A student does a series of tests using aqueous solution P and aqueous solution Q . (a) The tests the student does on P are shown in Table 3.1. Some of the observations for these tests are also shown. Table 3.1 test number test observations --- --- --- 1 flame test orange-red flame 2 add aqueous ammonia (i) Describe how the student does a flame test. \_\_\_\_\_\_ (ii) Describe one observation expected in test 2 based on the observation in test 1. \_\_\_\_\_\_ (iii) Describe…

(a)(i) Dip a wire/splint into P and place in a blue/roaring flame · (a)(ii) No precipitate (or slight white precipitate) · (a)(iii) test = add aqueous sodium hydroxide; result = white precipitate, insoluble in excess · (a)(iv) calcium / Ca2+ · (b)(i) red / orange / yellow · (b)(ii) effervescence · (b)(iii) hydrogen / H2 · (b)(iv) test = lighted splint; result = 'pop' sound · (b)(v) cation = hydrogen / H+; anion = sulfate / SO42-

Chemistry 5070/42 — October/November 2025

Cambridge O-Level · Alternative to Practical · worked solutions for every part, with the mark scheme

questions

marks

minutes

Topics Experimental Contexts · Use of Techniques, Apparatus and Materials · Analysis, Conclusions and Evaluation · Planning Experiments and Investigations · Observations and Measurements · Qualitative Analysis

Q16MMedium-EasyUse of Techniques, Apparatus and MaterialsExperimental ContextsAnalysis, Conclusions and EvaluationPlanning Experiments and Investigations

A student investigates the rate of a reaction.

Fig. 1.1 shows the apparatus the student uses.

Fig. 1.1

(a)

State the name of the piece of apparatus labelled A in Fig. 1.1.

______

1M Similar

(b)

The student adds a fixed volume and concentration of dilute hydrochloric acid to A, swirls the mixture in A and places it back on the cross.

The mixture in the flask slowly turns cloudy.
The student records the time taken for the solution to become so cloudy that the cross is no longer visible through the mixture.

State the name of a piece of apparatus needed which is not shown in Fig. 1.1.

______

1M Similar

(c)

The student repeats the experiment with the same fixed volume and concentration of dilute hydrochloric acid and with increasing concentrations of aqueous sodium thiosulfate, using a new cross drawn on paper for each experiment.

Explain why drawing a new cross on paper in each experiment reduces the accuracy of the results.

______

2M Similar

(d)

The student then:

uses the same cross drawn on paper
repeats the experiment with the same fixed volume and concentration of dilute hydrochloric acid and with increasing concentrations of aqueous sodium thiosulfate.

Suggest the effect of increasing the concentration of aqueous sodium thiosulfate on the time taken for the cross to become no longer visible.

______

1M Similar

(e)

Sulfur dioxide gas is produced during this reaction.

Sulfur dioxide causes breathing difficulties in humans.

Suggest one precaution necessary in this investigation to avoid breathing difficulties.

______

1M Similar

Q214MMediumObservations and MeasurementsExperimental ContextsAnalysis, Conclusions and EvaluationUse of Techniques, Apparatus and Materials

A student investigates the temperature changes when an acid neutralises aqueous potassium hydroxide.

The ionic equation for the reaction is shown.

\text{H}^+ + \text{OH}^- \rightarrow \text{H}_2\text{O}

The reaction is exothermic.

The temperature change is used to determine the concentration of hydrogen ions in an acid.

X is an acid of concentration $0.80\text{ mol / dm}^3$
Y is $2.0\text{ mol / dm}^3$ potassium hydroxide

The student:

step 1 fills the burette with Y
step 2 stands a plastic cup inside a beaker
step 3 uses a volumetric pipette to put $25.0\text{ cm}^3$ of X into the plastic cup
step 4 measures the initial temperature of X and records it in Table 2.1
step 5 adds $5.0\text{ cm}^3$ of Y from the burette to the plastic cup
step 6 stirs the mixture for 30 seconds and records in Table 2.1 the temperature of the mixture
step 7 repeats steps 5 and 6 until a total of $40.0\text{ cm}^3$ of Y is added.

