Chemistry 5070/32 — May/June 2024
Cambridge O-Level · Practical Test · worked solutions for every part, with the mark scheme
Topics Experimental Contexts · Planning Experiments and Investigations · Use of Techniques, Apparatus and Materials · Analysis, Conclusions and Evaluation · Observations and Measurements · Qualitative Analysis
You are going to investigate the temperature change when a solid, , is dissolved in water.
Read all the instructions below carefully before starting the experiment.
Instructions
You are going to do one experiment.
- Use a measuring cylinder to add of distilled water to a beaker.
- Use a thermometer to measure the initial temperature of the water in the beaker.
- Record this temperature to the nearest in Table 1.1 at time .
- Add all the sample of solid to the beaker and start the stopwatch.
Do not stop the stopwatch until the whole experiment is complete. - Carefully stir the mixture.
- Measure the temperature after approximately and again after approximately .
- Record the times, to the nearest second, and the temperatures, to the nearest , in Table 1.1.
- Continue stirring the mixture.
- Measure the temperature at approximately intervals for a total of .
- Record the times, to the nearest second, and the temperatures, to the nearest , in Table 1.1.
Table 1.1
Answer
Complete Table 1.1 with your own readings as follows:
- time 0 s and the initial temperature of the water, recorded to the nearest 0.5 °C
- subsequent times recorded to the nearest second, starting at approximately 30 s and then at approximately 60 s intervals up to approximately 300 s
- every temperature recorded to one decimal place ending in .0 or .5
- the temperature decreases, then becomes constant or starts to increase after a period of time.
No specific readings are given because these are your own experimental results.
Completed table: initial temperature at 0 s; times to nearest second at 30 s then 60 s intervals to 300 s; temperatures to 0.5 °C; temperature decreases then becomes constant/increases.
Walkthrough
This part is about carrying out the experiment and recording results properly. The first row must show time 0 s and the initial temperature. The stopwatch is started when solid X is added, so all later times are measured from that moment. The instructions ask for readings at about 30 s and then every 60 s up to 300 s; each time should be recorded to the nearest second. Temperatures must be read to the nearest 0.5 °C, so a value such as 21.5 °C is acceptable but 21.3 °C is not. The mark scheme also expects the pattern: the temperature falls as X dissolves, then levels off or starts to rise again.
Key Takeaways
- A results table needs a time column and a temperature column with units.
- Readings must be recorded to the precision stated in the instructions.
- A cooling curve usually falls, then flattens as the contents return toward room temperature.
Common Mistakes
- Recording times as 0, 30, 60, 90, 120, 150, 180, 210, 240, 270, 300 without noting that the first reading after 0 is at 30 s, then every 60 s.
- Recording temperatures to two decimal places, e.g. 21.35 °C, when the instruction says nearest 0.5 °C.
- Writing temperatures without units.
Things to Be Careful About
- The mark scheme requires the initial temperature at time 0 s.
- Times must be to the nearest second; temperatures must end in .0 or .5.
- The temperature should decrease, then become constant or start to increase after a period.
Describe the appearance of the mixture in the beaker at the end of the experiment.
______
Answer
Colourless solution.
Colourless solution
Walkthrough
After the experiment, look at the contents of the beaker. The solid X has dissolved in water, so the mixture should be a clear, colourless solution with no solid particles remaining. The mark scheme accepts exactly this observation.
Key Takeaways
- Qualitative observations describe what is seen, not what is measured.
- A dissolved solid gives a solution; if any solid remained, the word 'suspension' or 'solid remaining' would be used.
Common Mistakes
- Writing 'clear' without 'colourless', or 'colourless' without 'solution'.
- Saying 'white precipitate' when the solid has dissolved.
Things to Be Careful About
- The answer should be a single observation: colourless solution.
Calculate the maximum temperature change in the experiment.
maximum temperature change = ______
Working
From your completed Table 1.1, find the highest temperature and the lowest temperature.
maximum temperature change = highest temperature − lowest temperature
Answer
(highest temperature − lowest temperature) °C, using your own readings.
Highest temperature − lowest temperature (°C)
Walkthrough
The maximum temperature change is the difference between the highest and lowest temperatures recorded. Since the temperature decreases, the highest value is likely the initial temperature at 0 s and the lowest value is the minimum reached later. Subtract the lower value from the higher value and give the answer in °C.
Key Takeaways
- Temperature change = higher temperature − lower temperature.
