1. Planning an Investigation
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Planning an Investigation

3. Planning an investigation
As part of your GCSE Science course you will be assessed on your ability to plan, collect results, analyse the data, draw conclusions and evaluate investigations.
You will be expected to identify the variables in investigations and to explain how to collect reliable results that will allow you to draw valid conclusions.
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What should you consider when planning an investigation?

4. Deciding what to investigate
Scientists carry out experiments to test a hypothesis.
A hypothesis is an idea that could explain an observation. It can be tested by doing an investigation.
For example, one hypothesis could be: ‘Drinking caffeine makes you more alert.’
The first thing to decide is how to measure how alert someone is.
If they are alert, their reactions should be fast.
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Coffee contains caffeine, so you could measure the effect coffee has on their reaction times.

5. Deciding what to investigate
Jack notices the effects of acid rain on marble statues and decides to investigate the reaction between hydrochloric acid and marble chips. What specifically could he investigate?
Jack knows the reaction produces carbon dioxide, so he could investigate how acid concentration affects the rate at which it is produced.
CaCO3 + 2HCl  CaCl2 + CO2 + H2O
What could his hypothesis be?
“Carbon dioxide is produced faster when marble chips react with more concentrated acid.”

6. Deciding what to investigate
Samira has been reading about the use of thermistors in thermostats and decides to investigate the link between temperature and current. What could her hypothesis be?
“The current through a thermistor increases as its temperature decreases.”

7. What should be investigated?

9. Types of variable
A variable is a quantity or characteristic that can take different values. Every investigation involves three sorts of variable:
an independent variable, which is changed to test a hypothesis
a dependent variable, which is measured to show the effect of this change
Teacher notes
The illustration depicts an investigation to see how much carbon dioxide is produced when marble chips react with hydrochloric acid. Students could be encourage to think about the different types of variable involved.
control variables, which must be kept the same to make it a fair test.

10. What’s the variable (1)?

11. More types of variable The main types of variable are:
categoric variables, which can be described with words, like eye colour
continuous variables, which can have any value between certain limits, like the temperature of water.
There are also:
discrete variables, which can only be whole numbers, like the number of insects
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ordered variables, which can be ranked, like large, medium or small marble chips.

12. What’s the variable (2)?

14. Choosing appropriate equipment
When selecting equipment you need to consider the resolution (or accuracy) of the measuring instrument. Some variables need to be measured using a finer scale to make the results clear.
Consider these two ammeters:
Ammeter X measures current to the nearest 0.1 A up to 3.0 A. It could be used to measure the current in a 6 V, 12 W bulb.
Teaches notes
The resolution is the smallest change that the instrument’s scale can distinguish.
If the instrument gives repeat readings that are very similar, it is precise.
If the values on the scale are true, it is accurate.
Ammeter Y measures current to the nearest A up to 1.0 A. It could be used to measure the current in a 2.5 V, 2 W bulb.

15. Choosing appropriate equipment
To measure the volume of acid needed to neutralize an alkali, a small volume of acid needs to be measured accurately.
A burette is the best instrument to use.
It can measure up to 50 cm3 to the nearest 0.1 cm3.
In an investigation to measure the time taken for a given length of magnesium ribbon to completely react in excess hydrochloric acid, a 100 cm3 measuring cylinder that measures to the nearest 2 cm3 is good enough.

16. What equipment should be used?

18. What is reliability?
The results of an investigation are reliable if they are similar every time the investigation is carried out. Reliable data is data you can trust.
Holly timed how long it took magnesium to react with hydrochloric acid. To check the reliability of her results, she repeated the test five times. 20 22 21 19
repeat 5
repeat 4
repeat 3
repeat 2
repeat 1
time (s)
The results cover a narrow range (the difference between the lowest and highest value). This means they are repeatable, so they appear to be reliable.

19. Reliability: an example
Matt did the same investigation and compared his results with Holly’s.
Holly
Matt 20 22 21 19
repeat 5
repeat 4
repeat 3
repeat 2
repeat 1
time (s) 24 16 28
Which group’s results are the most reliable?
the range of Holly’s results = 22 – 19 = 3 s
the range of Matt’s results = 28 – 16 = 12 s
Holly’s results have a narrower range than those of Matt and are therefore more reliable.

20. Reliability question

21. Reliability and graphs
Another way to check reliability of a set of results is to see how well they follow a pattern when they are plotted on a graph.
The graphs show the results of two groups of students who have investigated the relationship between force and acceleration. Which group has the most reliable results?
group A
group B
Group A has the most reliable results.

22. Repeat measurements How many times should you repeat each experiment?
It depends on the nature of the experiment.
If the dependent variable has a steady value, one repeat to check is enough.
For example, the potential difference across a particular length of resistance wire should be steady and so one repeat to check this value would be sufficient.

23. How many repeats?
When the dependent variable changes quickly, more than one repeat is needed to balance out random errors.
For example, when measuring the volume of hydrochloric acid needed to neutralize a sample of sodium hydroxide, the acid is run into a flask of sodium hydroxide from a burette until the universal indicator in the flask turns green.
The exact point at which this happens has to be judged and so is likely to vary between attempts.
Several measurements would be made and, once any outliers have been removed, an average would be calculated.

24. One or multiple repeats?

25. What is validity? Teacher notes
Students usually need frequent reminders about the meanings of the terms reliable and valid.
If a fair test was carried out, the measurements were reliable, and the dependent variable was a suitable one, then conclusions based on the results will be valid.
Investigations should be designed to produce reliable results so that valid conclusions can be drawn.

