Cambridge IGCSE0620

Physical and chemical changes

Chemistry 0620 Chapter Notes

What this chapter covers

Physical and chemical changesRate of reactionReversible reactions and equilibriumRedox
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In this chapter

  1. 1What is a Physical Change?
  2. 2What is a Chemical Change?
  3. 3Signs of a Chemical Change
  4. 4Comparing Physical and Chemical Changes

1. What is a Physical Change?

A physical change is a change to a substance that does not alter its chemical identity. The atoms and molecules of the substance are not rearranged to form something new. Common physical changes include changes of state (like melting, boiling, freezing, or condensing), changes in shape or size (like crushing or tearing), and dissolving. For example, when ice melts, it becomes liquid water. Although its state has changed from solid to liquid, the substance is still water (H₂O). These changes are usually easy to reverse. You can freeze the water to get ice back. Similarly, if you dissolve sugar in water, you can get the sugar back by evaporating the water. No new chemical has been made.

Key term

Physical Change: A change in which no new chemical substance is formed and which is usually easy to reverse.

Common pitfall

Confusing dissolving with a chemical reaction. Dissolving is a physical change because the original substance can be recovered by simple physical means, like evaporation.

Fun fact

When you get a haircut, it's a physical change. The hair is still chemically hair, just in smaller pieces on the floor!

Worked example 12 marks

Is dissolving salt in water a physical or chemical change? Explain your answer.

  1. 1
    1. This is a physical change. [1 mark]
  2. 2
    1. The reason is that no new substance is formed. The salt and water are just mixed together, and the salt can be recovered by evaporating the water, meaning the change is easily reversible. [1 mark]

Worked example 22 marks

A sculptor carves a statue from a block of marble. Explain why this is a physical change.

  1. 1
    1. This is a physical change. [1 mark]
  2. 2
    1. Although the shape of the marble has been altered significantly, the substance itself is still marble (calcium carbonate). No new chemical substance has been created. [1 mark]

Recap

  • A physical change does not produce any new chemical substances.
  • The chemical properties of the substance remain unchanged.
  • Changes of state (melting, boiling, freezing) are common physical changes.
  • Dissolving a solute in a solvent is also a physical change.
  • Physical changes are generally easy to reverse and involve relatively small energy changes.

Quick check

  1. Give an example of a physical change involving a change of state.1 mark
  2. Is separating a mixture of sand and water a physical or chemical process?1 mark

2. What is a Chemical Change?

A chemical change, also known as a chemical reaction, is a process where the starting substances (reactants) are transformed into entirely new substances (products). These new products have different chemical properties and a different composition from the original reactants. For example, when iron rusts, it reacts with oxygen from the air to form iron oxide, a new substance with a reddish-brown, flaky appearance, unlike the original shiny, strong iron. Chemical changes are usually difficult to reverse and are often accompanied by a noticeable energy change, such as the release of heat, light, or sound. The process is represented by a chemical equation, for example: Reactant A + Reactant B → Product C.

Reactants → Products

Key term

Chemical Reaction: A process in which a chemical change takes place, where reactants are converted into new substances called products.

Examiner insight

Examiners award marks for correctly identifying the new substance formed, or the difficulty of reversal, as key evidence for a chemical change.

Fun fact

The browning of a cut apple exposed to air is a chemical change. An enzyme in the apple reacts with oxygen, producing a new brown-coloured substance.

Worked example 12 marks

When magnesium ribbon is heated, it burns with a dazzling white light to form a white powder called magnesium oxide. Explain why this is a chemical change, giving two reasons.

