IGCSE chemistry practical questions

IGCSE chemistry         Preparing for the practical questions

You will be assessed on your ability to:

1. Devise and plan fair investigations or practical procedures such as

·            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

2. Recognise and explain the use of common items of laboratory apparatus

    Examples of type of investigations for which you need to know the basic laboratory apparatus and the

    reason for their use:

Type of experiment	Main processes and items of laboratory apparatus
Separation technique	·         Step 1: add solute to solvent ·         Step 2: dissolving: warm to speed up dissolving ·         Step 3: filtration: filter funnel, filter paper ·         Step 4: crystallization: evaporating basin
Titration: making soluble salt using an acid and alkali	To improve accuracy and precision of a titration you should use: a burette for the acid use graduated pipette to measure out alkali which is to be placed in conical flask To know when neutralization is complete: ·         add indicator to solution in conical flask or ·         use a thermometer and stop adding acid when the temperature stops rising. If the salt is needed repeat the experiment with the same volumes of acid and alkali but without the indicator.
Energy changes	Polystyrene cups with lids to reduce heat loss as they are better insulators than glass beakers.
Making salts: excess base (making a soluble salt with a base)	Add base to warm acid (speed up reaction) until no more dissolves. Filter off excess base and heat and crystallize filtrate, wash and dry.
Preparation for gases	·         If gas is collected over water you rely on the gas being insoluble in water ·         Use burette or gas syringe to measure volume more accurately ·         Relate density of gas to method of collection o   hydrogen (less denser than air) use inverted test tube (downward displacement of air) o   carbon dioxide (denser than air) use test tube open at the top (upward displacement of air; it pushes the air up!!)
Tests for ions	Know the test and test result for the anions and cations – see section 2g.
Chromatography	·         Draw pencil line ·         Place sample of mixture on pencil line ·         Add solvent to a level below pencil line ·         Allow solvent to move up paper ·         Relies on difference in solubility In example to the right sample H is a mixture of F and G.
Making dry pure salt crystals	·         Heat solution until saturation ·         Allow to cool/crystallize ·         Wash with distilled water ·         Dry

      Others experiments you need to know (these are also listed on the wiki) :

• experiments (using iron, copper and phosphorus) used to provide evidence that air has 21% oxygen
• thermal decomposition of copper carbonate: the green stuff turning black!!!
• cracking alkanes
• dehydration of ethanol
• fermentation of sugar using yeast

3. Make observations and measurements with appropriate precision, record these methodically, and

    present them in a suitable form.

    This means reading scales to an appropriate degree of accuracy and designing your own results tables.

    Measuring tools you need to be able to read and record their values in a suitable form i.e. correct number of

    significant figures and decimal places (you should read the scale of an instrument to ± half of the smallest

    scale division)

• a digital balance,
• thermometer,
• measuring cylinder,
• burette  (you need two decimal places when recording data obtained from a burette e.g. 25.50 cm³).

     Mathematical operations:

• Example: calculation of the mean or average
• When calculating a mean only use concordant values i.e. values within 0.2 and do not include any anomalous results.
• Express your final result in the correct significant figures; this means your answer should have the same number of decimals as the numbers you used in the calculation.

4.  Plot graphs.

·      Draw graphs and bar charts.

·      Ignoring anomalous results

·      Plot and use graphs and draw straight lines and curves of best fit e.g. energy changes, thermometric 

titrations.

5. Analyse and interpret data to draw conclusions from experimental activities which are consistent

    with the evidence.

Identify relationships from graphs, identify anomalous results and explain how they may have arisen.

This involves:

• interpreting intersections of graph lines, finding values.
• recognizing a correlation: see table below.

Correlation

Correlation is a relationship between the two variables (independent and dependent) in a given experiment. This is often obtained from a graph. ·         If the gradient (slope) of a graph is positive (i.e. the graph slopes upwards) we can say there is a positive correlation. ·         If the gradient is negative, we can say there is a negative correlation between the variables. ·         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. 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.

6. Evaluate data and methods

·         Comment on the reliability/repeatability and accuracy of the data. This basically means: how can accuracy and reliability/repeatability be improved.

Accuracy	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
Precision	If the measurement gives the same result each time, it would be precise. The greater the number of decimals, the greater the precision e.g. ± 0.01g is more precise than ±.1 g. Precision can be improved by: Using an instrument with a smaller scale division e.g. a ruler with a millimeter scale is more precise than a ruler with a centimeter scale.
Validity	Validity is about how well an experiment measures what it says it is going to measure.   Validity is usually improved by: ·         Making the investigation a fair(er) test which means controlling variables you are not investigating. ·         Improving precision. Valid investigations are accurate and precise.
Reliability	Reliability or repeatability is about getting the same results if the experiment is repeated under the same conditions, if it does than the method is reliable.  Reliability is about the method; precision is about results. Reliability can be improved by: ·         Repeating anomalous results until they become concordant ·         Effective control of variables. ·         Increase precision.
Anomalous results	Anomalous readings are readings 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.
Concordant results	Concordant readings are obtained when any reading is repeated and all the readings are close, or identical. Example: If you obtained four measurements such as 6.20 cm, 6.10 cm, 6.30 cm and 6.20 cm, you would say they were concordant – they are within 0.2. If however, you obtained 6.20, 6.10, 7.10 and 5.30 for example, then these readings are not concordant. The 7.10 cm and 5.30 cm readings would probably be anomalous readings and these two readings should be repeated again. Anomalous results should not be included when averaging results. The more concordant your results are the more reliable they are likely to be.

  

·             Draw conclusions and offer explanations. Conclusions should relate to the data collected and  

             processed.

·             Suggest improvements to the procedures. Control of variables.

·             Suggest further experimental work that may be required

• how good was the experimental technique and how could this be improved?
• extending work e.g. a greater range of variables, other variables which affect the experiment.