Fig. 2.1 on page 6 shows two of the thermometer readings in $^\circ\text{C}$ during the experiment.

(a)

Complete Table 2.1.

You should:

record the student's results for $10.0\text{ cm}^3$ and $35.0\text{ cm}^3$ using Fig. 2.1
subtract the initial temperature of X from each temperature recorded to determine the temperature change.

All temperatures and temperature changes should be recorded to $+/- 0.5\,^\circ\text{C}$ .

Fig. 2.1

Table 2.1

volume of Y / $\text{cm}^3$	temperature / $^\circ\text{C}$	temperature change / $^\circ\text{C}$
0.0	20.5	0.0
5.0	23.5
10.0
15.0	29.5
20.0	31.5
25.0	30.5
30.0	29.0
35.0
40.0	26.5

4M Similar

DifficultyMedium-Easy

Worked solution

Answer

volume of Y / $\text{cm}^3$	temperature / $^\circ\text{C}$	temperature change / $^\circ\text{C}$
0.0	20.5	0.0
5.0	23.5	3.0
10.0	27.0	6.5
15.0	29.5	9.0
20.0	31.5	11.0
25.0	30.5	10.0
30.0	29.0	8.5
35.0	27.5	7.0
40.0	26.5	6.0

Final answer

Completed Table 2.1 with temperatures 27.0 and 27.5 °C and corresponding temperature changes 3.0, 6.5, 9.0, 11.0, 10.0, 8.5, 7.0, 6.0 °C

Detailed explanation

Walkthrough

Reading thermometer for $10.0\text{ cm}^3$ of Y:
Looking at the first thermometer in Fig. 2.1, the scale between $20\,^\circ\text{C}$ and $30\,^\circ\text{C}$ has 10 divisions, meaning each major subdivision represents $1.0\,^\circ\text{C}$ . The liquid level is exactly at the line for $27.0\,^\circ\text{C}$ .
Reading thermometer for $35.0\text{ cm}^3$ of Y:
Looking at the second thermometer in Fig. 2.1, the liquid level is midway between the $27.0\,^\circ\text{C}$ and $28.0\,^\circ\text{C}$ lines, which corresponds to $27.5\,^\circ\text{C}$ .
Calculating temperature change:
The initial temperature of acid X at $0.0\text{ cm}^3$ is $20.5\,^\circ\text{C}$ . Each temperature change is calculated by subtracting $20.5\,^\circ\text{C}$ from the recorded temperature:
- For $5.0\text{ cm}^3$ : $23.5 - 20.5 = 3.0\,^\circ\text{C}$
- For $10.0\text{ cm}^3$ : $27.0 - 20.5 = 6.5\,^\circ\text{C}$
- For $15.0\text{ cm}^3$ : $29.5 - 20.5 = 9.0\,^\circ\text{C}$
- For $20.0\text{ cm}^3$ : $31.5 - 20.5 = 11.0\,^\circ\text{C}$
- For $25.0\text{ cm}^3$ : $30.5 - 20.5 = 10.0\,^\circ\text{C}$
- For $30.0\text{ cm}^3$ : $29.0 - 20.5 = 8.5\,^\circ\text{C}$
- For $35.0\text{ cm}^3$ : $27.5 - 20.5 = 7.0\,^\circ\text{C}$
- For $40.0\text{ cm}^3$ : $26.5 - 20.5 = 6.0\,^\circ\text{C}$
All values must be recorded to the nearest $0.5\,^\circ\text{C}$ (i.e. ending in $.0$ or $.5$ ).

Key Takeaways

When taking measurements from analogue scales, read carefully using the smallest graduation.
Maintain consistent decimal precision across the entire data table.

Common Mistakes

Writing whole integers (e.g. $3$ or $11$ ) without the trailing $.0$ (e.g. $3.0$ or $11.0$ ).
Subtracting from previous row temperatures instead of the initial temperature ( $20.5\,^\circ\text{C}$ ).

Things to Be Careful About

Ensure every single calculated value ends with either $.0$ or $.5$ as requested by the $\pm 0.5\,^\circ\text{C}$ precision instruction.