26. Ensuring validity What makes conclusions valid?
They must be based on a fair test – only the independent variable must changed. The others should be controlled.
You must test as many values of the independent variable as possible, over as wide a range as possible, and the intervals between them must not be too big.
Any uncertainty in a measurement must be as small as possible compared to the size of the measurement itself.
Measuring instruments must be well calibrated and working properly.

27. Range of the independent variable
The range of values tested needs to be as wide as possible, and it must be possible to measure the dependent variable over that range.
For example, a student investigating how the resistance of a wire varies with thickness uses an ammeter measuring up to 1 A.
If one length of wire gives a current of 0.16 A and two lengths give 0.31 A, how many lengths of wire can he use?
The number of lengths = 1 / 0.16 = 6 lengths of wire.

28. Choosing an appropriate range

30. Risks and hazards
Whenever your teacher gives you an experiment to do they carry out a risk assessment beforehand. When designing your own experiment, this becomes your responsibility.
This involves identifying hazards and what safety precautions should be taken to reduce the risk of an accident.
A hazard is a possible danger or threat (e.g. HCl is hazardous because it is toxic and corrosive).
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Teacher notes
Students should also be reminded to use common sense and general rules about practical work (no eating or drinking, keeping bags out of the way, etc).
Risk is the probability of injury. The risk involved when using HCl can be reduced by wearing safety goggles.

31. Risk assessment: hazard symbols
A student is preparing a sample of copper chloride by adding an excess of copper oxide to hydrochloric acid, filtering out the excess copper oxide and then heating the resulting copper chloride solution until it becomes saturated.
The bottles of copper oxide and hydrochloric acid have the following hazard symbols on them:
copper oxide
hydrochloric acid
harmful
oxidizing
toxic
corrosive
Teacher notes
Hazcards carry more detailed information about chemicals and your teacher will make them available to you if you need them.
What should the student write in his risk assessment form?

32. Carrying out a risk assessment
Material/ procedure
Hazard
What could go wrong
Safety precautions
In case of accident
Risk (high, medium, low)
copper oxide
hydrochloric acid
heating
harmful if swallowed or breathed in
transferred to mouth by hand
use spatula; wash hands
wash mouth; see doctor
low
corrosive
transferred to eyes by hand
wear goggles
wash out eyes; see doctor
low
might cause asthma
fumes might be breathed in during heating
ensure excess copper oxide used
leave room; see doctor
medium
hot liquid and equipment
liquid or equipment knocked over
put stools under benches and stand
put affected area under cold tap
medium

33. Writing up experiments (1)
An account of an investigation should have certain features. It should start off by including:
the hypothesis being tested or purpose of the investigation
a labelled diagram showing how the apparatus was set up
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the range of the independent variable (and the intervals between) and the values measured.
What should come next?

34. Writing up experiments (2)
The account should also include:
the dependent variable, the equipment used to measure it, the number of repeats, and the steps taken to ensure accuracy
a list of control variables, and an explanation of how their values were controlled
a list of steps that were followed.
Where appropriate you should explain how you will take into account the effects of variables that can’t easily be controlled.
If you spot problems with your original plan, explain what changes you made and what effect they had.

35. Writing up an experiment: example
A student carried out an experiment to find the length of nichrome wire with a resistance of one ohm.
With 10 cm nichrome in the circuit, the voltage and current were measured.
The resistance was calculated using the equation: resistance = voltage  current.
This was repeated for further values of length up to 100 cm.
The whole experiment was repeated and the results were compared to check for anomalies before calculating averages.
We checked that the wire had a constant diameter along its length.

36. Glossary Teacher notes
accuracy – A measure of how close a single measurement is to the true value of the quantity being measured.
anomaly – A value that doesn’t appear to fall within the expected pattern or range of measurements. Also known as an outlier.
average – A single value that typifies a set of measurements. The most common type of average is the mean, which is equal to the sum of all the values in a data set divided by the total number of values in that set.
categoric – A variable that can be described with labels and which cannot be ranked. For example, the eye colours of students in a class.
continuous – A variable that can take any numerical value. For example, the temperature of water in a beaker.
control – A variable that could, in addition to the independent variable, affect the outcome of an investigation.
dependent – A variable that is measured during an investigation for each change of the independent variable.
discrete – A type of categoric variable that can only be whole numbers (integers). For example, the value of a slotted mass.
fair test – A test in which only the independent variable has been allowed to affect the dependent variable.
hazard – A possible danger or threat.
hypothesis – A proposed explanation for an observable, testable phenomenon.
independent – The variable that is changed during an investigation to see what effect it has on the dependent variable.
ordered – A type of categoric variable that produces data that can be ranked. For example, the size of marble chips in a rates of reaction experiment might be large, medium or small.
outlier – See anomaly.
random error – A type of error due to unpredictable factors that cannot be determined or controlled from one measurement to the next. The effect of random errors can be minimized by taking many measurements and finding the mean. The more random errors are present, the less reliable the data.
range – The difference between the smallest and largest values in a set of measurements. It is not the number of measurements taken.
reliability – The extent to which measurements of a quantity remain consistent after repeated measurements under the same conditions. The more consistent results are, the more reliable they are.
risk – The probability of an event (usually considered hazardous) happening.
risk assessment – The process of identifying hazards and what safety precautions should be taken to reduce the risk of an accident.
validity – The extent to which data or conclusions have been obtained from an investigation that has been suitably designed to test the proposed hypothesis. For laboratory-based investigations, validity depends on performing a fair test.
variable – A quantity or characteristic that can be changed and have different values. There are several types of variable.