contents

Chemistry

9.5 - Reading & Interpreting Graphs

7 min

note 1 of 6

What a graph-reading question looks like

Most graph questions ask you to read a value off the line or describe a trend.

What a graph-reading answer needs

A value read off the line that matches the graph, with construction lines drawn to show where it came from.
A trend given with a direction and the figures that back it up, in the right cause-and-effect order.
A rate, where asked, calculated from two points on the curve rather than read off as a single value.

Missing construction lines or giving figures without a direction (or a direction without figures) will lose marks.

A typical question

The graph shows the volume of gas produced against time.

(a) Calculate the average rate of reaction, in cm³ min^-1, between 1 and 4 minutes.

(b) Predict the final volume of gas that would be produced if the experiment were repeated at a higher temperature, using the same mass of calcium and the same volume of water.

note 2 of 6

Selecting information from a passage or table

Some questions give you all the information in a passage, table, or dataset, and simply ask you to find it.

Selecting vs. reasoning from your own knowledge

Selecting - The fact you need is written down in front of you, so pick it directly from the text (e.g. naming a gas identified in the passage).
Reasoning - The fact isn't in the passage, so you must use chemistry you already know (e.g. suggesting a flame test to detect potassium when the passage doesn't mention one).
When reading a table, check the specific row or column the question asks about rather than the first number you see.

note 3 of 6

Reading a value off the graph

To find a value, follow up from the given point on the x-axis to the line, then across to the y-axis and read off the value.
Always draw dashed construction lines on the graph to show where your reading came from.

Interpolation vs. extrapolation

Interpolation - Reading a value between plotted points.
Extrapolation - Reading a value beyond plotted points. This is unreliable because the trend may change (e.g. a reaction levelling off once a reactant is used up).

note 4 of 6

Describing a trend

A trend answer needs two things: the direction of the change and figures from the axes to back it up.
Use this structure to keep cause and effect in the right order:

As [the quantity changed] increases, [the quantity measured] increases / decreases, from [figure] to [figure].

If the line flattens out, describe both parts: the rise (or fall) and where it levels off.
Whichever quantity was changed in the experiment goes first in the sentence.

When to say "directly proportional"

Only use "directly proportional" when the line is straight and passes through the origin (0, 0).
If the line curves or levels off, do not describe it as proportional.

note 5 of 6

Reading a rate off a volume-against-time graph

To find the average rate between two times, read the volume off the curve at each time, subtract them, and divide by the time taken.

Formula for average rate

$Average rate = \frac{change in volume}{change in time}$

For example, if the volume is 32 cm³ at 1 minute and 48 cm³ at 4 minutes:

$Average rate = \frac{48 - 32}{4 - 1} = 5.3 {cm}^{3} {min}^{- 1}$

Once a reaction has finished and the graph levels off, the rate between any two points on the flat section is zero.

note 6 of 6

Common exam questions

Question 1

A passage describes a new indigestion tablet, stating that "the active ingredient neutralises stomach acid by reacting with hydrochloric acid to produce carbon dioxide gas." A question asks what gas is produced when the tablet reacts with stomach acid. Where does the answer come from?

Straight from the passage: it names carbon dioxide directly, so the answer is selected from the text in front of you rather than worked out from chemistry knowledge.

Question 2

What is the difference between interpolation and extrapolation?

Interpolation is reading a value between your plotted points, which is fine; extrapolation is reading a value beyond your last point, which is riskier because you have no readings out there to check the line still behaves the same way.

Question 3

A student reads a volume of 40 cm³ off a graph and writes just the number. What else is needed?

The construction lines: a line up from the time on the x-axis to the graph line, and a line across from there to the volume on the y-axis, so it can be seen where the value came from.

Question 4

A graph shows the volume of gas produced levelling off after four minutes. Describe what this shows and suggest why it happens.

The volume stops increasing and stays constant, showing the reaction has finished; this happens because one of the reactants, such as the calcium, has been fully used up.

Question 5

A graph of solubility against temperature shows the solubility falling as the temperature rises. Describe this trend.

As the temperature increases, the solubility decreases. Writing it the other way round, with the solubility named as the cause, would not be credited.

Question 6

A graph plots the volume of gas produced against the mass of magnesium reacted, using excess acid throughout, and gives a straight line through the origin. Describe the relationship.

The volume of gas produced is directly proportional to the mass of magnesium reacted, because the line is straight and passes through the origin.

Question 7

A graph shows the volume of gas at 1 minute as 32 cm³ and at 4 minutes as 48 cm³. Calculate the average rate of reaction between these times.

$\frac{48 - 32}{4 - 1} = 5.3 {cm}^{3} {min}^{- 1}$ .

Question 8

Why is a rate of zero a sensible answer for the section of a graph where the line has levelled off?

Because the volume is not changing over that stretch of time, so the change in volume, and therefore the rate, is genuinely zero; it does not mean the reading has gone wrong.

key terms

TermDefinition

InterpolationReading a value from a graph between the plotted points.

ExtrapolationReading a value from a graph beyond the range of the plotted points, by extending the line.

