1

18 Practical skills

●● Experimental chemistryTapputi Belatekallim is considered to be the world’s first chemist. She was a perfume maker who worked as a female palace overseer in the second millennium BC. She distilled flowers, oil, myrrh and other aromatic materials, filtered them and redistilled until she made the perfume she wanted. As a chemist, she was continually experimenting and developing her product.

Today, in the 21st century chemists continue to experiment as they search for and use new knowledge about chemicals to improve the way we live. Chemists develop synthetic fibres, drugs, foods and cosmetics. A chemist also creates processes, including oil refining and petrochemical processing that reduce future energy use and pollution. In addition, chemists analyse substances and determine their physical and chemical properties. Experimental and investigative work is at the forefront of chemistry.

Throughout the AQA AS Chemistry specification there will be opportunity for you, as a chemist to enjoy practical work and gain new skills through manipulating equipment and materials, investigating, recording measurements and processing data.

Figure 18.2 In heart surgery, doctors often crack the patient’s chest to operate. Closing the sternum again by winding wires around the bones to immobilise them as they heal is often more problematic than the surgery itself. Research chemists are developing an adhesive which is slow setting and can be used in sternal closure. Many different investigative experiments have been used to create a non-toxic polymer, with the correct properties for this purpose.

Figure 18.1 A cuneiform tablet which gives the recipe Tapputi Belatekallim used to make perfume in the second millennium BC.

1471807671_AQA_Chemistry_Ch18.indd 1 31/03/2015 10:31

18

P

ra

ctic

al

skil

ls

2

The practical work for AS will encompass a broad range of activities which will enable you to apply and extend your knowledge and understanding of chemistry. By stimulating interest and enjoyment, these experiments should enhance your learning, give insight into the scientific method, and help you develop expertise and scientific attitudes such as open mindedness and objectivity.

●● Practical skillsDuring your study of AS Chemistry, you are expected to acquire laboratory experience in carrying-out, interpreting, analysing and evaluating experiments. Your specification has included a range of different experiments to enable you to develop these skills.

1 Following written instructions.

Written instructions for chemistry practicals are often referred to as a ‘method’. For most practicals you will be provided with a written method, and simply have to follow these instructions sequentially. Make sure you do not skip any steps. It is also important that you understand specific chemical methods and instructions. For example, an instruction may state ‘set up the apparatus for reflux’ – in order to do this you need to remember that reflux means setting up the condenser in a vertical position. Other chemical techniques which you need to be familiar with are noted in Table 18.2, and are explained in detail in the relevant chapter.

table 18.2

technique chapter

Distillation including setting up glassware using retort stand and clamps

13

Reflux including setting up glassware using retort stand and clamps 13

Purifying a liquid product, including using a separating funnel 12

Fractional distillation 12

Titration 2

Use of a pipette 2

Use of a burette 2

How to make up a volumetric solution 2

2 Presenting information and data in a scientific way.

During experimental activities you must often record results in a table, of your own design. When drawing tables and recording data ensure that:

●● it is a ruled box with ruled columns and rows● there are headings and units for each column and/or row● there is room for repeat measurements and averages – remember the

more repeats you do, the more reliable the data● all data are recorded to an appropriate degree of accuracy taking into

consideration the apparatus used● you can draw a table for recording titration results, see page 57.Figure 18.3 Recording results during a

titration.

1471807671_AQA_Chemistry_Ch18.indd 2 31/03/2015 10:31

Practical skills

3

3 Making and recording observations.

Making and recording observations is an important skill in chemistry. Qualitative observations are what we see and smell during reactions. Important types of observations in chemistry, and notes on how to record these are shown in Table 18.3.

table 18.3

type of observation

Notes on recording observations Examples from the specification

Colour change Always state the colour of the solution before the reaction and after.

Figure 18.4

● Bubbling an alkene into bromine water – the colour change is from an orange solution to a colourless solution.

● Warming a primary or secondary alcohol with acidified potassium chromate(VI) – the colour change is from an orange solution to a green solution (Figure 18.4).

Bubbles produced

If a gas is produced, then bubbles are often observed in the liquid, and the solid reactant often disappears.

Figure 18.5

● When magnesium or calcium reacts with hydrochloric acid the observations are bubbles and the metal disappears.

Precipitate produced

When two solutions mix, often an insoluble precipitate forms. Ensure you use the word precipitate in your observation – a common mistake is to write – the solution became cloudy. Also state the colour of the precipitate and the colour of the solution before adding the reagent.

