O Level Chemistry 5070 Paper 42, Oct/Nov 2025

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

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.

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.

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.

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.

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 $\mathbf{X}$ from each temperature recorded to determine the temperature change. All temperatures and temperature changes should be recorded to $+\mathrm{/}-0.5{}^{\circ }\text{C}$.

Plot a graph of the temperature change (y-axis) against the volume of $\mathbf{Y}$ (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.

Find the point on the graph where the two lines cross. Determine the volume of $\mathbf{Y}$ at this point. volume of $\mathbf{Y}$ .................................................. ${\text{cm}}^3$

$\mathbf{Y}$ is $2.0{\text{ mol/dm}}^3$ potassium hydroxide. The formula of potassium hydroxide is $\mathrm{K}\mathrm{O}\mathrm{H}$. Calculate the number of moles of hydroxide ions, ${\mathrm{O}\mathrm{H}}^{-}$, in the volume of $\mathbf{Y}$ in (c). number of moles .........................................................

The volume of $\mathbf{Y}$ in (c) is the volume needed to completely neutralise $25.0{\text{ cm}}^3$ of $\mathbf{X}$. Use your answer to (d) to calculate the concentration of hydrogen ions, ${\mathrm{H}}^{+}$, in $\mathbf{X}$. $${\mathrm{H}}^{+}+{\mathrm{O}\mathrm{H}}^{-}\to {\mathrm{H}}_2\mathrm{O}$$ concentration .......................................... ${\text{mol/dm}}^3$

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

Suggest why the mixture is stirred in step 6.

The tests the student does on $\mathbf{P}$ are shown in Table 3.1. Some of the observations for these tests are also shown. Table 3.1

Describe how the student does a flame test.

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

Describe one additional test to confirm the identity of the cation in $\mathbf{P}$. Include the result of a positive test. test .................................................................................................................................... ........................................................................................................................................... result .................................................................................................................................

Identify the cation in $\mathbf{P}$. cation ...........................................................................................................................

Solution $\mathbf{Q}$ is acidic. The tests the student does on $\mathbf{Q}$ are shown in Table 3.2. Some of the observations for these tests are also shown. Table 3.2

Describe the expected colour change in test 3.

A gas is produced in test 4. Predict one other observation made by the student in test 4.

Predict the identity of the gas produced in test 4.

Describe a test to confirm the identity of this gas. Include the result of a positive test. test .................................................................................................................................... result .................................................................................................................................

$\mathbf{Q}$ contains one cation and one anion. Identify the cation and the anion in $\mathbf{Q}$. cation ........................................................ anion ........................................................