Directly proportionalA relationship between two quantities where doubling one doubles the other, so their graph is a straight line through the origin.

end of notes

Key points to remember

9.6 - Drawing ConclusionsNext up

contents

Chemistry

9.5 - Reading & Interpreting Graphs

7 min

note 1 of 6

What a graph-reading question looks like

Most graph questions ask you to read a value off the line or describe a trend.

What a graph-reading answer needs

A value read off the line that matches the graph, with construction lines drawn to show where it came from.
A trend given with a direction and the figures that back it up, in the right cause-and-effect order.
A rate, where asked, calculated from two points on the curve rather than read off as a single value.

Missing construction lines or giving figures without a direction (or a direction without figures) will lose marks.

A typical question

The graph shows the volume of gas produced against time.

(a) Calculate the average rate of reaction, in cm³ min^-1, between 1 and 4 minutes.

(b) Predict the final volume of gas that would be produced if the experiment were repeated at a higher temperature, using the same mass of calcium and the same volume of water.

note 2 of 6

Selecting information from a passage or table

Some questions give you all the information in a passage, table, or dataset, and simply ask you to find it.

Selecting vs. reasoning from your own knowledge

Selecting - The fact you need is written down in front of you, so pick it directly from the text (e.g. naming a gas identified in the passage).
Reasoning - The fact isn't in the passage, so you must use chemistry you already know (e.g. suggesting a flame test to detect potassium when the passage doesn't mention one).
When reading a table, check the specific row or column the question asks about rather than the first number you see.

note 3 of 6

Reading a value off the graph

To find a value, follow up from the given point on the x-axis to the line, then across to the y-axis and read off the value.
Always draw dashed construction lines on the graph to show where your reading came from.

Interpolation vs. extrapolation

Interpolation - Reading a value between plotted points.
Extrapolation - Reading a value beyond plotted points. This is unreliable because the trend may change (e.g. a reaction levelling off once a reactant is used up).

note 4 of 6

Describing a trend

A trend answer needs two things: the direction of the change and figures from the axes to back it up.
Use this structure to keep cause and effect in the right order:

As [the quantity changed] increases, [the quantity measured] increases / decreases, from [figure] to [figure].

If the line flattens out, describe both parts: the rise (or fall) and where it levels off.
Whichever quantity was changed in the experiment goes first in the sentence.

When to say "directly proportional"

Only use "directly proportional" when the line is straight and passes through the origin (0, 0).
If the line curves or levels off, do not describe it as proportional.

note 5 of 6

Reading a rate off a volume-against-time graph

To find the average rate between two times, read the volume off the curve at each time, subtract them, and divide by the time taken.

Formula for average rate

$Average rate = \frac{change in volume}{change in time}$

For example, if the volume is 32 cm³ at 1 minute and 48 cm³ at 4 minutes:

$Average rate = \frac{48 - 32}{4 - 1} = 5.3 {cm}^{3} {min}^{- 1}$

Once a reaction has finished and the graph levels off, the rate between any two points on the flat section is zero.

note 6 of 6

Common exam questions

Question 1

Straight from the passage: it names carbon dioxide directly, so the answer is selected from the text in front of you rather than worked out from chemistry knowledge.

Question 2

What is the difference between interpolation and extrapolation?

Question 3

A student reads a volume of 40 cm³ off a graph and writes just the number. What else is needed?

The construction lines: a line up from the time on the x-axis to the graph line, and a line across from there to the volume on the y-axis, so it can be seen where the value came from.

Question 4

A graph shows the volume of gas produced levelling off after four minutes. Describe what this shows and suggest why it happens.

The volume stops increasing and stays constant, showing the reaction has finished; this happens because one of the reactants, such as the calcium, has been fully used up.

Question 5

A graph of solubility against temperature shows the solubility falling as the temperature rises. Describe this trend.

As the temperature increases, the solubility decreases. Writing it the other way round, with the solubility named as the cause, would not be credited.

Question 6

A graph plots the volume of gas produced against the mass of magnesium reacted, using excess acid throughout, and gives a straight line through the origin. Describe the relationship.

The volume of gas produced is directly proportional to the mass of magnesium reacted, because the line is straight and passes through the origin.

Question 7

A graph shows the volume of gas at 1 minute as 32 cm³ and at 4 minutes as 48 cm³. Calculate the average rate of reaction between these times.

$\frac{48 - 32}{4 - 1} = 5.3 {cm}^{3} {min}^{- 1}$ .

Question 8

Why is a rate of zero a sensible answer for the section of a graph where the line has levelled off?

Because the volume is not changing over that stretch of time, so the change in volume, and therefore the rate, is genuinely zero; it does not mean the reading has gone wrong.

key terms

TermDefinition

InterpolationReading a value from a graph between the plotted points.

ExtrapolationReading a value from a graph beyond the range of the plotted points, by extending the line.

Directly proportionalA relationship between two quantities where doubling one doubles the other, so their graph is a straight line through the origin.

end of notes

Key points to remember

9.6 - Drawing ConclusionsNext up