Figure 18.6

● When testing for iodide ions with silver nitrate solution, the observation for a positive test is that a yellow precipitate forms in the colourless solution.

Heat produced Often in a reaction, the temperature changes – in an exothermic reaction it increases and heat is observed. In an endothermic reaction it decreases.

Figure 18.7

● The hydration of anhydrous copper sulfate is exothermic and the temperature increases.

Solubility of solids

When a spatula of soluble solid is added to water, the observation is often that the solid dissolves to form a solution. Make sure you state the colour of the solution formed.

Figure 18.8

● Copper(II) sulfate crystals dissolve in water to produce a blue solution.

1471807671_AQA_Chemistry_Ch18.indd 3 31/03/2015 10:31

18

P

ra

ctic

al

skil

ls

4

5 Safely and correctly use a range of practical equipment and materials.

All chemicals used in chemistry experiments must be labelled with a hazard warning label. You should be familiar with the following hazard warning labels.

corrosive flammable caution explosive toxic

Figure 18.11

clamp

test tube

water

HEAT

mixture of ethanol ethanoic acid and concentrated sulfuric acid

Figure 18.12 Flammable liquids can be heated in a water bath.

type of observation

Notes on recording observations Examples from the specification

Solubility of liquids

When a liquid is added to water, always record if it is miscible or immiscible with water.

Figure 18.9

● This mixture is immiscible and there are two layers.

4 Understanding how to use a range of experimental and practical instruments, equipment and techniques.

Practically you must be able to carry out the techniques of titration, preparation of a volumetric solution, reflux and distillation, with confidence. Measuring instruments, which you should be able to use correctly include a measuring cylinder, pipette, burette, stopclock, thermometer and balance. When recording measurements you must state the appropriate units.

The meniscus is the curve seen at the top of a liquid in response to its container. When you read a scale on a measuring cylinder, burette or volumetric flask, read it so the bottom of the meniscus (the centre of the meniscus) is level with the scale mark at eye level.

Figure 18.10 Always read to the bottom of the meniscus at eye level.

50

40

30

20

meniscus

eye level

1471807671_AQA_Chemistry_Ch18.indd 4 31/03/2015 10:31

Practical skills

5

You must always wear safety goggles during experiments. Other important safety rules for chemistry are given in Table 18.4.

table 18.4

safety rule When to implement this rule

Wear protective gloves When working with concentrated acid.

No naked flamesWhen working with flammable chemicals. For example alcohols are flammable and should be kept away from naked flames, and heated in a water bath.

Use a pipette filler A pipette filler must always be used with a pipette.

Use a fume cupboard For practicals where choking fumes may be produced.

In organic chemistry, particularly during reflux, a sand bath can be used to heat the reflux flask. A sand bath consists of a container filled with sand placed on a hot plate – it is a flameless method and useful when heating organic reactions; it can provide high temperatures.

6 Identifying variables.

An independent variable is one which you change during your experiment.

A dependent variable is one which may change as a result of changing the independent variable. It is measured during the experiment.

A controlled variable is one which remains constant during the experiment.

For example, when finding the effect of temperature on the rate of reaction of magnesium and excess hydrochloric acid by timing how long it takes until the magnesium disappears:

● the independent variable is the temperature● the dependent variable is the time taken for the experiment to finish● the controlled variables are the mass and surface area of the magnesium,

and the volume and concentration of the hydrochloric acid.

7 Plotting and interpreting graphs.

When plotting graphs the dependent variable should go on the y-axis and the independent on the x-axis. Details on how to draw graphs correctly are given on pages 360–361 in Chapter 17 of your textbook.

Experiments where you often have to plot graphs include determining enthalpy changes, and investigating the change in rate of reaction with temperature.

When interpreting graphs you need to describe the relationship between the independent and dependent variables.

Figure 18.13 A student using a fume cupboard to carry out an experiment.

Figure 18.14 Using a sand bath to heat a reaction flask.

1471807671_AQA_Chemistry_Ch18.indd 5 31/03/2015 10:31

18

P

ra

ctic

al

skil

ls

6

8 Researching.

The Oxford Dictionary defines research as ‘the systematic investigation of materials and sources in order to establish facts’. Another scientific skill which you should develop during your study of AS Chemistry, is the skill of researching. You should be able to use online and offline research skills. For example you could use websites, journals, research articles and textbooks to research the use of barium sulfate in medicine, to investigate the role of chemists in the legislation to ban the use of CFCs or to explore the use of fluoride in drinking water.

