Reporting Experimental Work (SQA National 5 Chemistry): Revision Note

Labelled diagrams

• A key skill is being able to draw a clear diagram of an experimental setup

• A good scientific diagram is a simple 2D drawing with clear lines and labels

  • For more detail on specific apparatus, see the Common Chemical Apparatus revision note

Rules for a good diagram

• Use a pencil and ruler for clear, straight lines

• Draw in 2D cross-section

• Label all important parts of the apparatus clearly

• Make sure the apparatus is functional

  • For example, do not draw a sealed container if a gas needs to escape

• Do not use shading

Results tables

• Tables are the clearest way to present experimental data

• A good table must have clear headings with units

Rules for a good results table

• Use a ruler for neat columns and rows.

• Every column must have a clear heading stating the quantity.

• The units must be included in the heading (usually after a /).

Rates of reaction example results table

• This table shows how the volume of a gas changes over time

• A common exam question might ask you to plot this data on a graph

Titration example results table

• This table shows the results from a titration experiment

• Common exam questions for this results table could be:

  • To calculate the titres

    • Titre = final volume - initial volume

  • To calculate the average titre, using only the concordant results

    • Concordant results are within 0.2 cm³

Graphs of results

• Graphs are a powerful way to visualise trends in your data

• It's important to choose the right type of graph for the data you have

Line graph or scatter graph

• Use a line or scatter graph when you are plotting the relationship between two numerical sets of data

• For example, plotting the volume of gas produced over time in a rates of reaction experiment

Bar chart

• Use a bar chart when you are comparing data across different, non-numerical categories

  • For example, comparing the melting points of different named plastics like PVC, PET and HDPE

Rules for drawing a good graph

• Drawing a graph correctly is a key skill that is often worth multiple marks in an exam

• Follow this checklist:

Axes

• Draw the independent variable on the x-axis

  • The independent variable is the one you control

  • For example, concentration of a reactant, mol l^-1

• Draw the dependent variable on the y-axis

  • The dependent variable is the one that you measure

  • For example, the volume of gas produced, cm³

Labels

• Both axes must be fully labelled with the quantity and its units

  • For example, "Time / s"

Scale

• Choose a simple, even scale for each axis

  • For example, going up in 2s, 5s or 10s

• Your scale must be chosen so that your graph covers at least half of the available graph paper

Plotting

• Plot your data points accurately using a sharp pencil

  • Use either neat crosses (x) or dots inside circles (⊙)

Line of best fit

• Draw a single, smooth line of best fit

  • This can be a straight line or a smooth curve

Calculating average (mean) values

• Repeating an experiment and calculating an average (or mean) of your results is a key way to improve the reliability of your data

• It helps to smooth out any small random errors made during the measurements

How to calculate an average

1. Identify your repeat readings for a particular measurement

2. Identify and discard any anomalous results

   • An anomalous result (or outlier) is a reading that does not fit the pattern of the others

3. Add the remaining, consistent readings together

4. Divide the total by the number of consistent readings you added

Average titres

• When calculating an average titre from a set of titration results:

  • You must only use the concordant results.

    • For SQA National 5, concordant results are titres that are within 0.2 cm³ of each other

  • The rough titre is always ignored

Improving experiments

• Exam questions often describe an experiment and ask you to suggest an improvement to the method

• A good answer needs to be more than just a simple suggestion; it must be fully explained

Identify-improve-justify

• To get full marks, your answer should have three parts:

  1. Identify

     • State a specific weakness or source of error in the original method

  2. Improve

     • Suggest a specific, practical change to the apparatus or procedure

  3. Justify

     • Explain why your change is an improvement

     • Examples include:

       • To make the measurement more accurate

       • To reduce heat loss

       • To see the end-point more clearly

Common scenarios for improvement

• Two of the most common experimental scenarios where improvements can be made are calorimetry and titrations

Calorimetry

1. Identify

   • A lot of heat energy is lost to the surroundings instead of heating the water

2. Improve

   • Use a lid on the beaker and place draught shields around the apparatus

3. Justify

   • This reduces heat loss, leading to a more accurate result

Titration

1. Identify

   • It can be difficult to see the exact point where the indicator changes colour

2. Improve

   • Place a white tile under the conical flask

3. Justify

   • This makes the colour change at the end-point much clearer and easier to see accurately