controlled may change the distance.
    not controlled
  2. Name one. The starting push or the surface could change the result.
    starting push
  3. Write the answer.
    Accept any one: the surface, the starting push, or only three repeats.

Check. A limit makes the evidence less certain. ✓

Watch out

Watch out: an evaluation is not just “it was good” or “it was bad”. Name a specific limit, and say how to improve it.

Exam tip

Exam tip. In an evaluation, name one limit, say what effect it has on the result, and give one improvement.

Try it (with your teacher, or a grown-up at home). Choose one experiment from this chapter and write three lines: one limit of the evidence, one anomaly you would check, and one improvement. Do not carry out any new heating or chemical test.

Exercise 1E  Evaluating evidence and models

1
FWhat is the word for the range in which the true value is likely to lie? Write the word.
2
FA ruler is read to the nearest millimetre. What is the uncertainty of one reading, in mm? Write the number.
3
FA reading is 12 mm with an uncertainty of 0.5 mm. What is the smallest possible value, in mm? Write the number.
4
FWhich word means how close a result is to the true value: accuracy or precision? Write the word.
5
FWhich word means how close repeated readings are to each other: accuracy or precision? Write the word.
6
FA pupil says the particle model is the true picture of matter. Is this a good scientific statement? Write yes or no.
7
FModels are changed when new evidence shows they are incomplete. Is this true? Write yes or no.
8
FWhich model explains that a sound is a vibration that is passed from particle to particle? Write the name of the model.
9
PSOmar removes an anomaly from his results without a reason. Name the risk this creates for the evidence. Write one word: bias.
10
PSWorking scientifically: name one limit of the ramp evidence and say how it could be improved. Accept any limit with an improvement, for example the surface was not checked; use the same surface.
11
RExplain how a model can be useful even when it is not perfect. Write one sentence.
12
RChallenge: a pupil says a result is accurate because the three repeats are close together. Explain why this is not enough to show accuracy. Write one sentence.

Chapter summary

SkillMethod
Fair testChange only the independent variable; measure the dependent; keep control variables the same.
Risk assessmentHazard (what could harm), risk (chance and seriousness), control measure (reduces it).
MeanTotal of repeat readings divided by the number of readings.
AnomalyA reading that does not fit the pattern: check it and state any removal.
GraphsBar chart for groups; scatter graph for two numbers; correlation is positive, negative or none.
Uncertainty and modelsUncertainty is a range; models explain and predict, and change with evidence.

Can you…?

  • I can name the independent, dependent and control variables in a fair test
  • I can name the hazard, risk and a control measure for an activity
  • I can work out the mean of repeat readings, and spot an anomaly
  • I can choose a suitable table or graph and describe the pattern
  • I can evaluate evidence, uncertainty and the limits of a model

Key terms

independent variable
the variable that is changed on purpose
dependent variable
the variable that is measured to see the effect
control variable
a variable that is kept the same to make a test fair
hazard
something that could cause harm
risk
the chance that a hazard causes harm, and how serious it could be
mean
the total of the readings divided by the number of readings
anomaly
a reading that does not fit the pattern of the others
correlation
a pattern showing how two variables change together: positive, negative or none
uncertainty
the range within which the true value is likely to lie

Exercise 1Z  Check-up

1
What is the independent variable in a fair test? Write the words.
2
What is the name of an action that reduces a risk? Write the words.
3
Find the mean of 60, 64 and 62. Write the number.
4
What is a reading that does not fit the pattern of the others called? Write the word.
5
A balance reads to the nearest 0.1 g. How many decimal places does each reading have? Write the number.
6
Which graph is best for two sets of numerical data? Write the words.
7
A pattern where y falls as x rises is called what correlation? Write the word.
8
A plant grows 20 cm in 5 weeks. What is its rate of growth, in cm per week? Write the number.
9
Name one control variable when testing how far a car rolls down a ramp. Accept any one: the car, the surface, or the starting point.
10
Explain why a scientist should say what limits the evidence in an evaluation. Write one sentence.

Total 15 marks. Allow about 15 minutes.

Exercise 1X  Practice test questions

1
FName the independent variable in an experiment to test how the mass of a car affects its speed. Write the words.[1 mark]
2
FWhat is the mean of 10, 12 and 14? Write the number.[1 mark]
3
PSWrite one hazard and one control measure for using a hot plate in a demonstration. Accept any sensible pair, for example hot surface, and stay a safe distance.[2 marks]
4
PSA reading of 25 mm is given to the nearest millimetre. What is the largest possible value, in mm? Write the number.[2 marks]
5
PSA pupil measures a reaction time three times: 0.40 s, 0.44 s and 0.42 s. Find the mean. Write the number of seconds.[2 marks]
6
FWhich graph would you use to show the number of pupils who chose each of four sports? Write the words.[1 mark]
7
FGive the correlation shown when the points on a scatter graph fall from left to right. Write the word.[1 mark]
8
RExplain why a pupil should not remove a result from a set of repeats without giving a reason. Write one sentence.[2 marks]
9
FWhich model explains that a gas exerts a pressure on the walls of its container? Write the name of the model.[1 mark]
10
RWrite one way to make a set of distance measurements more precise. Accept any one: use a more precise tool, or take more repeats.[2 marks]
Chapter 2: Organs, digestion and enzymes →

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