OCR GCSE Combined Science Chemistry, Higher tier sample questions and answers (J250)
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30 sample questions and answers
Taken from every topic of OCR GCSE Combined Science Chemistry, Higher tier, specification J250. The full guide has 916.
Paper 9 (Chemistry)
Chemical Reactions
276 questions in the guide, across 17 subtopics. More from this topic
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Writing formulae of elements and compounds
Sodium chloride is written NaCl even though it has no molecules. What does this formula represent?
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The simplest whole-number ratio of the ions in the giant lattice, one sodium ion to one chloride ion. -
Ionic equations, and state symbols
Write the ionic equation for magnesium reacting with a dilute acid.
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Mg(s) + 2H⁺(aq) → Mg²⁺(aq) + H₂(g). -
Conservation of mass, and changes of mass in open systems
4 g of hydrogen reacts completely with 32 g of oxygen. What mass of water is formed?
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36 g. -
Exothermic and endothermic reactions, and reaction profiles
Why is an instant cold pack for a sports injury based on an endothermic change?
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It takes in energy from its surroundings, so it cools the injury. -
Oxidation and reduction, by oxygen and by electrons
In Fe₂O₃ + 3CO → 2Fe + 3CO₂, name the reducing agent and explain your choice.
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Carbon monoxide, because it removes oxygen from the iron oxide and is itself oxidised to carbon dioxide.
Paper 9 (Chemistry)
Elements, Compounds and Mixtures
221 questions in the guide, across 15 subtopics. More from this topic
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Purity, and melting point data
A sample contains nothing but copper atoms. Is it an element, a compound or a mixture, and is it pure?
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It is an element, and it is pure because only one kind of atom is present. -
Relative formula mass and empirical formulae
A compound has the empirical formula CH₂O and a relative formula mass of 180. How many CH₂O units make up one molecule?
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Six, because one CH₂O unit has a relative mass of 30 and 180 ÷ 30 = 6. -
Paper and thin layer chromatography
When would a non-aqueous solvent such as ethanol be chosen instead of water?
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When the substances being separated do not dissolve in water, so they need an organic solvent to carry them. -
Interpreting chromatograms and Rf values
To how many significant figures should the answer to a calculation be given?
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To the same number as the value with the fewest significant figures among the data used in the calculation. -
Metals and non-metals in the Periodic Table
How does a non-metal oxide behave when it dissolves in water?
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It is acidic and gives a solution with a pH below 7.
Paper 10 (Chemistry)
Global Challenges
175 questions in the guide, across 11 subtopics. More from this topic
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Extracting metals by reduction and by electrolysis
Give a word equation for the reduction of iron(III) oxide by carbon.
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Iron(III) oxide + carbon → iron + carbon dioxide. -
Biological methods of metal extraction
Describe the stages of phytoextraction.
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Plants are grown on soil containing the metal, they absorb metal compounds as they grow, and the harvested plants are burned to leave an ash rich in the metal. -
Life cycle assessment
A plastic bag is buried in landfill and does not break down. Which stage of a life cycle assessment does this belong to?
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Disposal at the end of the product's life. -
Recycling, and the decisions behind it
Give one reason a recycled material is often used for a lower-grade purpose than the original product.
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Reprocessing can shorten polymer chains or leave impurities, so the material is weaker or less pure. -
Fractional distillation of crude oil
What is kerosene used for?
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It is used as fuel for aircraft.
Monitoring and Controlling Chemical Reactions
123 questions in the guide, across 8 subtopics. More from this topic
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Mass of solute, concentration and dilution calculations
State the relationship between the mass of solute, the volume of solution and the concentration in g/dm³.
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Concentration in g/dm³ equals the mass of solute in grams divided by the volume of the solution in dm³. -
Measuring the rate of a reaction, and rate graphs
Give the units in which a rate of reaction is expressed when gas volume is measured, and when mass loss is measured.
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Gas volume gives a rate in cm³/s or cm³/min, and mass loss gives a rate in g/s or g/min. -
Factors affecting rate, and collision theory
Which of the factors that change rate applies only to reactions involving gases?
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Pressure, because only gases are compressed enough for pressure to change how closely the particles are packed. -
Surface area and rate
A cube of side 3 cm is cut into twenty-seven cubes of side 1 cm. Calculate the new total surface area and the new surface area to volume ratio.
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Each small cube has a surface area of 6 cm², so the total is 162 cm² for the same 27 cm³, giving a ratio of 6:1, three times the original. -
Catalysts and how they work
Give two reasons why catalysts are used so widely in industry.
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They allow a useful rate at a lower temperature or pressure, which saves energy and money, and they increase the amount of product made per day from the same plant.
Paper 9 (Chemistry)
Particles
68 questions in the guide, across 5 subtopics. More from this topic
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The particle model and changes of state
Name the change of state in which a liquid becomes a gas, and the reverse change.
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A liquid becoming a gas is boiling or evaporating, and a gas becoming a liquid is condensing. -
The particle model and changes of state
Explain why burning magnesium in air is a chemical change.
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The magnesium atoms join with oxygen atoms to form a new substance, magnesium oxide, which has different properties from the magnesium. -
Limitations of the particle model
Explain why leaving out the forces between particles makes the model poor at explaining boiling points.
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Boiling requires enough energy to overcome the attraction between the particles, so a model with no forces cannot explain why different substances boil at different temperatures. -
Limitations of the particle model
Explain why real particles cannot be solid spheres.
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An atom is mostly empty space, with a tiny nucleus and electrons in the space around it, so it is not solid throughout. -
How the atomic model changed, and the structure of the atom
Explain how Rutherford interpreted the results of the alpha scattering experiment.
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He concluded that an atom is mostly empty space with its positive charge and nearly all its mass concentrated in a tiny nucleus at the centre.
Predicting and Identifying Reactions
53 questions in the guide, across 4 subtopics. More from this topic
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Groups 1, 7 and 0, and why they behave as they do
In what form do the elements of Group 7 exist?
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As diatomic molecules of two atoms sharing a pair of electrons, such as Cl₂ and Br₂. -
Groups 1, 7 and 0, and why they behave as they do
How many outer electrons does a halogen atom have, and what ion does it form?
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Seven outer electrons, and it gains one more electron to form a 1− ion. -
Predicting reactivity from the Periodic Table
Predict the formula of the compound formed between potassium and iodine.
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KI, because potassium loses one electron and iodine gains one. -
Predicting reactivity from the Periodic Table
An unknown element is a soft metal that is stored under oil and fizzes vigorously on cold water. Which group does it belong to?
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Group 1. -
Reactivity of metals with water and acids
Compare how magnesium reacts with cold water and with steam.
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It reacts extremely slowly with cold water, but burns vigorously in steam to form magnesium oxide and hydrogen.
OCR GCSE Combined Science Chemistry, Higher tier Active Recall Guide
The other 886 questions, with the answers printed after each section.