Techniques used

read thermometer scales with appropriate precisioncalculate temperature changes by subtractionrecord data consistently to one decimal place

(b)

Plot a graph of the temperature change ( $y$ -axis) against the volume of Y ( $x$ -axis) on Fig. 2.2.

Draw a line of best fit through the points where the temperature change is increasing.

Draw a line of best fit through the points where the temperature change is decreasing.

Extend both lines so that they cross.

Fig. 2.2

5M Similar

DifficultyMedium

Worked solution

Answer

$y$ -axis labelled as temperature change / °C.
Suitable linear scale chosen for the $y$ -axis starting at $0.0$ and going up to at least $12.0\,^\circ\text{C}$ (e.g., $2\text{ cm} = 2.0\,^\circ\text{C}$ ), so plotted points occupy more than half of the vertical grid.
All 9 points correctly plotted to within $\pm\frac{1}{2}$ small square:
$(0.0, 0.0)$ , $(5.0, 3.0)$ , $(10.0, 6.5)$ , $(15.0, 9.0)$ , $(20.0, 11.0)$ , $(25.0, 10.0)$ , $(30.0, 8.5)$ , $(35.0, 7.0)$ , $(40.0, 6.0)$ .
One straight line of best fit drawn through the increasing points (from $0.0$ to $20.0\text{ cm}^3$ ).
A second straight line of best fit drawn through the decreasing points (from $25.0$ to $40.0\text{ cm}^3$ ).
Both straight lines extended until they cross cleanly at an intersection point.

Final answer

Graph plotted with labelled axes, correct points, and two intersecting straight lines of best fit.

Detailed explanation

Walkthrough

Labelling and Scaling:
- The $x$ -axis is pre-labelled volume of Y / cm³ from $0$ to $40$ .
- Label the $y$ -axis with the quantity and unit: temperature change / °C.
- Choose an appropriate linear scale for the $y$ -axis so the maximum value ( $11.0\,^\circ\text{C}$ ) occupies more than half of the vertical height of the grid (e.g. $1$ large square $= 1.0\,^\circ\text{C}$ or $2\text{ cm} = 2.0\,^\circ\text{C}$ ).
Plotting Points:
- Plot all 9 pairs of values $(x, y)$ precisely using small crosses $(\times)$ or encircled dots $(\odot)$ to within $\pm\frac{1}{2}$ small square.
Drawing Best-Fit Lines:
- Thermometric titrations produce two distinct regions: an increasing temperature phase while neutralisation occurs (exothermic reaction generating heat), followed by a cooling/dilution phase once all acid has reacted and excess cold alkali is added.
- Draw one straight line using a ruler through the rising points.
- Draw another straight line through the falling points.
- Extend both lines until they intersect. The point of intersection represents the exact equivalence (neutralisation) point.

Key Takeaways

In thermometric titrations, two intersecting straight lines represent the reaction phase and the post-equivalence dilution phase.
Scales must be linear and utilize at least half the available grid space.

Common Mistakes

Drawing a single smooth curved line instead of two distinct intersecting straight lines.
Forgetting units on the $y$ -axis label.
Joining points dot-to-dot instead of using a line of best fit.

Things to Be Careful About

Ensure ruler lines are drawn sharply and extended sufficiently so the intersection point is clearly visible.

Techniques used

select and label linear graph axes with unitsplot experimental data points accuratelydraw intersecting straight lines of best fit

(c)

Find the point on the graph where the two lines cross.

Determine the volume of Y at this point.

volume of Y = ______ $\text{cm}^3$

1M Similar

(d)

Y is $2.0\text{ mol / dm}^3$ potassium hydroxide.

The formula of potassium hydroxide is $\text{KOH}$ .

Calculate the number of moles of hydroxide ions, $\text{OH}^-$ , in the volume of Y in (c).

number of moles = ______

1M Similar

(e)

The volume of Y in (c) is the volume needed to completely neutralise $25.0\text{ cm}^3$ of X.

Use your answer to (d) to calculate the concentration of hydrogen ions, $\text{H}^+$ , in X.

