Look inside the OCR GCSE Combined Science Chemistry, Foundation tier guide (J250)

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Look inside the OCR GCSE Combined Science Chemistry, Foundation tier guide

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OCR · GCSE · J250
Combined Chemistry Foundation
Active Recall Guide
797 questions
Combined Chemistry FoundationContents
Contents
6 topics, 53 subtopics
  1. Chemical Reactions193
  2. Elements, Compounds and Mixtures221
  3. Global Challenges165
  4. Monitoring and Controlling Chemical Reactions105
  5. Particles60
  6. Predicting and Identifying Reactions53
ii
Elements, Compounds and MixturesQuestions
Elements, Compounds and Mixtures
Purity, and melting point data
  1. In science, what does it mean to say that a substance is pure?
  2. What does the word 'pure' usually mean when it is used about food or drink in everyday language?
  3. Why would a carton labelled 'pure orange juice' not count as pure to a chemist?
  4. Is bottled spring water a pure substance in the scientific sense?
  5. Can a mixture of two elements ever be described as pure in the scientific sense?
  6. A sample contains nothing but copper atoms. Is it an element, a compound or a mixture, and is it pure?
  7. Give an example of a substance that is chemically pure but that many people would not call pure in everyday speech.
  8. Why do chemists need a stricter definition of purity than the everyday one?
  9. How does a pure substance behave when it is heated up to its melting point?
12
Elements, Compounds and MixturesAnswers
Answers
Purity, and melting point data
  1. A pure substance is a single element or a single compound, with nothing else mixed into it.
  2. In everyday use it means nothing has been added, so the product is natural or unadulterated.
  3. Because orange juice is a mixture of water, sugars, acids and many other compounds rather than one single compound.
  4. No, because it is a mixture of water with dissolved minerals and gases.
  5. No, because a pure substance contains only one element or only one compound.
  6. It is an element, and it is pure because only one kind of atom is present.
  7. Distilled water containing nothing but H₂O, which some people would describe as processed rather than pure.
  8. Because properties such as melting point and boiling point only take fixed, predictable values when a single element or compound is present.
  9. It melts sharply at one fixed temperature.
13
Global ChallengesQuestions
Global Challenges
Life cycle assessment
  1. Name the four stages considered in a life cycle assessment of a product.
  2. What is the purpose of carrying out a life cycle assessment?
  3. A quarry is dug to obtain the clay for a brick. Which stage of a life cycle assessment does this belong to?
  4. Electricity is used to run a washing machine for ten years. Which stage of a life cycle assessment does this belong to?
  5. Lorries carry bottles from the factory to shops across the country. Which stage of a life cycle assessment does this belong to?
  6. A plastic bag is buried in landfill and does not break down. Which stage of a life cycle assessment does this belong to?
  7. Give two things measured at the raw material stage of a life cycle assessment.
  8. Why must the disposal stage of a plastic product take account of how long the material lasts?
  9. Product A takes more energy to manufacture than product B but uses far less energy over ten years of service. Which is likely to have the lower total energy?
34
Global ChallengesAnswers
Answers
Life cycle assessment
  1. Making the material and the product from raw materials, using the product, transporting it, and disposing of it at the end of its life.
  2. To judge the total use of resources and the environmental impact of a product across its whole life.
  3. Obtaining the raw material and making the product.
  4. The use of the product.
  5. Transport of the product.
  6. Disposal at the end of the product's life.
  7. The quantity of raw material taken and the energy used to extract and process it.
  8. A material that does not break down stays in landfill for a very long time, taking up space and possibly releasing harmful substances.
  9. Product A, provided the saving made while it is in use outweighs the extra energy used to make it.
35
ParticlesQuestions
Particles
The particle model and changes of state
  1. What does the particle model assume about the particles that make up a substance?
  2. Describe the arrangement and movement of the particles in a solid.
  3. Describe the arrangement and movement of the particles in a liquid.
  4. Describe the arrangement and movement of the particles in a gas.
  5. Name the change of state in which a solid becomes a liquid, and the reverse change.
  6. Name the change of state in which a liquid becomes a gas, and the reverse change.
  7. What is the name for a solid changing directly into a gas without becoming a liquid first?
  8. Explain, in terms of particles, why energy must be supplied to melt a solid.
  9. Explain why a gas can be compressed easily but a solid cannot.
56
ParticlesAnswers
Answers
The particle model and changes of state
  1. It assumes that all substances are made of tiny particles that are constantly moving and are attracted to one another.
  2. They are packed closely together in a regular arrangement and vibrate about fixed positions.
  3. They are still close together but arranged randomly, and they move around and slide past one another.
  4. They are far apart and arranged randomly, and they move quickly in all directions.
  5. A solid becoming a liquid is melting, and a liquid becoming a solid is freezing.
  6. A liquid becoming a gas is boiling or evaporating, and a gas becoming a liquid is condensing.
  7. Sublimation.
  8. Energy is needed to make the particles vibrate more strongly and to overcome the forces of attraction holding them in their fixed positions.
  9. In a gas there are large spaces between the particles that can be squeezed out, whereas in a solid the particles are already touching.
57
Keep going
797
questions
53 subtopics

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797 questions
Across 53 subtopics, every one of them from specification J250.
One PDF
Print it, or work from a screen with a notebook beside you.
Answers after, not beside
Covering the answers is a matter of not turning the page, which is what makes the closed-book pass possible.

How the guide is worked

Answering a question from memory stores it far better than reading the answer again. The guide runs that as a fixed procedure on one subtopic at a time, about twenty minutes a session.

  1. Step 1 · Closed book

    Cover the answers. Work through one subtopic and write down what you can. Leave blanks where you have nothing.

  2. Step 2 · Open book

    Go back to the top. Read each printed answer and write it out in full, including the ones you had right.

  3. Step 3 · Closed book again

    Same questions, same order, from memory. The gap between pass one and pass three is the session result.

Read the full method, the return schedule and the research behind it.

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