OCR GCSE Combined Science Chemistry, Foundation 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, Foundation tier, specification J250. The full guide has 797.
Paper 3 (Chemistry)
Chemical Reactions
193 questions in the guide, across 14 subtopics. More from this topic
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Writing formulae of elements and compounds
What is the formula of carbon dioxide, and what does the subscript tell you?
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CO₂, where the subscript 2 shows that each molecule contains two oxygen atoms. -
Balanced equations, and deducing formulae from ions
Deduce the formula of sodium sulfate from the ions Na⁺ and SO₄²⁻.
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Na₂SO₄, because two sodium ions are needed to balance one sulfate ion. -
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
Between which two points on a reaction profile is the activation energy measured?
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From the energy level of the reactants up to the top of the curve. -
Oxidation and reduction in terms of oxygen
In 2CuO + C → 2Cu + CO₂, state which substance is oxidised and which is reduced.
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Carbon is oxidised because it gains oxygen, and copper oxide is reduced because it loses oxygen.
Paper 3 (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 4 (Chemistry)
Global Challenges
165 questions in the guide, across 10 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. -
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. -
The fractions, and crude oil as a feedstock
Name the alkane with the formula C₅H₁₂.
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Pentane.
Paper 4 (Chemistry)
Monitoring and Controlling Chemical Reactions
105 questions in the guide, across 6 subtopics. More from this topic
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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. -
Measuring the rate of a reaction, and rate graphs
How should a results table for a rate of reaction experiment be set out?
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The independent variable goes in the first column and the readings, their repeats and the mean in the columns after it, with each heading giving the quantity and its unit, such as "Time (s)", so no units are written beside the numbers. -
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. -
Factors affecting rate, and collision theory
Explain why only a small fraction of the collisions in a reaction mixture actually produce a reaction.
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Most colliding particles have less than the activation energy, so they simply bounce apart unchanged. -
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.
Paper 3 (Chemistry)
Particles
60 questions in the guide, across 4 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. -
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. -
How the atomic model changed, and the structure of the atom
How does the radius of the nucleus compare with the radius of the whole atom?
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The nucleus is very much smaller, with a radius of the order of ten thousand times less than that of the atom. -
The size of atoms, and the subatomic particles
Put a nucleus, a whole atom and a small molecule in order of increasing size.
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The nucleus is smallest, then the atom, then the small molecule.
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, Foundation tier Active Recall Guide
The other 767 questions, with the answers printed after each section.