- The result has the unit °C.
Common Mistakes
- Subtracting the wrong way round, giving a negative value.
- Forgetting the unit °C.
Things to Be Careful About
- Use your own readings; there is no single correct number.
- The mark scheme says 'correctly subtracts the lowest temperature from the highest temperature'.
Answer
The temperature decreases at first, then becomes constant (or starts to increase again) after a period of time.
Temperature decreases, then becomes constant or starts to increase again.
Walkthrough
Look down the temperature column. The first clear trend is a decrease: the temperature falls as the solid dissolves. After some time the temperature stops falling; it either stays the same or begins to rise again as the mixture exchanges heat with the surroundings. Both parts are needed for full marks.
Key Takeaways
- A trend description should cover the whole set of results, not just one point.
- Dissolving can be endothermic, so the temperature falls; afterwards the contents return toward room temperature.
Common Mistakes
- Only saying 'temperature decreases' and missing the later levelling off or increase.
- Saying 'temperature increases' without mentioning the initial decrease.
Things to Be Careful About
- The mark scheme awards one mark for the decrease and one for becoming constant / starting to increase.
Suggest the temperature of the mixture if it is left for 60 minutes.
Explain your answer
temperature after 60 minutes = ______
explanation ______
Answer
temperature after 60 minutes = initial temperature (about room temperature)
Explanation: after the solid has fully dissolved, the mixture gradually returns to room temperature because it exchanges energy with the surroundings — it cools down if it was colder, or warms up if it was warmer.
Initial temperature (room temperature); returns to room temperature by heat exchange with surroundings.
Walkthrough
Once the solid has dissolved, no more energy is being taken in for dissolving. The beaker is open to the room, so the mixture will slowly gain or lose energy until it reaches the same temperature as the surroundings. That is why the temperature after 60 minutes should be close to the initial temperature recorded at time 0 s.
Key Takeaways
- A system left in contact with its surroundings eventually reaches room temperature.
- The initial temperature is a good estimate of room temperature.
Common Mistakes
- Predicting a temperature that keeps decreasing forever.
- Giving a temperature without explaining the return to room temperature.
Things to Be Careful About
- The mark scheme wants a temperature equal to the value at 0 s plus a reasonable explanation of returning to room temperature.
Describe the energy change when dissolves in water.
Explain how the results in Table 1.1 support your answer.
description ______
explanation ______
Answer
Description: the dissolving of X is endothermic — it absorbs energy from the surroundings.
Explanation: the temperature of the mixture decreases, which shows that energy is being taken in from the water and the surroundings during dissolving.
Endothermic; energy absorbed from surroundings; temperature decreases.
Walkthrough
When a solid dissolves, energy is either released (exothermic, temperature rises) or absorbed (endothermic, temperature falls). The table shows the temperature decreasing, so the process must be endothermic: the dissolving takes in energy from the water and the surroundings, cooling the mixture.
Key Takeaways
- A temperature decrease indicates an endothermic change.
- A temperature increase indicates an exothermic change.
- The results in the table are the evidence for the energy change.
Common Mistakes
- Writing 'exothermic' because dissolving is often thought to release energy.
- Saying 'temperature decreases' without linking it to energy absorption.
Things to Be Careful About
- The mark scheme requires both 'endothermic / absorbs energy' and 'temperature decreases'.
The maximum temperature change calculated is not the true value for this experiment.
This may be because the volume of water and the temperature are not measured precisely.
Answer
Pipette (or burette).
Pipette / burette
Walkthrough
A measuring cylinder measures volume only to about ±0.5 cm3. A pipette or burette can deliver a volume much more precisely, so either is an acceptable answer. A burette is also used for measuring variable volumes in titrations.
Key Takeaways
- Pipettes and burettes are more precise than measuring cylinders for measuring volumes.
- Precision means how closely repeated measurements agree and how finely the scale is divided.
Common Mistakes
- Writing 'beaker' — a beaker is not precise for measuring volume.
- Writing 'balance' — that measures mass, not volume.
Things to Be Careful About
- The mark scheme accepts pipette or burette.
Answer
Use a thermometer with smaller scale divisions, for example one graduated in 0.1 °C or 0.2 °C intervals, so the temperature can be read more precisely.
Use a thermometer graduated in intervals smaller than 1 °C (e.g. 0.1 °C).
