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

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

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OCR · GCSE · J250
Combined Chemistry Higher
Active Recall Guide
916 questions
Combined Chemistry HigherContents
Contents
6 topics, 60 subtopics
  1. Chemical Reactions276
  2. Elements, Compounds and Mixtures221
  3. Global Challenges175
  4. Monitoring and Controlling Chemical Reactions123
  5. Particles68
  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
Biological methods of metal extraction
  1. Name two biological methods used to obtain metals from low-grade ores.
  2. Describe how bioleaching obtains copper from a low-grade ore.
  3. Name two ways copper can be recovered from a leachate solution.
  4. Describe the stages of phytoextraction.
  5. Why is the ash produced by phytoextraction useful?
  6. Give one environmental advantage of biological extraction over traditional mining.
  7. Give one economic advantage of biological methods of extraction.
  8. Give the main disadvantage of bioleaching and phytoextraction compared with smelting.
  9. Give one disadvantage of bioleaching linked to the solutions it produces.
34
Global ChallengesAnswers
Answers
Biological methods of metal extraction
  1. Bioleaching using bacteria and phytoextraction using plants.
  2. Bacteria break down the ore and produce a leachate solution containing dissolved copper compounds, from which the copper is then recovered.
  3. By displacement using scrap iron and by electrolysis.
  4. 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.
  5. It contains a far higher concentration of the metal compound than the soil or low-grade ore the plants grew in.
  6. It avoids the large-scale digging, tipping and landfill that traditional mining causes.
  7. They recover metal from low-grade ores that would be uneconomic to mine and smelt.
  8. They are very slow, so the rate of metal production is low.
  9. Toxic or acidic leachate can escape and damage rivers and soil.
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
916
questions
60 subtopics

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916 questions
Across 60 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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