6.1Chemical Reactions
Physical and chemical changes — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions
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Lesson summary
6.1 Physical and chemical changes — Notes — IGCSE Chemistry notes: Distinguish physical changes from chemical changes and recognise the signs of a chemical reaction.Physical change — a change in which no new substance is formed.Physical changes are often easy to reverse and the s…
- Dissolving a salt
- Chemical change (reaction)
- Representation
- Displacement of silver
- Halide test
- Exothermic examples
Section 1 of 11Physical changes
Physical change
- Physical change a change in which no new substance is formed.
Physical changes are often easy to reverse and the substances are relatively easy to separate.
The energy change is usually small.
Examples: changing state (melting, freezing, boiling), making a mixture and dissolving a solute in a solvent.
Dissolving a salt
- Dissolving a salt it is a physical change that can be shown by a state change, for example NaCl(s) → NaCl(aq).
Section 2 of 11Chemical changes
1Chemical change (reaction)
- Chemical change (reaction) a change that forms one or more new substances with very different properties from the reactants.
2
Most chemical changes are difficult to reverse.
3
The energy change is often large (heat, light, sound).
4
Mass is conserved in a chemical reaction.
5
Examples: burning magnesium, rusting, neutralisation, burning and cooking.
6Representation
- Representation chemical changes are shown by balanced equations with state symbols.
Section 3 of 11Signs of a chemical reaction
| Sign | What you observe | Example | Chemical or physical? |
|---|---|---|---|
| Gas given off | Fizzing, bubbles | Metal + acid | Chemical |
| Colour change | New colour | Iron rusting; copper in silver nitrate solution | Chemical |
| Temperature change | Gets hot or cold | Burning, neutralisation | Chemical |
| Precipitate | Solid appears in a liquid | Mixing two solutions | Chemical |
| Light or sound | Flash or bang | Fireworks | Chemical |
| Change of state only | Melting, freezing | Ice melting | Physical |
Signs of a chemical reaction
Boiling also produces bubbles, but it is a physical change because no new substance forms.
A new smell or a change in pH are other signs.
Section 4 of 11Colour and temperature changes
Displacement of silver
- Displacement of silver orange-brown copper added to colourless silver nitrate gives silver crystals and a blue solution.
Halide test
- Halide test gives a cream, white or yellow precipitate depending on the halide ion.
Exothermic examples
- Exothermic examples calcium oxide with water and sodium with water.
Endothermic examples
- Endothermic examples photosynthesis and dissolving ammonium chloride in water, as used in cold packs.
Section 5 of 11Gas production
Effervescence
- Effervescence fizzing shows that a gas is made, often when acids react.
Alkali metals with water
- Alkali metals with water metal + water → metal hydroxide + hydrogen.
Section 6 of 11Solubility rules and precipitation reactions
| Type of salt | Solubility in water |
|---|---|
| Sodium, potassium and ammonium salts | All soluble |
| Nitrates | All soluble |
| Chlorides | Soluble except silver chloride and lead chloride |
| Sulfates | Soluble except barium, calcium and lead sulfates |
| Carbonates | Insoluble except sodium, potassium and ammonium carbonates |
| Hydroxides | Insoluble except sodium and potassium hydroxides, and calcium hydroxide is slightly soluble |
Precipitation
- Precipitation it occurs when two solutions are mixed and an insoluble solid forms.
Silver nitrate with sodium chloride gives a white precipitate of silver chloride.
Lead(II) nitrate with potassium iodide gives a bright yellow precipitate of lead(II) iodide.
Section 7 of 11Writing ionic equations for precipitation
1
Write the full balanced equation with state symbols, such as AgNO3(aq) + NaCl(aq) → AgCl(s) + NaNO3(aq).
2
Split all aqueous ionic compounds into ions.
3
Cross out the spectator ions, which are unchanged on both sides (Na+ and NO3−).
4
Write the remaining ions, Ag+(aq) + Cl−(aq) → AgCl(s).
Checks
- Checks both atoms and charges must balance, and the precipitate is written as a solid with no charge.
Barium sulfate
- Ba2+(aq) + SO42−(aq) → BaSO4(s).
Lead(II) iodide
- Pb2+(aq) + 2I−(aq) → PbI2(s).
