Valid measurements

Planning to collect measurements that are valid

Controlling factors

The conclusions that can be drawn from the experiment are valid if the method makes sure that the effects observed and measured are due to the cause claimed.

It is important that other factors are accounted for. The factors that need to be controlled should be identified and the plan should include how these will be controlled.

Question

Two students investigate how the concentration of acid affects the rate of reaction between marble chips and acid. They test the first two concentrations on one day. They test the other concentrations the following week when the weather is much hotter.

A sealed conical flask connected to a gas syringe. The flask contains a reaction mixture that is giving off bubbles of gas.

Why is any conclusion from this experiment not valid?

The conclusion cannot be valid because the surrounding temperature was different on the two days. An increase in the rate of reaction could be caused by the reaction mixture being much hotter, not just by a change in concentration.

Question

What factors should the students have controlled?

Surface area and temperature also affect rate of reaction, so the students should have controlled these.

Question

How could the students make their results repeatable?

They should also use the same volume of acid and the same mass of marble chips each time.

Deciding range

The range of measurements made must be enough to draw a valid conclusion. If the measurements are too close together, it is difficult to be confident in any trend shown.

Question

The students record the volume of gas produced at 20 second intervals.

Their first set of results are:

Time (s)Volume of gas (cm3)
201
402
603
805
1006

How could the students improve the experiment to collect a broader range of results?

The students could take measurements over a longer time period so that the volumes collected are larger. They could, if this is safe to do, use higher concentrations of acid. They could also use a larger mass of marble chips.

Planning to collect measurements that are precise and accurate

Precision

Measurements are precise if the results are very close in value. The choice of measuring apparatus can affect the precision of data collected.

For example, during titration, the volume of sodium hydroxide in the flask should be measured with a pipette. If a measuring cylinder were to be used, the volume would vary between repeats, so the titration reading would differ. They would not be precise.

Accuracy

The accuracy of a numerical result is how close it is to the true value.

For example, if following a rate of reaction by loss of mass, reading from a balance with two decimal places would be closer to the true value than readings from a balance with no decimal places. The data would be less accurate.

curriculum-key-fact
Results are precise if they are close in value, but they must also be close to the true value to be accurate.

Identify hazards and suggest ways to minimise the risk

The substances used in a chemistry experiment may be a hazard. They could be harmful to the eyes or skin, or cause harm if breathed in.

The risk of harm from these hazards may be minimised by using the lowest concentration of any solutions, wearing eye protection, wearing gloves and using a fume hood.

Procedures can also be hazardous. For example, care must be taken not to touch very hot equipment. Loose clothing or hair should be kept away from flames.

All experiments should be risk assessed. Always follow teacher instructions and safety advice.