Thinking and Working Scientifically: Investigations, Variables, Tables and Graphs

British & IGCSE › Checkpoint 6 › Checkpoint 6 Science — Complete Course

Almost every Checkpoint Science question has a part about how science is done: naming a variable, completing a table, reading a graph, finding an odd result or suggesting an improvement. These marks are easy to win once you know the rules on this page.

1. Scientific questions and the five types of enquiry

A scientific question is one you can answer by collecting evidence. "Which is the most beautiful rock?" is not scientific (it is an opinion). "Which rock soaks up the most water?" is scientific — you can test it. Once you have a question, choose the type of enquiry that can answer it.

How to choose: Can you change one thing and keep everything else the same? → fair test. Is it about change over time? → observing over time. Is it about many living things or places that you cannot control? → pattern seeking. Are you sorting or naming? → identifying and classifying. Can you not test it yourself? → research.

2. Models and diagrams

A model represents something real so that we can understand it. Models are useful when the real thing is too small (particles), too big or too far (the Solar System), hidden inside the body (heart and lungs), or too slow (the rock cycle). A diagram is a kind of model.

In the test you may be asked to use a model ("draw the particles", "draw the force arrows", "complete the circuit diagram") or to say how a model helps ("it shows … that we cannot see") and one way it is not like the real thing.

3. Predictions and hypotheses

A prediction says what you think will happen and why, using science you know. A good prediction has two parts: I predict that … because …. A hypothesis is the scientific idea behind it — an explanation that can be tested.

• Weak: "I think the sugar will dissolve." (no comparison, no reason)
• Good: "I predict that sugar will dissolve faster in hotter water, because the water particles move faster and break up the sugar more quickly."
• In a new (unfamiliar) context, still use science: "I predict the plant in sandy soil will wilt first, because sand does not hold much water."

4. Variables and fair tests

In a fair test you change only one variable, so you know that it caused any difference in your results.

1. Write the independent variable as "the ___ of the ___" (the temperature of the water) — never just "the water".
2. The dependent variable is usually something you read from an instrument: a time, a distance, a temperature, a mass.
3. For control variables, name a quantity, and if possible give its value: "the same volume of water, 100 ml".
4. If a question says "Why is it a fair test?", answer: "Only one variable (the ___) was changed; everything else was kept the same."

5. Risks and safety

A hazard is something that could cause harm (hot water, glass). A risk is the chance that harm happens. Before practical work, describe each risk and say how to minimise it.

Exam tip: name the hazard AND the action. "Be careful" earns nothing. "Hot water could burn you, so let an adult pour it" earns marks.

6. Choosing equipment and measuring accurately

Choose equipment with a suitable scale: use a 100 ml measuring cylinder (not a bucket) for 50 ml of water; use a stopwatch, not a wall clock, for a 10-second time. Then read it carefully:

1. Work out what one small division is worth (count the gaps between two numbers).
2. Read with your eye level with the reading (for a liquid, the bottom of the curved surface, called the meniscus).
3. Start a ruler at 0, not at the edge of the ruler. Set a balance to zero before adding the object.
4. Write the value and the unit.

7. Units you must know

The prefixes tell you the size: milli (m) = one thousandth, centi (c) = one hundredth, kilo (k) = one thousand.

To change a big unit into a small one, multiply (2.5 kg = 2.5 × 1000 = 2500 g). To change a small unit into a big one, divide (350 cm = 350 ÷ 100 = 3.5 m).

8. Repeating measurements

Measurements are rarely exactly the same each time (you press the stopwatch a bit late, the car leaves the ramp slightly differently). Repeating a measurement (usually three times) and working out the mean makes the data more reliable. Repeat when results vary a lot, when a result looks odd, or when the measurement is quick and easy to get slightly wrong (like timing).

Mean = add the values ÷ how many values. Leave out any anomalous result before you work out the mean.

9. Sorting, classifying and using keys

We can sort and classify objects, materials and living things by testing (Does a magnet attract it? Does it let electricity through?), by observing (Does it have legs? Is it shiny?) and by using secondary information (a book says whales breathe with lungs, so they are mammals, not fish). A key is a set of yes/no questions about features that are easy to see. Each question splits the group in two until each thing has its own name.

1. Use features you can see (legs, shell, colour, wings), not ones you cannot ("Is it fast?", "Is it dangerous?").
2. Each question must have only two answers (yes / no).
3. When you complete a key, check that every living thing ends in its own box.

10. Secondary sources

Secondary sources are information that someone else collected: books, reliable websites, encyclopaedias, videos from science organisations, data tables, or an expert (a doctor, a farmer, a geologist). Use them when you cannot test something yourself.

• Use more than one source and check that they agree.
• Check who wrote it (a science organisation, a museum or a university is more reliable than an advert or a random post) and when (is it up to date?).
• Pick only the evidence that is relevant to your question, and write it in your own words.

11. Recording results in tables

1. Put the independent variable in the first column, in order (20, 40, 60).
2. Put the dependent variable in the next columns, with a column for each repeat and one for the mean.
3. Write the unit in the heading (in °C, in s), not after every number.
4. Use a ruler and give every column a heading.

Check the means: (118 + 122 + 120) ÷ 3 = 360 ÷ 3 = 120; (75 + 78 + 75) ÷ 3 = 228 ÷ 3 = 76; (50 + 47 + 47) ÷ 3 = 144 ÷ 3 = 48. For some enquiries a labelled diagram is better than a table — for example, drawing the Moon's shape each night.

