OCR GCSE Combined Science B (21st Century) Paper 4 June 2025 question paper

This question is about acids and metals.

This is the symbol equation for the reaction between magnesium, and hydrochloric acid:

Which gas is made when magnesium reacts with hydrochloric acid?

Tick (✓) one box.

Which salt is made when magnesium reacts with hydrochloric acid?

Tick (✓) one box.

Hydrochloric acid is made by dissolving hydrogen chloride in water.

What forms when acids dissolve in water?

Tick (✓) one box.

Plot the results from the table on the graph.

Three points have already been plotted.

Draw a curve of best fit on the graph.

Concentration of hydrochloric acid (mol / dm³)

Describe the effect of changes in concentration on the time taken.

A student is looking at stomata from a leaf using an optical microscope.

This is the student’s method but it is not in the correct order:

A Focus using the coarse focus knob.

B Spread nail varnish on the underside of a leaf and allow to dry.

C Place the glass slide in the middle of the stage.

D Focus using the fine focus knob.

E Peel off the dried nail varnish and place it on a glass slide.

Write the letters in the boxes to show the correct order of the statements.

The first one has been done for you.

The magnification of the microscope’s eyepiece is ×10. The objective lens is ×40.

What is the total magnification?

Use the equation: total magnification = magnification of eyepiece lens × magnification of objective lens

Tick (✓) one box.

The student looks down the microscope and counts how many stomata they can see. They repeat this measurement in four other areas of the leaf.

The table shows their results:

Calculate the mean number of stomata.

Scientists have examined how the size of a stomatal opening changes during the day. Their results are shown in the graph.

Describe the trend shown by the graph.

Stomata may open and close in response to environmental conditions.

Use the graph to suggest one environmental condition that may cause stomata to open.

Studies have shown that some plant species in warming climates have reduced the number of stomata on their leaves.

Complete the sentences to explain how this change occurred through evolution.

Use terms from the list.

chloroplasts die
natural selection no change

nucleus selective breeding survive variation

In a population of plants, there is........ in the number of stomata

each plant has.

Plants with lower numbers of stomata lose less water. This makes them more likely to

........ and reproduce. Some plant species have evolved to have fewer

stomata through the process of........

Plants rely on stomata for gas exchange.

Carbon dioxide is used in the........ as a reactant in photosynthesis.

A student is using a ripple tank to investigate how the speed of water waves depends on the depth of the water.

The waves are generated by a vibrating bar.

The mean number of waves leaving the vibrating bar in five seconds is 18.3.

Calculate the frequency of the waves.

The student repeats the experiment with a different depth of water, and obtains new results:

Calculate the speed of these water waves.

Give your answer in cm / s.

Use the Data and Equation Sheet.

Speed of water waves =........ cm / s

This question is about cardiovascular health and fitness.

A student is measuring their pulse rate. They count their pulse over 60 seconds.

Describe how the student can measure their pulse.

The students conclude that student B had a greater fitness level.

Which two statements support this conclusion?

Tick (✓) two boxes.

To be more confident in their conclusion, they decide to repeat their experiment.

Suggest one improvement to their method to increase the validity of their conclusion.

High blood pressure is a risk factor for cardiovascular disease.

Enalapril is a drug that can lower blood pressure. A doctor has recommended a patient begin taking this drug.

Suggest one health-related question the patient may ask the doctor before they begin taking this drug.

Suggest one question the doctor should ask the patient before they prescribe them this drug.

A student is investigating how the resultant force acting on a trolley affects its acceleration.

The diagram shows the apparatus used by the student in their investigation.

Describe how the pulling force of the string on the trolley can be increased.

Describe how the friction of the wheels of the trolley can be reduced.

The student measures the time taken for the trolley to travel a distance of 1.0 m.

Why does the student repeat this time measurement three times and then calculate the mean?

Tick (✓) one box.

For a particular resultant force, it takes the trolley a mean time of 2.0 s to travel a distance of 1.0 m.

Calculate the acceleration of the trolley.

Acceleration =........ m / s²

The student raises one end of the table to compensate for friction so that the resultant force acting on the trolley is equal to the pulling force of the string.

The diagram shows this change.

When the pulling force of the string is zero, the trolley travels at a constant speed down the slope.

The student repeats the experiment by changing the pulling force of the string multiple times.

Draw a line on the axes to show the expected results of the student’s experiment.

Explain your answer to (c)(i).

A student is investigating the composition of some ink samples A, B and X, using chromatography.

The student sets up the experiment as shown in the diagram.

Identify two mistakes that the student has made.

The student corrects their mistakes.

Ink Sample A is pure.

Ink Sample X is a mixture.

One of the substances in Ink Sample X travels 24 mm on the chromatogram. The solvent travels 39 mm.

Calculate the Rf value of this substance in Ink Sample X.

Give your answer to 2 significant figures.

How could you conclude from the chromatogram that Ink Sample X is a mixture?

Another substance in Ink Sample X does not travel up the chromatography paper.

Suggest one change to the experiment that would cause this substance to travel up the paper.

Ink Sample B is also a mixture. All of the components of Ink Sample B have similar solubility in the solvent, but have different boiling points.

Explain why simple distillation should be used instead of chromatography to separate this ink sample.

Carbon-15 is a radioactive isotope that emits beta particles. The graph shows the activity of a sample of carbon-15.

Use the graph to find the half-life of carbon-15.

* A patient goes to a radiology department in a hospital to have a kidney scan.

A radioactive tracer is injected into the patient’s arm. After a few hours, a set of images are taken on a camera. The camera uses the radiation emitted by the radioactive tracer to form an image of the kidney.

The table shows information about some radioactive isotopes that are used in radioactive tracers.

Which isotope should be used for injection into the patient’s arm?

Explain your answer.

You should also consider:

• penetrating power

• ionising power.

A skater wants to buy a new skateboard.

The table shows information about three different skateboards.

Suggest why the skater chooses to buy Skateboard C.

State why the development of composite materials is beneficial when designing new products.

The skater decides to recycle the old skateboard so that it does not take up space in landfill.

State one other factor that should also be considered when disposing of a skateboard.

The skateboards are transported to shops in lorries that burn diesel.

One gas that forms when diesel burns is sulfur dioxide.

Explain one problem caused by increased amounts of sulfur dioxide in the atmosphere.

Carbon monoxide and nitrogen monoxide are also formed when diesel burns.

Catalytic converters help to decrease these emissions into the atmosphere by converting:

• nitrogen monoxide into nitrogen

• carbon monoxide into carbon dioxide.

Complete the equation to show the reaction that occurs in a catalytic converter.

A student is investigating the rate of anaerobic respiration in yeast using this method and the apparatus shown:

• Add glucose to a yeast solution.

• Add a layer of oil on top to prevent oxygen reaching the yeast.

• Measure the volume of carbon dioxide made in 20 minutes.

• Repeat using sucrose and maltose.

The student controls the temperature of the yeast and sugar solution to make sure their comparisons are valid.

Describe how the student should control the temperature.

Calculate the rate of carbon dioxide production for the yeast and glucose solution.

Rate of carbon dioxide production =........ cm³ / minute

Yeast cells absorb oxygen across their cell membranes.

An axolotl is a multicellular animal that lives in water. It has external gills for gas exchange.

Explain why an axolotl needs external gills for gas exchange.

Use ideas about surface area to volume ratios in your answer.