  1. 1
    1. A new substance, magnesium oxide, is formed. This is the primary evidence of a chemical change. [1 mark]
  2. 2
    1. A large amount of energy is released in the form of bright light and heat, which is characteristic of a chemical reaction. [1 mark]
  3. 3
    1. (Alternative reason) The change is not easily reversible; it is very difficult to turn magnesium oxide back into magnesium and oxygen. [1 mark]

Recap

  • A chemical change always produces one or more new substances with different properties.
  • The starting materials are called reactants and the new substances are called products.
  • Chemical changes are often accompanied by a significant energy change (e.g., heat or light).
  • Chemical changes are generally difficult to reverse.
  • Another name for a chemical change is a chemical reaction.

Quick check

  1. What is the general name for the substances you start with in a chemical reaction?1 mark
  2. Is the process of a bicycle rusting a physical or chemical change?1 mark

3. Signs of a Chemical Change

Since chemical changes create new substances, we can often observe the effects of this transformation. There are several key signs to look for that suggest a chemical reaction has taken place:

  1. A distinct colour change: For example, when green copper(II) carbonate is heated, it turns into black copper(II) oxide.
  2. A gas is produced: This is often visible as bubbles or fizzing. For instance, when an indigestion tablet (containing a carbonate) is dropped in water, it fizzes, releasing carbon dioxide gas.
  3. A precipitate is formed: A precipitate is an insoluble solid that forms when two solutions are mixed. For example, mixing clear solutions of lead(II) nitrate and potassium iodide produces a bright yellow solid precipitate of lead(II) iodide.
  4. A significant energy change: The reaction mixture may get noticeably hotter (an exothermic reaction) or colder (an endothermic reaction). The production of light or sound, like in an explosion, is also a clear sign of an energy change from a chemical reaction.

Key term

Precipitate: An insoluble solid that forms from a solution during a chemical reaction.

Common pitfall

Assuming any colour change is chemical. For example, adding purple potassium manganate(VII) crystals to water causes the water to turn purple, but this is dissolving and mixing—a physical change.

Worked example 12 marks

When dilute sulfuric acid is added to a piece of zinc metal in a test tube, the metal disappears, bubbles are seen, and the test tube gets hot. State two signs that a chemical reaction has occurred.

  1. 1
    1. A gas is produced, which is seen as bubbles or fizzing. [1 mark]
  2. 2
    1. There is an energy change, as the test tube gets hot (an exothermic reaction). [1 mark]

Worked example 22 marks

A student mixes a clear, colourless solution of silver nitrate with a clear, colourless solution of sodium chloride. A white, cloudy solid is formed. Explain how this observation proves that a chemical change has taken place.

  1. 1
    1. A new substance has been formed, which is the white solid (a precipitate). [1 mark]
  2. 2
    1. The formation of an insoluble solid from two solutions is a clear indication of a chemical reaction, as the product has different properties (insolubility) from the soluble reactants. [1 mark]

Recap

  • A permanent colour change can indicate a chemical reaction.
  • Bubbles or fizzing are a sign that a gas is being produced.
  • A cloudy solid forming in a solution is called a precipitate and indicates a reaction.
  • A change in temperature (getting hotter or colder) is a strong sign of a chemical change.
  • The production of light or sound also indicates a chemical reaction.

Quick check

  1. What is the scientific term for a reaction that gives out heat?1 mark
  2. What would you observe if a chemical reaction was producing a gas?1 mark

4. Comparing Physical and Chemical Changes

It is crucial to be able to distinguish between physical and chemical changes. The key differences can be summarised by considering four main points:

1. Identity of Substances:

  • Physical Change: No new chemical substances are formed. The particles of the substance are the same before and after the change (e.g., solid H₂O particles become liquid H₂O particles).
  • Chemical Change: One or more new chemical substances are formed. The atoms of the reactants rearrange to form products with different chemical formulas and properties.

2. Reversibility:

  • Physical Change: Usually easy to reverse. For example, you can freeze water to get ice, and you can evaporate water from a salt solution to get the salt back.
  • Chemical Change: Usually difficult to reverse. It would require another, often complex, series of chemical reactions to get the original reactants back.