12. Choosing the right chart or graph

Drawing a graph: independent variable on the x-axis (across), dependent on the y-axis (up); label both axes with the unit; choose a scale that goes up in equal steps and uses most of the grid; plot points as small crosses in pencil, exactly; draw bars of equal width with gaps for a bar chart.

13. Patterns, anomalous results and conclusions

Describe a pattern with "As … increases, … increases / decreases", then add numbers from the data. From the line graph: "As the temperature of the water increases, the time for the sugar to dissolve decreases — from 120 s at 20 °C to 48 s at 60 °C."

An anomalous result does not fit the pattern (80 s at 50 °C should be about 60 s). It may be caused by a mistake: the timer started late, the sugar was not stirred, or less hot water was used. What to do: repeat that measurement, and leave it out of the mean and the line.

A conclusion answers the question of the investigation and explains it with science: "Sugar dissolves faster in hotter water, because the particles of water move faster and break up the sugar more quickly." Then compare with your prediction: "My prediction was correct: I said hotter water would dissolve sugar faster, and the time went down from 120 s to 48 s." If only part of the prediction was right, say which part.

14. Evaluating and improving an investigation

To improve an investigation, say what to change and why it helps:

Worked examples

Worked example 1 (variables)

Mona investigates how the mass of plasticine on a paper boat affects how deep the boat sinks into water. She adds 10 g, 20 g, 30 g and 40 g of plasticine and measures how far the boat sinks below the water surface. (a) Name the independent variable. [1] (b) Name the dependent variable. [1] (c) Name two variables Mona must keep the same. [2]

1. (a) What does Mona change? → the mass of plasticine (on the boat).
2. (b) What does she measure? → how deep the boat sinks (the distance below the water surface).
3. (c) Things that could also change the result: the same paper boat (size and shape), the same type of paper, the same water (type and depth), plasticine put in the same place on the boat. Any two = 2 marks.

Common mistake: writing "the plasticine" for (a). Say what about it changes: its mass.

Worked example 2 (prediction and its accuracy)

Before the dissolving investigation (line graph above), Sami predicted: "Sugar will dissolve faster in hotter water." (a) Add a scientific reason to Sami's prediction. [1] (b) Was his prediction correct? Use the results to explain. [2]

1. (a) "… because the particles move faster in hot water."
2. (b) Yes, the prediction was correct [1]: the time to dissolve went down as the temperature went up, from 120 s at 20 °C to 48 s at 60 °C [1].
3. A full answer can also mention that the result at 50 °C did not fit and should be repeated.

Worked example 3 (repeats, anomalous result and mean)

Ali times how long an ice cube takes to melt in three places. He repeats each one three times.

(a) Which result is anomalous? [1] (b) Work out the mean for the classroom table, leaving out the anomalous result. [1] (c) Why did Ali repeat each measurement? [1]

1. (a) 27 min — the other two classroom results are 19 and 20, so 27 does not fit.
2. (b) (20 + 19) ÷ 2 = 39 ÷ 2 = 19.5 min.
3. (c) To make the results more reliable / to spot an anomalous result.

Common mistake: dividing by 3 after leaving out a value. You only added two values, so divide by 2.

Worked example 4 (choosing a graph and describing a pattern)

(a) Yara measured the distance a toy car rolled on four surfaces. Which type of graph should she draw? Give a reason. [1] (b) Describe the pattern shown in the scatter graph of height and arm span. [1]

1. (a) A bar chart, because the type of surface is a word (category), not a number.
2. (b) As height increases, arm span increases (taller children have longer arm spans).

Worked example 5 (risks)

Ahmed uses water from a kettle at 60 °C in glass beakers. Describe one risk and how to reduce it. [2]

1. Risk [1]: hot water could scald / burn his skin (or: the glass beaker could break and cut him).
2. Action [1]: an adult pours the hot water / keep the beaker on a heatproof mat away from the edge (or: handle glass carefully and report breakages).

Worked example 6 (improving an investigation)

Sara added "one spoon of sugar" to each beaker and timed each beaker once. Suggest one improvement and explain why it helps. [2]

1. Improvement [1]: measure the same mass of sugar (for example 5 g) with a balance. Why [1]: spoons hold different amounts, so this keeps a control variable the same and makes the test fair.
2. Or: repeat each temperature three times and calculate a mean [1], because this makes the results more reliable [1].

Practice

All pages in this course

Start here

• Checkpoint 6 Science: The Test and How to Prepare
• Thinking and Working Scientifically: Investigations, Variables, Tables and Graphs

Biology

• The Heart and Circulatory System
• The Lungs and Respiratory System
• Human Reproductive System and Puberty
• Diseases, Hygiene and the Body's Defences
• Food Chains, Food Webs, Energy and Toxins

Chemistry

• Properties of Substances: Melting and Boiling Points, Gases, Conductors
• Reversible Changes and Dissolving
• Boiling and Evaporation
• Chemical Reactions: Reactants, Products and Evidence

Physics

• Mass, Weight and Gravity
• Force Diagrams and Forces on Moving Objects
• Floating and Sinking
• Light: Reflection and Refraction
• Electric Circuits, Symbols, Series and Parallel

Earth and space

• Rocks and Fossils
• Types of Soil
• The Rock Cycle
• The Solar System and the Phases of the Moon

Practice tests

• Checkpoint 6 Science — Practice Test 1
• Checkpoint 6 Science — Practice Test 2
• Checkpoint 6 Science — Practice Test 3