Practical work brings science to life, helping make sense of the universe around us. Practical work at the heart of the chemistry A level. Practical science allows scientific theory to transform into deep knowledge and understanding – scientific thinking. Through investigation, you will uncover the important links between your observations and scientific ideas.

It's important to understand the language of measurement - here are some key terms

Accuracy: A measurement result is considered accurate if it is judged to be close to the true value.

Calibration: Marking a scale on a measuring instrument. This involves establishing the relationship between indications of a measuring instrument and standard or reference quantity values, which must be applied. E.g. placing a thermometer in melting ice to see whether it reads 0⁰C, in order to check if it has been calibrated correctly.

Measurement error: The difference between a measured value and the true value.

Anomalies: These are values in a set of results which are judged not to be part of the variation caused by random uncertainty.

Random error: These cause readings to be spread about the true value, due to results varying in an unpredictable way from one measurement to the next. Random errors are present when any measurement is made, and cannot be corrected. The effect of random errors can be reduced by making more measurements and calculating a new mean.

Systematic error: These cause readings to differ from the true value by a consistent amount each time a measurement is made. Sources of systematic error can include the environment, methods of observation or instruments used. Systematic errors cannot be dealt with by simple repeats. If a systematic error is suspected, the data collection should be repeated using a different technique or a different set of equipment, and the results compared.

Zero error: Any indication that a measuring system gives a false reading when the true value of a measured quantity is zero, eg the needle on an ammeter failing to return to zero when no current flows. A zero error may result in a systematic uncertainty.

Fair test: A fair test is one in which only the independent variable has been allowed to affect the dependent variable.

Precision: Precise measurements are ones in which there is very little spread about the mean value. Precision depends only on the extent of random errors – it gives no indication of how close results are to the true value. 

Repeatable: A measurement is repeatable if the original experimenter repeats the investigation using same method and equipment and obtains the same results. 

Reproducible: A measurement is reproducible if the investigation is repeated by another person, or by using different equipment or techniques, and the same results are obtained. 

Resolution: This is the smallest change in the quantity being measured (input) of a measuring instrument that gives a perceptible change in the reading.

Uncertainty: The interval within which the true value can be expected to lie, with a given level of confidence or probability, e.g. “the temperature is 20 °C ± 2 °C, at a level of confidence of 95 %.

Validity: Suitability of the investigative procedure to answer the question being asked. For example, an investigation to find out if the rate of a chemical reaction depended upon the concentration of one of the reactants would not be a valid procedure if the temperature of the reactants was not controlled. Valid conclusion A conclusion supported by valid data, obtained from an appropriate experimental design and based on sound reasoning.

Categoric variables: Categoric variables have values that are labels. E.g. names of plants or types of material. 

Continuous variables: Continuous variables can have values (called a quantity) that can be given a magnitude either by counting (as in the case of the number of shrimp) or by measurement (e.g. light intensity, flow rate etc). 

Control variables: A control variable is one which may, in addition to the independent variable, affect the outcome of the investigation and therefore has to be kept constant or at least monitored. 

Dependent variables: The dependent variable is the variable of which the value is measured for each and every change in the independent variable. 

Independent variables: The independent variable is the variable for which values are changed or selected by the investigator. 

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The Required Practical work is assessed against the Common Practical Assessment Criteria (CPAC) - more information can be found here