OCR GCSE Combined Science
Topic questions Chemical analysis Pure substances
9 questions · 39 marks · about 45 minutes
Which statement describes a pure substance?
A A mixture of substances which melts at a single temperature.
B A mixture of substances which melts over a range of temperatures.
C One substance which melts at a single temperature.
D One substance which melts over a range of temperatures.
Which statement describes a pure substance?
A A mixture of substances which melts at a single temperature.
B A mixture of substances which melts over a range of temperatures.
C One substance which melts at a single temperature.
D One substance which melts over a range of temperatures.
A student investigates mixtures of salt and water.
The student has a solution of salt dissolved in water. The diagram shows how they separate pure water from the solution.
What name is given to this method of separation? Tick (✓) one box.
The particle diagrams represent changes of state.
Which diagram represents what happens in the boiling tube?
Tick (✓) one box.
After several minutes the glass tube becomes hot, and no more water is collected in the test tube.
Give one reason why the glass tube becomes hot.
The student wants the experiment to work for longer.
What other piece of equipment can they use instead of the glass tube?
The student also investigates how adding salt to ice changes the melting point of the ice.
The table shows their results.
| Mass of salt (g) | Melting point (°C) |
| 1 | – 2 |
| 2 | – 4 |
| 3 | – 6 |
| 4 | – 8 |
The student draws a bar chart of the results.
Draw a bar on the bar chart for the results when 4 g of salt are added to the ice.
Describe how changing the mass of salt changes the melting point.
The student thinks the results show that pure ice will have a melting point of 0 
C.
Explain why the student is correct.
The graph shows how the temperature of two samples of a substance, Y and Z, changes as they melt from solids to liquids.
Which row describes Y and Z?
| Y | Z | |
| A | impure | impure |
| B | impure | pure |
| C | pure | impure |
| D | pure | pure |
Which substance is described as pure by a scientist?
A A solution of sodium chloride
B An alloy
C Dilute sulfuric acid
D Distilled water
Which substance is described as pure by a scientist?
A A solution of sodium chloride
B An alloy
C Dilute sulfuric acid
D Distilled water
Complete each sentence to describe the formation of nitrogen dioxide.
Use the words or phrases in the list.
The molecules of nitrogen and oxygen........
The atoms then........ to form nitrogen dioxide.
This is called a........
The equation shows the formation of nitrogen dioxide.
Complete the balanced symbol equation for the reaction.
Use Fig. 11.1.
The table shows the melting point and boiling point of pure nitrogen dioxide.
| Melting point (°C) | Boiling point (°C) |
| –9 | 21 |
Which state symbol shows the physical state of nitrogen dioxide at –2 °C?
Tick (✓) one box.
A scientist measures the melting point of a sample of nitrogen dioxide.
The result shows that the sample of nitrogen dioxide is impure.
Which temperature could be the melting point of the impure nitrogen dioxide?
Tick (✓) one box.
A molecule of nitrogen dioxide has the formula NO2.
Calculate the relative formula mass for nitrogen dioxide.
Relative atomic mass (Ar): N = 14.0 O = 16.0
Nitrogen dioxide is made from the reaction of nitrogen with oxygen.
Fig. 11.2 shows the bonding in a molecule of oxygen, O2.
Fig. 11.2
Fig. 11.3 shows an incomplete dot and cross diagram for oxygen.
Fig. 11.3
Complete Fig. 11.3 to show the arrangement of the electrons in a molecule of oxygen.
A heating curve shows how the temperature of a substance changes as it is heated.
Fig. 14.1 shows the heating curve for ice.
Look at the sentences, A – D. They describe sections 1, 2, 3 and 4 of the heating curve in Fig. 14.1.
A Heat energy is used to break the forces between the particles.
B Heat energy is used to loosen the forces between the particles.
C Heat energy is used to make the particles move past each other more.
D Heat energy is used to make the particles vibrate faster.
Match the sentences A, B, C or D to the parts of the heating curve labelled 1, 2, 3 and 4 putting the correct letter in each box.
A student wants to produce the heating curve for ice.
Fig. 14.2 shows the equipment the student uses.
The student slowly heats up the ice and measures the temperature every minute.
The student found they could not produce part X of the heating curve in Fig. 14.1.
Explain why.
The table shows the melting points of four different substances, A – D.
| Substance | Melting point (°C) |
| A | –14 |
| B | 74–79 |
| C | 89 |
| D | 121–123 |
State the letter, A, B, C or D, of one pure substance in the table.
State a reason for your answer to (c)(i) using information from the table.
Aspirin is a widely used painkiller and is sold as tablets.
One of the chemicals in aspirin is salicylic acid. The structure of salicylic acid is shown below.
Salicylic acid
What is the empirical formula of salicylic acid?
It is important that aspirin is pure for it to be safely used in tablets.
Explain the meaning of the term pure.
Three students, A, B and C make aspirin and check its purity. The students test the purity of the aspirin by measuring the melting point.
Their results are shown in the table.
| Student | Melting point (°C) |
| A | 160 |
| B | 139 |
| C | 137 |
The melting point of pure aspirin is in the range 138–140 °C.
Which student made pure aspirin?
Tick (✓) one box.
Another method of checking the purity of a substance is paper chromatography.
Look at the diagram of the apparatus used for paper chromatography.
Label the chromatography diagram.
Choose your labels from the list.
active phase
inactive phase
gas phase
mobile phase
stationary phase
* This is the method the students follow for the paper chromatography experiment.
1. Draw a pen line near the bottom of the paper.
2. Put a spot of the substance to be tested onto the line.
3. Stand the paper in the solvent. The solvent should be at the same level as the spot.
4. Cover the beaker.
5. Remove the paper from the beaker before the solvent reaches the top.
6. Calculate the Rf value of the substance by using this equation:
Their teacher notices some mistakes with this method.
Describe and explain the mistakes in the method and suggest improvements that could be made.
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