4.1Chemical Reactions
Introduction for Reactions — AP CHEMISTRY Revision Notes & Topical Questions
5 min read 8 parts Lesson 4 of 12 1 quiz
Lesson summary
4.1 Introduction for Reactions — Notes — AP CHEMISTRY notes: Distinguish physical from chemical change and recognise the evidence for a chemical reaction.A physical change alters properties but not composition.Changes of state and the forming or separating of mixtures are physica…
- Precipitate
- Conservation of mass
- State symbol
- Coefficient
- Changes of state and the forming or separating of mixtures are physical changes
- Only intermolecular or interparticle interactions are overcome or formed
Part 1 of 6Physical change
1
A physical change alters properties but not composition.
2
Changes of state and the forming or separating of mixtures are physical changes.
3
Only intermolecular or interparticle interactions are overcome or formed.
4
Examples are melting ice and dissolving sugar.
Part 2 of 6Chemical change
Chemical change
A chemical change turns substances into new substances with different compositions.
Atoms are rearranged by breaking and forming chemical bonds.
Part 3 of 6Evidence of a chemical reaction
1
Colour change.
2
Formation of a precipitate when solutions are mixed.
3
Gas evolution (bubbles not due to boiling).
4
Temperature change, with heat released or absorbed.
5
Emission of light, or a change in odour.
Part 4 of 6Limits of the evidence
Limits of the evidence
None of these observations proves a chemical change on its own.
- Boiling produces bubbles.
- Dissolving ammonium nitrate causes a temperature drop without forming a new substance.
The strongest evidence is showing that a new substance with different properties has formed, such as identifying the gas or solid.
Part 5 of 6Conservation of mass and representation
Conservation of mass and representation
Mass is conserved in all reactions, so the total mass of reactants equals that of products in a closed system.
Reactions are shown by balanced equations with state symbols (s), (l), (g) and (aq), where coefficients give mole ratios.
The main AP reaction types are precipitation, acid-base (proton transfer) and oxidation-reduction (electron transfer).
| Example | Type | Evidence | New substance |
|---|---|---|---|
| Ice melting | Physical | State change | No |
| Iron rusting | Chemical | Colour change | Yes, Fe2O3 |
| Sugar dissolving | Physical | Disappears | No |
| Burning methane | Chemical | Heat and light | Yes, CO2 and H2O |
| Precipitation | Chemical | Solid forms | Yes, such as AgCl |
Part 6 of 6More from the specification
1Chemical formula
- Symbols and numbers showing the relative number of atoms of each element.
2Covalent bond
- A shared pair of electrons between two atoms, giving noble gas configurations.
3Hydrogen
- A gas that gives a squeaky pop with a lighted splint.
4Oxygen
- A gas that relights a glowing splint.
5Fraction
- A group of hydrocarbons with similar chain length and boiling points.
6Matter
- Everything you can see and feel; it exists as solid, liquid or gas.
7Aqueous
- Dissolved in water.
8Ionic equation
- An equation showing only the ions that react.
Key terms
| Term | Meaning |
|---|---|
| Physical change | Change in properties without change in composition. |
| Chemical change | Change in which new substances form. |
| Precipitate | Insoluble solid formed when solutions are mixed. |
| Conservation of mass | Total mass of reactants equals total mass of products. |
| State symbol | Label (s), (l), (g) or (aq) showing the phase of a species. |
| Coefficient | Number in front of a formula giving the mole ratio. |
Avoid these mistakes
1
Saying a temperature change alone proves a reaction — dissolving can change temperature without a reaction
2
Treating all bubbles as evidence of a reaction — boiling also produces bubbles
3
Saying mass changes in a reaction — mass is conserved
4
Treating a change of state as a chemical change
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