Practical Skills: Prescribed Practicals and Analysis

Thinking like a scientist

A quarter of the marks in this course come from practical skills. The good news is that the same way of working applies to biology, chemistry and physics alike. This module teaches that shared method: - Planning a fair test: choosing what to change, what to measure, and what to keep the same.\n- Measuring well: getting readings that are both accurate and precise.\n- Building reliability: repeating readings and taking a mean.\n- Making sense of it: spotting anomalies, drawing conclusions and evaluating the method. The hands-on practicals themselves cannot be done on a screen, but the thinking behind them, which the written practical exam tests, is exactly what this module builds.

Words for practical work

Five terms you need before you use them. Learn what each one means.

Match each practical idea to what it means

Accuracy against precision

These two words sound similar but mean different things, and telling them apart is a common exam point.

Which variable is this?

You change the temperature of some water and measure how fast sugar dissolves in it, keeping the amount of sugar the same. What is the temperature in this investigation?

• The independent variable, because it is the thing you change
• The dependent variable, because it is the thing you measure
• A control variable, because it is kept the same
• An anomaly

Tap the two ways to make results more reliable

Tap the TWO actions that make a set of results more reliable.

• Repeating each reading several times and taking a mean
• Spotting and setting aside a clear anomaly before averaging
• Changing several variables all at once
• Taking a single reading and stopping there

Recording and analysing results

Good data is only useful if it is recorded and read properly, and marks are won and lost here more often than anywhere else. Write results into a table with clear headings and the correct units. Then plot a graph to let the pattern show itself, and look hard for any reading that sits off the line. Describe the trend you see plainly. Finally, make sure the conclusion answers the original question using the data, and does not wander beyond what the results actually show. A neat table, a clear graph and a conclusion tied to the evidence are the marks of strong practical work.

Pick the true practical facts

Select the TWO statements that are true.

• Changing only one variable at a time keeps a test fair
• Repeating readings and taking a mean improves reliability
• An anomaly should always be treated as a correct result
• Accuracy and precision mean exactly the same thing

Order how to run a fair investigation

Put the stages of planning and running a fair investigation in a sensible order.

• Write a hypothesis and pick the one variable to change
• Decide what to measure and what to keep the same
• Take repeated readings and record them in a table
• Work out a mean and plot the results
• Draw a conclusion and evaluate the method

One experiment, done properly

Follow how one student investigated whether warmer water dissolves sugar faster, working carefully at every step. Planning: she chose to change only the water temperature, decided to measure the time for the sugar to vanish, and kept the amount of sugar and the stirring the same, so the test was fair. Measuring: at each temperature she timed the dissolving three times over and wrote every reading into a neat table with its units. Checking: one of her three times was far longer than the other two, so she set that odd reading aside and averaged the rest. Concluding: her averaged times fell as the temperature rose, so she concluded that warmer water does dissolve sugar faster, just as she had predicted. Careful planning, repeated readings and an honest look at the odd result gave her a conclusion she could truly stand behind.

Complete the practical skills facts

The one thing you deliberately change is the _____ variable. The thing you measure to see the effect is the _____ variable. A result that clearly does not fit the pattern is an _____. Repeating readings and taking a mean makes a set of results more _____.

Build a practical-skills point

Choose the words that complete this statement about a fair investigation.

Work out the mean reading

In a practical, three repeat readings of a length are taken: 12 centimetres, 17 centimetres and 19 centimetres. Add the three readings together, then divide by how many readings there are, to find the mean. What is the mean length in centimetres?

Improve the investigation

Read each problem with an investigation and choose the best fix, then think about why.

• A student changes both the temperature and the amount of sugar at the same time. What is wrong with this?
• A student takes just one reading at each temperature. How could the results be made more reliable?
• One result sits far off the smooth line that all the other results follow. What should the student do?

Explain how to plan and evaluate a fair experiment

A student in the year below does not understand how to plan a good science experiment. Explain how using what this module has covered.

• Explain the difference between independent, dependent and control variables
• Explain what makes a test fair
• Explain how to make a set of results more reliable
• Explain the difference between accuracy and precision
• Finish by explaining how to spot an anomaly and reach a fair conclusion