Metal hydroxide
- Cu2+(aq) + 2OH−(aq) → Cu(OH)2(s).
Displacement
- Zn(s) + Cu2+(aq) → Zn2+(aq) + Cu(s).
Carbonate with acid
- CO32−(aq) + 2H+(aq) → H2O(l) + CO2(g).
Neutralisation
- H+(aq) + OH−(aq) → H2O(l), which is not a precipitation but is written in the same way.
Section 8 of 11Uses of precipitation and hydroxide precipitates
1Preparing insoluble salts
- Preparing insoluble salts mix two soluble salts, filter, wash with distilled water and dry.
2Testing for ions
- Testing for ions chloride gives a white precipitate with acidified silver nitrate and sulfate gives a white precipitate with acidified barium chloride.
3Acidifying
- Acidifying use dilute nitric acid for the chloride test and dilute hydrochloric acid for the sulfate test,
- so that the acid does not itself form a precipitate.
4Removing harmful ions
- Removing harmful ions heavy metals can be precipitated from water as hydroxides.
| Cation | Hydroxide precipitate with NaOH | In excess NaOH |
|---|---|---|
| Cu2+ | Blue | Insoluble |
| Fe2+ | Green | Insoluble |
| Fe3+ | Red-brown | Insoluble |
| Ca2+ | White | Insoluble |
| Al3+ and Zn2+ | White | Dissolves to a colourless solution |
Ionic equation
- Ionic equation metal ions combine with OH− ions, for example Fe3+(aq) + 3OH−(aq) → Fe(OH)3(s).
Aqueous ammonia
- Aqueous ammonia it gives similar precipitates for most of these cations,
- and in excess the Cu(OH)2 precipitate dissolves to give a deep blue solution and the Zn(OH)2 precipitate dissolves.
Section 9 of 11More from the specification
More from the specification
Physical reversibility as a diagnostic means
- Physical reversibility as a diagnostic means checking whether matter returns to its original state through cooling or evaporation provides a simple diagnostic means.
Collision Theory and Activation Energy
- For a chemical reaction to occur, reactant particles must collide with the correct geometric orientation and with kinetic energy equal to or exceeding the activation energy.
Effect of Concentration on Collision Frequency
- Increasing solute concentration increases the number of particles per unit volume, multiplying the frequency of collisions between reactant particles and accelerating the reaction rate.
Effect of Surface Area to Volume Ratio
- Subdividing a solid reactant into smaller pieces or powder greatly expands the accessible surface area, exposing more reactant particles simultaneously to collisions.
Pressure
- Force of gas particles colliding with a surface.
Theory
- A tested hypothesis that is widely accepted.
Successful collision
- A collision with the correct orientation and enough energy to react.
Activation energy
- The minimum energy colliding particles need to react.
Collision theory
- Particles react only if they collide with at least the activation energy and the correct orientation.
Kinetic energy
- The energy of motion of a particle.
Particle
- A tiny portion of matter, too small to see.
Concentration
- Moles ÷ volume in dm³, or g/dm³.
Volume
- The space taken up by an object.
Catalyst
- A substance that speeds up a reaction and is not used up.
Gradient
- The change in y divided by the change in x on a graph.
Section 10 of 11Key terms
| Term | Meaning |
|---|---|
| Physical change | a change in which no new substance is formed. |
| Chemical change | a change in which one or more new substances are formed. |
| Effervescence | fizzing caused by a gas being given off. |
| Precipitate | an insoluble solid formed in a solution. |
| Exothermic | a change that transfers thermal energy to the surroundings. |
| Endothermic | a change that takes in thermal energy from the surroundings. |
| Spectator ion | an ion present in the mixture but not involved in the reaction. |
| Ionic equation | an equation showing only the ions that react. |
| Soluble | able to dissolve in a solvent. |
Section 11 of 11Exam technique
1
Saying all bubbling is a chemical reaction: boiling also makes bubbles but forms no new substance.
2
Saying dissolving is a chemical changeit is a physical change.
3
Saying mass is lost in a reaction: mass is conserved.
4
Saying all chemical changes are easily reversed: most are difficult to reverse.
5
Including spectator ions or writing insoluble solids as separate ions in an ionic equation — only the ions that react are shown, and the precipitate is written as a solid
Marked practiceQuizzes for this lesson
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