\text{H}^+ + \text{OH}^- \rightarrow \text{H}_2\text{O}

concentration = ______ $\text{mol / dm}^3$

1M Similar

DifficultyMedium-Easy

Worked solution

Working

From the equation $\text{H}^+ + \text{OH}^- \rightarrow \text{H}_2\text{O}$ , the mole ratio of $\text{H}^+$ to $\text{OH}^-$ is $1 : 1$ .

\text{moles of } \text{H}^+ = \text{moles of } \text{OH}^- = 0.040\text{ mol}

\text{concentration of } \text{H}^+ = \frac{\text{moles}}{\text{volume in dm}^3} = \frac{0.040}{\frac{25.0}{1000}} = 0.040 \times 40 = 1.6\text{ mol / dm}^3

Answer

$1.6\text{ mol / dm}^3$

Final answer

1.6

Detailed explanation

Walkthrough

From the ionic equation $\text{H}^+ + \text{OH}^- \rightarrow \text{H}_2\text{O}$ , $1\text{ mol}$ of $\text{OH}^-$ neutralises $1\text{ mol}$ of $\text{H}^+$ .
Therefore, moles of $\text{H}^+$ in $25.0\text{ cm}^3$ of acid X = moles of $\text{OH}^-$ from part (d) = $0.040\text{ mol}$ .
Calculate the concentration of $\text{H}^+$ ions: $[\text{H}^+] = \frac{\text{moles}}{\text{volume (dm}^3\text{)}} = \frac{0.040}{0.0250} = 1.6\text{ mol / dm}^3$ (Alternatively, $\text{moles from (d)} \times 40 = 0.040 \times 40 = 1.6\text{ mol / dm}^3$ ).

Key Takeaways

In a $1 : 1$ neutralisation, $\text{moles of } \text{H}^+ = \text{moles of } \text{OH}^-$ .
$\text{Concentration} = \frac{\text{moles}}{\text{volume (dm}^3\text{)}}$ .

Common Mistakes

Dividing by $25$ without converting to $\text{dm}^3$ .
Using the concentration of acid X given in the stem ( $0.80\text{ mol / dm}^3$ ) instead of calculating the concentration of hydrogen ions from the experimental results.

Things to Be Careful About

Allow error carried forward (ecf) from part (d): $\text{answer from (d)} \times 40$ .

Techniques used

apply stoichiometry of neutralisationcalculate concentration from moles and volume

(f)

Suggest why $25\text{ cm}^3$ of X is measured using a volumetric pipette and not a measuring cylinder.

______

1M Similar

(g)

Suggest why the mixture is stirred in step 6.

______

1M Similar

Q314MMedium-EasyQualitative AnalysisUse of Techniques, Apparatus and MaterialsExperimental ContextsObservations and Measurements

A student does a series of tests using aqueous solution P and aqueous solution Q.

(a)

The tests the student does on P are shown in Table 3.1.

Some of the observations for these tests are also shown.

Table 3.1

test number	test	observations
1	flame test	orange-red flame
2	add aqueous ammonia

(i)

Describe how the student does a flame test.

______

2M Similar

(ii)

Describe one observation expected in test 2 based on the observation in test 1.

______

1M Similar

(iii)

Describe one additional test to confirm the identity of the cation in P.

Include the result of a positive test.

test = ______
result = ______

3M Similar

(iv)

Identify the cation in P.

cation = ______

1M Similar

(b)

Solution Q is acidic.

The tests the student does on Q are shown in Table 3.2.

Some of the observations for these tests are also shown.

Table 3.2

test number	test	observations
3	add five drops of universal indicator	colour change
4	add a piece of magnesium ribbon	magnesium ribbon disappears
5	add dilute nitric acid followed by aqueous barium nitrate	white precipitate

(i)

Describe the expected colour change in test 3.

______

1M Similar

(ii)

A gas is produced in test 4.

Predict one other observation made by the student in test 4.

______

1M Similar

(iii)

Predict the identity of the gas produced in test 4.

______

1M Similar

(iv)

Describe a test to confirm the identity of this gas.

Include the result of a positive test.

test = ______
result = ______

2M Similar

(v)

Q contains one cation and one anion.

