Physical and chemical changes
A physical change is a change in the state or properties of matter without any accompanying change in its chemical composition. The particles (atoms, molecules, or ions) remain the same; only their arrangement, spacing, or energy changes. No new substances are formed.
A chemical change (or chemical reaction) occurs when one or more substances are converted into different substances. New chemical bonds are broken and/or formed, resulting in the creation of new substances with different chemical properties from the original reactants.
| Feature | Chemical Change |
|---|---|
| New Substances? | |
| Particle Level | |
| Reversibility |
A change in which the identity of the substance does not change. The same molecules or atoms are present before and after the change, merely in a different arrangement or state.
Key Examples:
- Melting/Freezing: Solid \rightarrow Liquid (e.g., ice melting to water). H_2O remains H_2O.
- Boiling/Condensing: Liquid \rightarrow Gas (e.g., water boiling to steam). H_2O remains H_2O.
- Dissolving: A solute disperses in a solvent (e.g., sugar dissolving in water). The sugar molecules are still sugar molecules, just separated from each other. This is not a chemical reaction because no new bonds are formed between the sugar and water that change the sugar's identity.
- Breaking/Crushing: Changing shape or size does not change chemical identity.
A process where new substances are formed with different chemical properties. This involves the rearrangement of atoms.
Key Examples:
- Combustion: Burning methane (CH_4) produces carbon dioxide (CO_2) and water (H_2O). New bonds form.
- Decomposition: Heating calcium carbonate (CaCO_3) breaks it down into calcium oxide (CaO) and carbon dioxide (CO_2).
- Rusting: Iron (Fe) reacts with oxygen and water to form hydrated iron(III) oxide (rust). The iron atoms are chemically altered.
- Neutralisation: An acid reacts with a base to form salt and water.
Scenario 1: Dissolving Salt (NaCl) in Water
- Observation: The solid disappears, and the water becomes salty.
- Analysis: If you evaporate the water, the salt crystals reappear unchanged. No new chemical bonds were formed between Na^+ and Cl^- ions and the water molecules that alter their identity; they are simply hydrated (surrounded) by water molecules.
- Conclusion: Physical Change.
Scenario 2: Burning Magnesium Ribbon
- Observation: Bright white light, heat released, and a white powder (magnesium oxide) remains.
- Analysis: The shiny grey metal magnesium has reacted with oxygen in the air to form a new white solid. You cannot get the magnesium ribbon back by cooling the powder.
- Conclusion: Chemical Change.
Scenario 3: Heating Calcium Carbonate (Limestone)
- Observation: A gas is given off, and a white solid residue remains.
- Analysis: The calcium carbonate breaks down into calcium oxide and carbon dioxide. The chemical formula changes from CaCO_3 to CaO + CO_2.
- Conclusion: Chemical Change (specifically, thermal decomposition).
Why it is wrong: Dissolving is a physical change. The solute particles are separated from each other and dispersed among solvent particles, but their chemical identity remains intact. No new chemical bonds are formed that change the molecular structure of the solute.
Correct Understanding: To be a chemical change, the atoms must rearrange to form new substances with different chemical formulas. In dissolving NaCl, you still have Na^+ and Cl^- ions; they haven't turned into something else. In burning wood, carbon atoms bond with oxygen to form CO_2, which is a completely new substance.
The Examiner's Expectation: Cambridge examiners look for specific observable indicators that suggest new substances have formed. While the definition of a chemical change is 'formation of new substances', you must often provide the evidence for this in your answer.
Accepted Evidence (Markscheme Phrases):
- Colour change: e.g., 'The solution turned from colourless to blue.'
- Gas evolution / Effervescence: e.g., 'Bubbles were observed' or 'A gas was given off.' (Note: Do not just say 'gas produced' unless you identify it; 'effervescence' is the physical observation).
- Precipitate formation: e.g., 'A solid formed from two clear liquids.'
- Temperature change: e.g., 'The container became hot (exothermic)' or 'cold (endothermic).' Note: Temperature change alone is not definitive proof (dissolving NaOH is exothermic but physical), so combine it with other evidence.
- Permanent odour change: e.g., 'A sweet smell was detected.'
Why examiners accept this: These observations are macroscopic manifestations of microscopic bond breaking and forming. For example, a colour change indicates new electronic energy levels in the product molecules compared to the reactants.
B. Rusting iron
C. Melting ice
D. Cooking an egg
B. The shape of the substance changes.
C. New substances with different properties are formed.
D. The mass of the system changes.