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CHEMISTRY · Stage 7 · Chapter 4: Chemical Reactions 1

Chemistry 4.3 Investigating a chemical reaction — Practical Observations

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4.3 Investigating a chemical reaction — Practical Observations — Alhussan International Academy — Chemistry (Stages 7–9) notes: Plan and carry out an investigation into a chemical reaction, record observations and use them and the conservation of mass to draw a conclusion.Chemica…

  • Physical change
  • Evidence of a chemical reaction
  • Hypothesis
  • Evidence needed
  • Prediction
  • Explanation

Section 1 of 8Chemical change versus physical change

  • Chemical reaction
    • new substances form with different properties from the starting materials.
  • Physical change
    • no new substance forms, for example melting or dissolving.
  • Evidence of a chemical reaction
    • a colour change, bubbles of gas (effervescence), a precipitate forming, a temperature change, light or a flame, or a new smell.
    • None on its own is absolute proof, because boiling also makes bubbles, but several together are good evidence.

Section 2 of 8Hypothesis and prediction

  • Observation
    • big pieces of iron do not burn, but thin strands called iron wool burn well.
  • Hypothesis
    • if iron wool is heated in air, the iron will react with oxygen to make a compound, iron oxide.
    • It is based on scientific knowledge that magnesium burns by reacting with oxygen.
  • Evidence needed
    • if the mass of the product is greater than the mass of the iron, it shows that iron reacts with oxygen from the air.
  • Prediction
    • iron atoms and oxygen atoms will join, so the mass of iron oxide will be greater than the mass of iron at the start.
    • A ruler balanced with iron wool on one end should tilt down at the iron end.

Section 3 of 8Planning the method

  • Apparatus
    • a metal ruler balances the iron wool, and a hot flame heats it.
    • A wooden ruler might catch fire, so a metal ruler is used.
  • Safety
    • wear gloves because iron wool can cut skin, wear eye protection, and keep spare iron wool in a jar with a lid because it catches fire easily.
  • Heating
    • heat the iron wool for one minute.

Section 4 of 8Results and conclusion

  • Result
    • the iron wool end of the ruler tilted down, so the prediction was correct.
  • Explanation
    • iron atoms joined with oxygen atoms from the air to make the compound iron oxide, which had a greater mass than the iron.
  • Improving the investigation
    • take measurements.
    • Mass of iron before heating was 4.1 g and mass of iron oxide after heating was 5.6 g.
    • Total mass of reactants equals total mass of products, so mass of oxygen = 5.6 g − 4.1 g = 1.5 g.
  • Steps of an enquiry
    • make a hypothesis and prediction, choose equipment, work safely, make a conclusion and suggest improvements.

Section 5 of 8Making observations

  • Describe what you see, hear, smell (only if safe) and feel, using precise words.
    • Write that the blue solution turned colourless, a grey solid appeared and the test tube became warm.
  • Observations are facts, while conclusions are explanations.
    • The observation is that gas bubbled off, and the conclusion is that a gas was produced.
  • Smell
    • waft vapours towards you instead of smelling chemicals directly.
  • Temperature
    • measure with a thermometer, since exothermic reactions raise it and endothermic reactions lower it.
    • Use a results table with headings and units.

Section 6 of 8Typical practicals

1Magnesium with acid3 points
  • magnesium ribbon in dilute hydrochloric acid bubbles, the ribbon shrinks and disappears and the tube gets warm.
  • A lighted splint at the mouth gives a squeaky pop, showing hydrogen.
  • magnesium + hydrochloric acid → magnesium chloride + hydrogen.
2Iron with copper sulfate2 points
  • iron nails become coated in brown solid and the blue colour fades.
  • iron + copper sulfate → iron sulfate + copper, which is a displacement reaction.

Section 7 of 8Writing a conclusion and evaluating

  • Three parts
    • say what happened (observation), say what it shows, and explain why (atoms rearranged).
      • (a chemical reaction because a new substance formed)
  • Errors
    • impure chemicals, imprecise measuring cylinders or reading the thermometer too early.
  • Improvements
    • repeat to check reliability and suggest ways to reduce errors.

Section 8 of 8Planning a fair test

  • Example
    • to investigate how the amount of acid changes the temperature rise, the independent variable is the volume of acid.
  • Dependent variable
    • the temperature change.
  • Control variables
    • the mass of magnesium, the starting temperature and the container used.
  • Reporting
    • include units, label every column of the results table and say how many times each measurement is repeated.

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