3. Energy Changes:

  • Physical Change: Involve relatively small amounts of energy being absorbed or released (e.g., the energy required to melt ice is much less than the energy released by burning methane).
  • Chemical Change: Involve significant energy changes. Many reactions are strongly exothermic (give out heat) or endothermic (take in heat).

4. Mass:

  • In both physical and chemical changes, mass is conserved. The total mass of the substances before the change is equal to the total mass of the substances after the change. Atoms are not created or destroyed, only rearranged.

Key term

Conservation of Mass: The principle that mass is not created or destroyed in any chemical reaction or physical change.

Examiner insight

For comparison questions, directly contrast the two types of change. Use comparative words like 'whereas' or 'on the other hand' to show you are clearly distinguishing between them for each point of comparison.

Worked example 14 marks

Compare the process of melting a candle (the wax turning to liquid) with the process of burning a candle (the wick burning and producing light). Classify each as a physical or chemical change and justify your answer.

  1. 1
    1. Melting the wax is a physical change. [1 mark] This is because the wax is only changing state from solid to liquid; no new substance is formed and the process is easily reversed by cooling. [1 mark]
  2. 2
    1. Burning the wick and wax is a chemical change. [1 mark] This is because new substances (carbon dioxide and water vapour) are formed, energy is released as heat and light, and the change is not reversible. [1 mark]

Recap

  • Physical changes alter form, not chemical identity; chemical changes create new substances.
  • Physical changes are easily reversed, while chemical changes are not.
  • Energy changes in chemical reactions are much larger than in physical changes.
  • Mass is always conserved in both physical and chemical changes.
  • In an exam, always give a reason for your classification.

Quick check

  1. Is the total mass of the products of a chemical reaction greater than, less than, or equal to the total mass of the reactants?1 mark
  2. Which type of change, physical or chemical, involves the breaking and forming of chemical bonds?1 mark

End-of-chapter exercise

Test yourself on the whole chapter. Work through these before moving on.

  1. State whether each of the following is a physical or a chemical change: (a) Frying an egg. (b) Bending a copper wire. (c) Dissolving sugar in tea. (d) A firework exploding.4 marks
  2. A student gently heats blue copper(II) sulfate crystals. They turn into a white powder and steam is given off. When water is added to the white powder, it turns blue again and feels hot. Is heating the blue crystals a physical or chemical change? Give two reasons for your answer.3 marks
  3. Describe two observations you would make that show a chemical change is occurring when a piece of magnesium ribbon is added to a test tube of dilute hydrochloric acid.2 marks
  4. Photosynthesis is the process by which plants make food. The word equation is: carbon dioxide + water → glucose + oxygen. Explain why photosynthesis is a chemical change.2 marks
  5. A mixture of iron filings and sulfur powder is grey and yellow. When heated, it glows red-hot even after the heat source is removed, and a black solid is formed. The black solid is not magnetic. Use this information to give three reasons why a chemical change has occurred.3 marks
  6. Compare a physical change and a chemical change with respect to (a) the substances involved and (b) reversibility.4 marks
  7. When you light a gas hob to cook, methane gas (CH₄) reacts with oxygen (O₂) from the air. The products are carbon dioxide (CO₂) and water (H₂O) vapour. (a) Write a simple word equation for this reaction. (b) Give three pieces of evidence that show this combustion is a chemical change.4 marks
  8. A student claims that when a puddle of water evaporates, the water is destroyed. Explain why this student is incorrect, referring to the type of change and the particles involved.3 marks
  9. Classify each of the following processes as either a physical change or a chemical change. For each, provide one reason for your classification. (a) Making toffee from sugar by heating it. (b) Weaving cotton threads to make a bedsheet.4 marks
  10. Some chemical reactions are reversible, for example: hydrated copper(II) sulfate (blue) ⇌ anhydrous copper(II) sulfate (white) + water. A student states that because this reaction is reversible, it must be a physical change. Explain why this student's reasoning is incorrect.3 marks

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