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Tables & Graphs for Practical Chemistry (PDO)

PDO (Paper 3): tables and graphs with correct headings/units, consistent decimal places, best-fit lines, and gradients (rate graphs).

  • SEC G3 Pure Chemistry 2027
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Learning objectives

  • make and record observations, measurements and estimates;
  • interpret and evaluate observations and experimental results;

Tables and graphs should let another person read the measurements without guessing. Units, consistent precision, sensible scales and clear plotting all contribute to that goal.

1. Definition

Presentation of data and observations (PDO) is recording results in clear tables and graphs with correct headings, units, and sensible precision.

2. Key Ideas

  • Every table column needs a heading + unit.
  • Keep decimal places consistent down a column.
  • For graphs: label both axes + units, use a good scale, and draw a best-fit line/curve.
  • For rate experiments: gradient can represent a rate (e.g., volume vs time).

3. Detailed Explanations

Quick Recall (what goes where?)
  • IV (what you change) → table first column, x-axis on graphs.
  • DV (what you measure) → table second column, y-axis on graphs.
  • Put units in the heading/axis label, and keep decimal places consistent down a column.

A. Tables (Headings, Units, Precision)

Good table headings look like this:

independent variabledependent variable
concentration / mol dm⁻³time / s
time / svolume of gas / cm³
Exam Trap: Units belong in the heading, not in every cell

Write “time / s” as the heading. Do not write “s” after every number.

B. Graphs (Axes, Scale, Best Fit)

RuleWhat to do
AxesIV on the x-axis, DV on the y-axis
Scaleuse most of the graph area (no tiny graph in a corner)
Pointsplot points accurately (use crosses)
Linedraw a best-fit line/curve (do not join dot-to-dot unless told)

Example: a scatter plot with a best-fit line (axes + units must be labelled):

Example Graph: Volume of Gas vs Time

Example of a scatter plot with a best-fit line, correctly labelled axes and units.

Scroll across the graph to read all labels.

Example of a scatter plot with a best-fit line, correctly labelled axes and units.Example of a scatter plot with a best-fit line, correctly labelled axes and units.
Plot points accurately, then draw a best-fit line/curve. The gradient of a straight-line section can represent a rate.
Open full-size graph
View figure data
Values and uncertainty for Example Graph: Volume of Gas vs Time
SeriesTime (s)Time uncertaintyVolume of gas (cm³)Volume of gas uncertainty
Data points00
Data points1019
Data points2035
Data points3055
Data points4071
Best-fit line (example)00
Best-fit line (example)4072

If your x-axis is categories (not a number line), use a bar chart:

Example: Categorical Comparison (Bar Chart)Example bar chart comparing results for different conditions (categorical x-axis).Example: Categorical Comparison (Bar Chart)ConditionVolume of gas in 60 s (cm³)
Use bar charts when the x-axis is categories (e.g., different conditions). Use line/scatter graphs for continuous variables.
Data table
ConditionResult
No catalyst12
Catalyst A45
Catalyst B30

C. Gradients (When a Gradient is a “Rate”)

If you have a straight-line graph, gradient is:

gradient = (Δ y)/(Δ x)

Example: volume (cm³) vs time (s) → gradient has units cm³/s (rate of gas production).

4. Common Mistakes

  • Missing units in table headings or axes labels.
  • Using mixed decimal places in one column (e.g., 24.0, 24.05, 24.1).
  • Putting DV on the x-axis.
  • Drawing dot-to-dot when a best-fit line is expected.
  • Ignoring anomalous points (or deleting them without comment).

5. Exam Tips

How to handle an anomaly

If one point is far from the pattern, circle it and state “anomalous”. Do not force the best-fit line to pass through it.

6. Worked Examples

Modelled example 1

Fix the Table Headings

Core

Problem

A results table has headings “Time” and “Volume”. Rewrite both headings so they meet PDO requirements for gas-volume readings taken over time.
Study the worked solution

  1. Name each measured quantity

    Method

    Use “time” and the more specific “volume of gas”.

    Reason

    A reader must know exactly which quantity each column records.

    Working

    Quantities: time; volume of gas.

  2. Add units to the headings

    Method

    Write time / s and volume of gas / cm³.

    Reason

    Units belong once in each heading rather than after every data value.

    Working

    time / s; volume of gas / cm³.

Guided practice 2

Choose the Correct Graph

About 5 min

Problem

An investigation changes concentration, a continuous variable, and measures the time for a cross to disappear. Choose the graph type and assign both axes.

Classify variables before plotting

Graph type
x-axis
y-axis

Hints

Hint 1: axes
Put the independent variable on x and the dependent variable on y.
Hint 2: variable type
A bar chart is for categories; concentration and time are numerical continuous variables.
View solution step by step

  1. Assign axes

    Method

    Place concentration on x and time on y.

    Reason

    Concentration is deliberately changed; disappearance time is measured.

    Working

    x: concentration; y: time.

  2. Choose the graph

    Method

    Plot a line/scatter graph with an appropriate best-fit line or curve.

    Reason

    Both variables are continuous and the graph should show their trend.

    Working

    Concentration against time, with labelled units and best fit.

Common misconception 3

Decimal Places in Burette Readings

Find and correct the mistake

Learner table

A learner records burette readings as 12.2, 12.25 and 12.250 and says the extra digits make some readings more accurate. Identify and correct the PDO issue.

Apply one justified precision throughout

Issue
Corrected set

View solution step by step

  1. Identify the issue

    Method

    Identify inconsistent decimal places.

    Reason

    All values in one measurement column should reflect the same apparatus resolution.

    Working

    Given precision varies from one to three decimal places.

  2. Standardise the records

    Method

    Record each value to two decimal places.

    Reason

    This is the expected precision for standard burette readings.

    Working

    12.20, 12.25, 12.25.

Challenge 4

Gradient Calculation

Minimal support

Graph-to-rate transfer

A gas-volume best-fit line passes through (10 s,24 cm³) and (40 s,84 cm³). Calculate its gradient with units.

Use changes across a large triangle

Change in volume
Change in time
Gradient

Hints

Hint 1: formula
Use change in y divided by change in x.
Hint 2: units
Divide the y-axis unit, cm³, by the x-axis unit, s.
View solution step by step

  1. Find coordinate changes

    Method

    Subtract the two y-values and the two x-values in matching order.

    Reason

    A gradient measures vertical change per horizontal change.

    Working

    Δ V = 84-24 = 60 cm³; Δ t = 40-10 = 30 s.

  2. Calculate and unit the gradient

    Method

    Divide 60 by 30 and attach cm³/s.

    Reason

    Volume is the y quantity and time is the x quantity.

    Working

    gradient = 60/30 = 2.0 cm³/s.

7. Mind Stretchers

Mind stretcher 1: Bad Scale ChoiceExtension

Question: Your x-axis goes 0 to 100, but your data only goes 0 to 20. Why is this a problem and what should you do?

Show Answer

Answer: The graph becomes too small to read accurately, so plotting and gradient become inaccurate. Choose a scale that uses most of the graph paper, e.g., 0 to 20 (or slightly above).

Mind stretcher 2: “Join-the-dots” vs Best FitExtension

Question: Your data should form a straight-line trend, but one point is off due to a leak. Should you join dots, or draw a best-fit line?

Show Answer

Answer: Draw a best-fit line based on the overall trend and treat the off point as an anomaly.

8. Quiz

Quiz time

Ready to check your understanding? Try the interactive quiz, then review any questions you missed.

Go to quiz