Practical+skills

back to IGCSE chemistry experiments you need to know

Recognise and explain the use of common items of laboratory apparatus
Common items of laboratory apparatus such as stirring rods, spatula, thermometer, measuring cylinder, burette (measuring volumes accurately to +/- 0.05 cm3), evaporating basin, watch glass, condenser, electronic balance, ...

Know which pieces of apparatus measure volumes accurately – burette (2 decimals) and graduated pipette are very accurate, burette measures any volume but a pipette measure a specific volume. Measuring cylinders are not as accurate and beakers should never be used! If you want to measure something, such as a height of a liquid or volume of gas, make sure that whatever you use is graduated; the smaller the divisions the more precise you are.

Plan practical procedures

 * Fair investigations such as comparing reactivity of metals, factors which affect the rate of a reaction, comparing enthalpy changes in the combustion of alcohols or comparing enthalpy changes in displacement reactions, investigating conditions for rusting.
 * Practical procedures such:
 * o measuring the percentage of oxygen in an oxide e.g. copper oxide and magnesium oxide
 * o measuring enthalpy changes
 * o making soluble and insoluble salts
 * o making carbon dioxide, oxygen and hydrogen
 * o decomposing metal carbonates
 * o testing for unsaturation in hydrocarbons
 * o determining formula of copper oxide by reduction with hydrogen

Use the idea of a fair test (independent, dependent and control variables)
In fair investigations you need to identify the 3 types of variables; independent variable (the one you change), dependent variable (the one you will measure) and the control variables that you will keep the same because they might also affect the dependent variable. The help sheet on the wikipage "Design an investigation" should help you how to determine all three types of variables.

Read scales to an appropriate degree of accuracy and perform simple mathematical operations (including finding the mean) on results obtained

 * When calculating a mean only use concordant values i.e. values within 0.20 (=concordant results), in particular when you are calculating an average volume of a titration, and do not include any anomalous results e.g when calculating average value.
 * Express your final result in the correct significant figures (the examiner often tells you how many decimals you need); this means your answer should have the same number of decimals as the number of decimals in the result with the least number of decimals.
 * Using concordant results means your values have high precision.

Use correct units for values (standard unit is in blue)

 * quantity || units and symbols ||  || quantity || units and symbols ||   || quantity || units and symbols ||
 * length || km, m, cm, ||  || temperature || C, K ||   || concentration || mol/dm3 ||
 * volume || m3, dm3 (l), cm3 (also ml) ||  || mass || kg, g , mg ||   || current || A ||
 * time || h, m, s ||  || amount of chemical || mol ||   ||   ||   ||

Record results in tables and use data presented in a variety of formats
When you have to draw a table:

a) use a ruler b) label all headings c) add units d) put all data in the table e) be consistent with the number of decimal places; if the figures in a column in a table have 2 decimal places, keep the same even if the answer does not. e.g. 27.1 = 27.10 if all the rest have two decimal places.

Plot and use graphs and draw straight lines and curves of best fit

 * Draw line graphs.
 * If you have to make your own scale, select a scale that uses as much of the sheet as possible.
 * Don't forget to label each axis including a unit
 * Line graphs need at least 5 data points to be able to draw a valid line of best fit which should either be a curve (drawn with the free hand) or a straight line (drawn with a ruler).
 * You need to check if the line needs to start from the origin; you do this by checking if the point with (0,0) as coordinates would make sense e.g a solution with a concentration of 0 mol dm-3 would have no time for a reaction as no reaction would happen so therefore a graph showing the relationship between concentration and time would start at the origin..
 * Ignoring anomalous results when drawing lines of best fit.

Identify relationships from graphs
Correlation is a relationship between the two variables (independent and dependent) in a given experiment. This is often obtained from a graph Just because two factors correlate well and there is a good line of best fit, does not necessarily mean that one variable is causing the other variable to increase or decrease.
 * If the gradient (slope) of a graph is positive (i.e. the graph slopes upwards) we can say there is a positive correlation e.g. Y increases when X increases.
 * If the gradient is negative, we can say there is a negative correlation between the variables e.g Y decreases when X increases.
 * If a straight line goes through the origin of a graph and the gradient is positive, we can say that the variables are directly proportional to each other.
 * Inverse proportional: Y halves when Y doubles.

Comment on the reliability and accuracy of data
An ** accurate ** measurement is a measurement that is close to the true or accepted value. An 'accepted value’ comes from the work of many scientists who have measured that value, agreed with it and published the value. We can refer to a textbook, data tables, or the internet to find these values. __ Accuracy can be improved by: __
 * Using measuring devices with smaller scale divisions.
 * Improve design to eliminate errors such as heat loss.
 * Allow a precipitate to settle before measuring its height
 * Measure the highest temperature not just a higher temperature
 * Allowing a salt to dry before you measure its mass
 * Ensuring reaction is complete e.g in energetics

An experiment is **reliable** if it produces the same results every (or nearly every) time it is repeated. If all or most results (ignore the ones that are not - anomalous results) are concordant or in close agreement the experiment is reliable. __ Reliability can be improved by __ :
 * Repeating an experiment until three or more results are close together or concordant
 * Ignore anomalous results and repeat until they become concordant
 * Effective control of variables.
 * ==== Increase precision of the results i.e. the more concordant they can be. ====

Draw conclusions and offer explanations for results
The conclusion should be an answer to the research question and should be supported by the data you have collected

Identify anomalous results and explain how they may have arisen
Anomalous results are results that fall outside the normal, or expected, range of measurements. If we take a large number of readings, we can be more certain about saying which readings are anomalous (do not fit the pattern) and which are not anomalous. Anomalous readings will show on a graph as a point, or points, standing clearly away from a line of best fit. We do not use anomalous results in calculations. Anomalous results are usually caused because a control variable was not kept the same. Possible reasons could be:
 * used a different concentration of solution to the other tests
 * did not allow the precipitate to settle
 * did not allow reaction to go to completion
 * a different temperature
 * too much of a reactant was added

When suggesting an improvement describe clearly what step needs improving and how; the how will include values e.g 50 cm3 or 100g or heat for another 5 minutes or repeat steps 1 to 6 five times.

Suggest further experimental work that may be required.
This will usually involve investigating other variables, usually one of those that is controlled in the investigation in the question.

The safety precaution should relate to the experiment although wearing goggles applies to many. Tying hair back, standing up, following instructions, reporting spills, .. are also all general safety precautions that always have to be obeyed so avoid using those if you can.


 * experiment involving heating || * let the apparatus cool before putting it away
 * let the crucible cool before moving it
 * when the volume of the solution has been halved turn down heat to avoid spitting ||
 * cracking || remove delivery tube from water before closing the bunsen burner to avoid suck back which would crack the test tube ||
 * salt formation || * when the volume of the solution has been halved turn down heat to avoid spitting ||
 * working with flammable substances || * keep away from naked flames; use electric heaters if heating is needed e.g in fractional distillation
 * do not pour down the sink in the case of flammable liquids ||
 * acid and alkali experiments || clear up spills with a lot of water ||

More on practical skills in Appendix A in your textbook and section F in the revision guide!!
Use the revision guide to read up on the difference between observations and deductions.