9 Correctly cite sources of information.

In your examination papers you are provided with a Chemistry Data Sheet. This is available on the AQA website and you should ensure that you are familiar with it. In addition to a Periodic Table, infrared absorption data are given. You should be able to use the infrared data on the Chemistry Data Sheet to suggest possible structures for molecules. In your answer you must cite the bond and the exact wavenumber given on the Data Sheet, for example for the C=C bond the wavenumber is 1620–1680 cm−1.

10 Considering margins of error, accuracy and precision of data and evaluating results with reference to measurement uncertainties and errors.

In an experiment, when you make a measurement of whatever kind, you cannot be sure just how close it is to the true value, that is, how accurate it is. There is an experimental uncertainty often called ‘experimental error’.

Experimental error arises because of:

● limits in how exact the measuring apparatus is; this is referred to as the precision of the apparatus

● imperfections in experimental procedures● judgements made by the operator.

When recording your results, make sure you record to the precision allowed by the apparatus. For example, if the balance reads to 0.01 g, make sure you do not round up a measurement of 3.58 g to 3.6 g or 4 g.

tiPYou must be able to calculate the percentage error involved in each measurement. For examples of this type of calculation check page 55 of your textbook.The experiments which often refer to percentage error and uncertainties include titrations and making up volumetric solutions, finding Mr of a volatile liquid, finding percentage yield, finding empirical formula and determining enthalpy changes.

Figure 18.17 Public Health England has published ‘Water fluoridation: health monitoring report for England 2014’. The report says there is no evidence of harm to health in fluoridated areas and that 5 year olds in fluoridated areas are 28% less likely to have had tooth decay than those in non-fluoridated areas.

Rat

e/m

ol d

m–3

s –1

Concentration/mol dm–3

Figure 18.15 The relationship shown by this graph is a positive correlation. As the concentration increases the rate increases.

Co

nce

ntr

atio

n/m

ol d

m–3

Time/s

Figure 18.16 The relationship shown by this graph is a negative correlation. As the concentration decreases the rate increases.

1471807671_AQA_Chemistry_Ch18.indd 6 31/03/2015 10:32

Required practicals

7

11 Process and analyse data using appropriate mathematical skills.

For many of your chemical practicals you need to be able to process and analyse the data you have obtained. For example, in a titration you may need to calculate the concentration of a solution, or in an enthalpy experiment you may need to use q = mcΔT to work out enthalpy change. By referring to the list of required practicals shown below, you should check out the appropriate chapter and review the different types of calculation needed.

●● Required practicalsYou are required to carry out six practicals across the one year course. These will be carried out at different points throughout the school calendar, in your centre. Questions in the papers will be written, in the expectation that you have carried out at least these six required practicals, and 15% of the marks in the AS papers will relate to practical work. The required practicals for AS are shown, with examples, in Table 18.5. Refer to the particular chapter to read about each type of practical and answer the activities within the chapter; this will help you prepare for the practical assessment.

table 18.5

required practical Examples chapter

1 Be able to make up a volumetric solution and carry out a simple acid–base titration.

•   Find the concentration of ethanoic acid in vinegar•   Find the mass of calcium carbonate in an indigestion tablet•   Find the Mr of MHCO3•   Find the Mr of succinic acid•   Find the mass of aspirin in an aspirin tablet 

2

2 Be able to measure an enthalpy change. •   Find ΔH for the reaction by calorimetry of the following examples:

– solution of KCl– solution of Na2CO3– neutralisation: NaOH + HCl– displacement: CuSO4 + Zn– combustion of alcohols

4

3 Be able to investigate how the rate of reaction changes with temperature.

•   Investigate the effect of temperature on the rate of reaction between sodium thiosulfate and hydrochloric acid

5

4 Carry out simple test-tube reactions to identify cations and anions in aqueous solution.

•   Identification of– cations: Group 2 and ammonium– anions: Group 7 (halide), OH–, CO2–

3 , SO2–4

10

5 Be able to distil a product from a reaction. •   Prepare cyclohexene from cyclohexanol and purify by distillation

•   Prepare ethane from ethanol and purify by distillation.

13

156 Tests for alcohol, aldehyde, alkene and

carboxylic acid.•   Using acidified potassium dichromate(VI), bromine water, 

sodium carbonate, Fehling’s solution and Tollens’ reagent.16

1471807671_AQA_Chemistry_Ch18.indd 7 31/03/2015 10:32