Walkthrough
The precision of a temperature reading is limited by the size of the divisions on the thermometer. A thermometer marked in 1 °C intervals can only be read to about 0.5 °C. A thermometer graduated in 0.1 °C or 0.2 °C intervals allows a more precise reading.
Key Takeaways
- Smaller scale divisions give more precise readings.
- Precision is improved by choosing better apparatus, not by reading more carefully with the same apparatus.
Common Mistakes
- Saying 'read the thermometer more carefully' — the mark scheme wants a thermometer with smaller intervals.
- Saying 'use a digital thermometer' — acceptable only if it gives more decimal places, but the mark scheme specifically mentions smaller scale divisions.
Things to Be Careful About
- The mark scheme: 'use a thermometer graduated to less than 1 °C intervals'.
The maximum temperature change calculated is less than the true value for this experiment.
Suggest a reason for this, other than the precision of measurements.
Describe an improvement to the method which reduces this error.
reason ______
improvement ______
Answer
Reason: heat is gained from (or lost to) the surroundings during the experiment, so the measured temperature change is smaller than the true value.
Improvement: insulate the beaker, for example use a polystyrene cup, and/or cover it with a lid to reduce heat exchange with the surroundings.
Heat gain from surroundings; insulate/cover with lid/polystyrene beaker.
Walkthrough
The experiment is open to the room, so energy can move between the mixture and the surroundings. If the mixture is colder than the room, it gains heat; if it is warmer, it loses heat. This makes the measured maximum temperature change smaller than the true value. To reduce this error, insulate the beaker (polystyrene cup) or put a lid on it, so less heat is exchanged with the surroundings.
Key Takeaways
- Heat loss/gain to surroundings is a common source of error in temperature-change experiments.
- Insulation and lids reduce heat exchange.
- Improvements should directly address the identified error.
Common Mistakes
- Giving a reason about precision when the question says 'other than the precision of measurements'.
- Suggesting an improvement that does not reduce heat exchange, such as stirring faster.
Things to Be Careful About
- The mark scheme accepts 'heat gain (from the surroundings)' and 'more insulation / lid / use a polystyrene beaker'.
You are provided with solution .
You will do a series of experiments on .
You should:
- record your observations and conclusions for each of these experiments
- test and name any gases evolved.
To prepare for the experiment in (b), place depth of in a test-tube. Place a wooden splint into the test-tube and leave it while doing the experiments in (a).
To depth of in a boiling tube, add depth of dilute nitric acid. Keep this solution for use in (a)(ii) and (a)(iii).
observations ______
conclusions ______
Answer
Observations: effervescence / fizzing / bubbles; gas turns limewater milky.
Conclusions: carbon dioxide formed; solution contains carbonate / ions.
Observations: effervescence; gas turns limewater milky. Conclusions: carbon dioxide; carbonate ions present.
Walkthrough
R is an unknown solution. Adding dilute nitric acid is a test for carbonate ions. If carbonate is present, the acid reacts to release carbon dioxide gas, seen as effervescence (fizzing or bubbles). To confirm the gas is , it is bubbled through limewater, which turns milky or cloudy. The effervescence earns the first mark, the limewater result the second, and the conclusions — carbon dioxide formed and carbonate present — the third and fourth.
Key Takeaways
Carbonates react with acids to give carbon dioxide: . Carbon dioxide turns limewater milky. This is the standard test for carbonate ions.
Common Mistakes
- Not testing the gas with limewater — just saying "bubbles" is not enough to identify .
- Writing "carbonate" without the gas test evidence.
- Confusing the observation (effervescence) with the conclusion (carbonate present).
Things to Be Careful About
The observation and conclusion are separate marks. The gas must be tested with limewater to confirm it is before concluding carbonate is present.
To depth of the solution from (a)(i) in a test-tube, add depth of aqueous barium nitrate.
observations ______
conclusions ______
Answer
Observations: white precipitate formed.
Conclusions: solution contains sulfate / ions.
White precipitate formed; sulfate ions present.
Walkthrough
The solution from (a)(i) has been acidified with nitric acid. Adding barium nitrate is the test for sulfate ions. A white precipitate of barium sulfate forms if sulfate is present. The white precipitate is the observation; the conclusion is that sulfate ions are present.
Key Takeaways
(white precipitate). Barium nitrate is the standard test for sulfate ions. The solution is acidified with nitric acid first to prevent carbonate interference.
Common Mistakes
- Confusing sulfate with sulfite or sulfide.