Identify the cation and the anion in Q.

cation = ______ anion = ______

2M Similar

Q46MMediumPlanning Experiments and InvestigationsExperimental Contexts

The reaction between a metal and dilute sulfuric acid is exothermic.

Plan an experiment to determine which metal, magnesium or zinc, releases more thermal energy per gram of metal when reacting with excess dilute sulfuric acid.

You are provided with magnesium powder, zinc powder, dilute sulfuric acid and common laboratory apparatus. No other chemicals should be used.

Your plan should include:

the apparatus needed
the method to use and the measurements to take
how the measurements are used to determine which metal releases more thermal energy per gram.

You may draw a diagram to help answer the question.

DifficultyMedium

Worked solution

Answer

Apparatus:

Polystyrene cup with a lid (and beaker to support it)
Measuring cylinder (or pipette / burette)
Thermometer
Balance

Method and Measurements:

Measure a known volume (e.g. $25\text{ cm}^3$ ) of dilute sulfuric acid using a measuring cylinder and pour it into a polystyrene cup.
Measure and record the initial temperature of the acid using a thermometer.
Weigh a known mass (e.g. $0.50\text{ g}$ ) of magnesium powder using a balance.
Add the magnesium powder to the acid, stir the mixture, and record the maximum temperature reached (or temperature when effervescence stops).
Repeat the entire procedure using the same volume of dilute sulfuric acid and the zinc powder (either using the same mass or a measured mass of zinc).

Determination / Processing:

Calculate the temperature rise for each metal: $\Delta T = \text{maximum temperature} - \text{initial temperature}$
Calculate the temperature rise per gram of metal: $\frac{\Delta T}{\text{mass of metal}}$
The metal with the greater value of $\frac{\Delta T}{\text{mass}}$ releases more thermal energy per gram.

Final answer

See working

Detailed explanation

Walkthrough

To compare the thermal energy released per gram of two different metals reacting with excess acid, we design a simple calorimetry experiment using a polystyrene cup:

Apparatus selection:
- A measuring cylinder, burette, or pipette to measure the volume of dilute sulfuric acid.
- A thermometer to measure the initial and maximum temperature.
- A balance to determine the mass of metal powder used.
- An insulated container such as a polystyrene cup with a lid to reduce heat loss to the surroundings.
Experimental procedure & variables controlled:
- Measure a fixed volume of sulfuric acid (which must be in excess) and record its initial temperature.
- Weigh a known mass of magnesium powder.
- Add the metal to the acid, stir continuously to ensure complete mixing, and monitor the temperature until it reaches its highest point (when effervescence stops and temperature stops rising).
- Repeat the exact same method with zinc powder, keeping the acid volume and concentration constant.
Processing results:
- Find the temperature change $\Delta T = T_{\text{final}} - T_{\text{initial}}$ .
- Calculate the energy released per gram by dividing the temperature change by the mass of metal used ( $\frac{\Delta T}{m}$ ), or directly compare $\Delta T$ if identical masses were weighed out.
- The metal with the larger value of $\frac{\Delta T}{m}$ (or larger $\Delta T$ for the same mass) releases more thermal energy per gram.

Key Takeaways

Calorimetry experiments require controlling the volume of liquid and measuring both initial and peak temperatures.
To determine energy per gram, either the mass must be held constant or the temperature rise must be divided by the mass used ( $\frac{\Delta T}{\text{mass}}$ ).
Highlighting the exact point when the reaction finishes (maximum temperature reached / effervescence ceases) is essential for accurate thermal measurements.

Common Mistakes

Forgetting to specify measuring the initial temperature of the acid before adding the metal.
Omitting the specific measuring apparatus needed (e.g. stating 'measure acid' without naming a measuring cylinder or pipette).
Failing to state how the final conclusion is reached from the data gathered.

Things to Be Careful About

Ensure sulfuric acid is present in excess so that all of the weighed metal reacts completely.
Clearly mention repeating the experiment with the second metal (zinc) under the same conditions.

Techniques used

design a calorimetry investigationselect appropriate measuring apparatuscontrol experimental variablesprocess temperature change and mass data