5.3.4Tools
Graphing in Biology — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 10 of 10
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Section 1 of 5Choosing the Graph
Bar chart
- Bar chart Used to compare values for separate categories, with gaps between the bars, such as the mass of different tissues.
Histogram
- Histogram Used to show the frequency of continuous data grouped into classes, with bars touching, such as the heights of plants.
Line graph
- Line graph Used when both variables are continuous and the points are connected, for example enzyme activity over a range of temperatures.
Scatter graph
- Scatter graph Used to look for a correlation between two measured variables, with a line of best fit added if a trend exists.
| Data | Best graph |
|---|---|
| Categories | Bar chart |
| Frequency of continuous data | Histogram |
| Two continuous variables | Line or scatter graph |
Section 2 of 5Drawing a Graph
1
Place the independent variable on the x-axis and the dependent variable on the y-axis, and label both axes with the quantity and unit.
2
Choose scales that use more than half of the graph area, with equal intervals and an origin only if it is needed.
3
Plot each point accurately, using small crosses or dots, and use different symbols for different data sets.
4
Add error bars, such as plus or minus one standard deviation, where repeats were taken.
5
Draw a smooth curve or a straight line of best fit through the trend and give the graph a descriptive title.
Section 3 of 5Reading and Interpreting Graphs
Reading and Interpreting Graphs
Gradient
- Gradient The gradient of a line equals the change in y divided by the change in x and represents a rate of change.
Tangent
- Tangent For a curve the rate at a particular point is found from the gradient of a tangent drawn at that point.
Trends
- Trends Describe the overall pattern, quoting data values, and note any plateau, peak or outlier.
Error bars
- Error bars If the error bars of two points overlap, the difference between them may not be significant.
Correlation
- Correlation A trend shows association only; a causal link needs controlled experiments or a known mechanism.
Section 4 of 5Worked Example
Question
- A graph of oxygen produced against time passes through the points (2 min, 4.0 cm³) and (10 min, 20.0 cm³).
- Calculate the rate of oxygen production.
- Change in volume = 20.0 − 4.0 = 16.0 cm³.
- Change in time = 10 − 2 = 8 min.
- Gradient = 16.0 ÷ 8 = 2.0 cm³ min⁻¹.
Answer The rate of oxygen production is 2.0 cm³ min⁻¹.
Section 5 of 5Exam Checklist & Tips
5.3.4 – Key Points for the Exam
- Independent variable The variable that is deliberately changed, plotted on the x-axis.
- Dependent variable The variable that is measured, plotted on the y-axis.
- Line of best fit A line that shows the trend of plotted points.
- Error bar A bar showing the uncertainty or spread of a plotted value.
Common Mistakes
- Joining the points of a scatter graph with straight lines instead of drawing a best-fit line.
- Putting the dependent variable on the x-axis.
- Leaving out the units on the axis labels.
- Drawing a histogram with gaps between the bars or a bar chart with the bars touching.
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