- Not recognising that the nitric acid from (a)(i) acidifies the solution.
Things to Be Careful About
The white precipitate must be noted as the observation. The conclusion names the sulfate ion.
To depth of the solution from (a)(i) in a test-tube, add depth of aqueous silver nitrate.
observations ______
conclusions ______
Answer
Observations: colourless solution / no change / stays colourless.
Conclusions: no halide ions present.
Colourless solution / no change; no halide ions present.
Walkthrough
Adding silver nitrate to the acidified solution tests for halide ions. If chloride, bromide or iodide were present, a precipitate of the silver halide would form. Here no precipitate forms — the solution stays colourless — so no halide ions are present.
Key Takeaways
(precipitate). No precipitate means no halide. The solution was acidified with nitric acid, which is correct for the halide test.
Common Mistakes
- Expecting a precipitate when there is none.
- Not recording "no change" as a valid observation.
Things to Be Careful About
"Colourless solution" or "no change" is the correct observation. The conclusion is that no halide ions are present.
Place the end of the wooden splint which has been in into the flame of a Bunsen burner with the air hole open. Record the first flame colour seen.
first flame colour seen ______
conclusions ______
Answer
First flame colour seen: yellow flame produced.
Conclusions: solution contains sodium ions / .
Yellow flame; sodium ions present.
Walkthrough
The wooden splint was left in R, so it is coated with the solution. Placing it in a blue Bunsen flame vaporises the solution and the metal ions colour the flame. A yellow flame indicates sodium ions.
Key Takeaways
Sodium compounds give a yellow flame in the flame test. This is the standard test for sodium ions.
Common Mistakes
- Not recording the first flame colour.
- Using a yellow Bunsen flame (which would mask the result).
Things to Be Careful About
The first flame colour seen is the important one. Yellow flame = sodium.
Explain why the air hole on the Bunsen burner must be open when doing this flame test.
______
Answer
The Bunsen flame is blue with the air hole open, so the yellow Bunsen flame does not mask the flame colour.
The Bunsen flame is blue with the air hole open, so the yellow flame does not mask the test colour.
Walkthrough
With the air hole open, the Bunsen burner gives a blue flame. A yellow flame (air hole closed) would itself be yellow, masking the yellow sodium flame. So the air hole must be open to see the true flame colour.
Key Takeaways
A blue Bunsen flame is needed for flame tests so the yellow flame does not mask the test colour.
Common Mistakes
- Saying "to make the flame hotter" — the key point is the masking.
- Not mentioning the yellow flame masking.
Things to Be Careful About
The mark is for the masking idea: the Bunsen flame must be blue / not yellow.
Solution is made from a mixture of two different ionic compounds.
Suggest the names of these two compounds.
______
Answer
Sodium sulfate and sodium carbonate.
Sodium sulfate and sodium carbonate.
Walkthrough
Combining the results: (a)(i) showed carbonate ions, (a)(ii) showed sulfate ions, (b)(i) showed sodium ions. R is made from two ionic compounds, so the two compounds are sodium carbonate and sodium sulfate.
Key Takeaways
Multiple qualitative tests can be combined to identify the ions present and hence the compounds.
Common Mistakes
- Naming only one compound.
- Naming compounds with the wrong ions.
Things to Be Careful About
Both compounds must be named. The ions come from the three tests.
A student tests a different solution, , and finds it difficult to decide whether the solution contains chloride ions or bromide ions.
The student also has aqueous potassium chloride and aqueous potassium bromide.
Suggest how the student could use the aqueous potassium chloride and aqueous potassium bromide to make it easier to decide whether contains chloride ions or bromide ions.
______
Answer
Do the silver nitrate test on the aqueous potassium chloride and aqueous potassium bromide. Compare the colour of the precipitates with the precipitate from solution P.
Do the silver nitrate test on KCl and KBr, compare the precipitate colours with P.
Walkthrough
To decide whether P contains chloride or bromide, the student should do the silver nitrate test on known samples. Adding silver nitrate to potassium chloride gives a white precipitate (), while with potassium bromide it gives a cream precipitate (). Comparing the precipitate from P with these known colours tells which halide is present.
Key Takeaways
Silver halide precipitates have different colours: white, cream, yellow. Comparing with known samples removes doubt.
Common Mistakes
- Not comparing with known samples.
- Only testing P without a reference.
Things to Be Careful About
The comparison must be with known chloride and bromide samples.
The student adds dilute hydrochloric acid to another solution and a gas is produced. The gas is passed through limewater.
Describe how the gas can be passed through limewater.
You may draw a labelled diagram to help answer the question.
Answer
Use a delivery tube from the reaction vessel to bubble the gas through the limewater.
Use a delivery tube to bubble the gas through the limewater.
Walkthrough
To pass the gas through limewater, use a delivery tube connected to the reaction vessel. The gas travels through the tube and bubbles through the limewater in a test-tube. If the gas is , the limewater turns milky.
Key Takeaways
A delivery tube is used to pass gas into a test reagent. The gas bubbles through the limewater.
Common Mistakes
- Not using a delivery tube.
- Just holding limewater near the reaction.
Things to Be Careful About
The gas must be bubbled through the limewater, not just brought near it.
You are not expected to do any experimental work for this question.
Copper(II) carbonate reacts with dilute sulfuric acid at room temperature.
The word equation for the reaction is shown.
Plan an experiment to determine the volume of carbon dioxide formed when a known mass of copper(II) carbonate completely reacts with dilute sulfuric acid.
Your plan must include the use of common laboratory apparatus, dilute sulfuric acid and copper(II) carbonate. No other chemicals should be used.
Your plan must include:
- the apparatus needed
- the method to use and the measurements to take
- procedures to ensure that the volume measured is as accurate as possible.
You may draw a diagram to help answer the question.
Answer
Apparatus
- (conical) flask fitted with a bung and delivery tube
- gas syringe (or an inverted measuring cylinder/burette in a trough of water)
- balance
- known mass of copper(II) carbonate and excess dilute sulfuric acid
Method and measurements
- Weigh a known mass of copper(II) carbonate on a balance and place it in the conical flask.
- Add an excess of dilute sulfuric acid so that all the carbonate reacts.
- Seal the flask with the bung and connect the delivery tube to the gas syringe.
- Start the reaction and wait until no more effervescence is seen and the volume reading is constant (no solid remains).
- Record the final volume of collected.
- Repeat the experiment and calculate the average volume.
Accuracy
- Use a bung/cork to prevent gas escaping.
- Add the acid without opening the flask, e.g. by placing it in a small test tube on a string inside the flask and tipping the flask, or put the bung on quickly after adding the acid.
- Wait until the reaction is complete before taking the reading.
- Repeat and take an average to improve reliability.
Plan: react a known mass of copper(II) carbonate with excess dilute sulfuric acid in a sealed flask, collect the carbon dioxide in a gas syringe, wait until the reaction is complete, repeat and average the volume.
Walkthrough
This is a planning question, so the answer must be a complete method that another student could follow. The mark scheme gives up to two marks in each of three sections: apparatus, method and measurements, and accuracy. To get all 6 marks, include at least two valid points from each section.
First, choose a container for the reaction: a conical flask is ideal because it can be sealed with a bung. The gas must be collected and measured, so choose a gas syringe or collect over water in an inverted measuring cylinder or burette. A balance is needed to measure the mass of copper(II) carbonate.
The method must include measuring the mass of the carbonate, using excess acid so that all the carbonate reacts, and measuring the volume of gas produced. The flask must be sealed with a bung to stop gas escaping. The acid can be added without opening the flask by using a small test tube on a string, or the bung can be replaced quickly. Wait until no more effervescence and the volume is constant, so the reaction is complete. Repeat and average to improve reliability.
Key Takeaways
- A plan needs apparatus, method, measurements, and accuracy points.
- Excess reactant ensures complete reaction of the limiting reactant.
- Gas collection must be gas-tight and measured with suitable apparatus.
- Waiting for completion and repeating improve accuracy/reliability.
Common Mistakes
- Not naming the container or gas-collection apparatus (M1/M2 lost).
- Forgetting to measure the mass of the carbonate (M3).
- Using insufficient acid, so not all carbonate reacts (M4).
- Not sealing the flask, allowing gas to escape (M5).
- Not waiting until reaction complete before reading volume (M8).
- Not repeating the experiment (M9).
Things to Be Careful About
- The mark scheme says maximum two marks per section; include at least two points in each.
- No other chemicals should be used, so do not suggest an indicator or catalyst.
- The volume measured should be the volume of carbon dioxide, not the volume of acid.
- If using a measuring cylinder over water, read the volume at eye level and allow the water level to settle.
- "Repeat and average" is an accuracy/reliability point, not just a method step.