Sources of Energy
Heating something requires an energy source. The main ones are:
| Source | Notes | Renewable? |
|---|---|---|
| Biomass | Organic material — grains, wood chips, animal waste; used for heating/cooking | Yes |
| Fossil fuels | Coal, crude oil, natural gas — formed from ancient living matter | No |
| Solar | Harvested via solar panels | Yes |
| Wind | Turbines drive generators | Yes |
| Geothermal | Steam from hot rocks beneath the Earth drives turbines | Yes |
| Wave | Ocean wave motion drives turbines | Yes |
| Hydro | Water from a dam turns generators; costly to build | Yes |
| Nuclear | Energy released by nuclear reactions, e.g. from uranium; safety and waste disposal are concerns | No |
Fossil fuels and biomass both release large amounts of CO₂. Renewable sources are essentially unlimited in supply; non-renewable ones are finite and don’t replenish on a human timescale.
Decomposition vs. Dissociation
When some substances are heated, they break down into two or more new substances that do not recombine on cooling — this is thermal decomposition, e.g. CaCO₃ → CaO + CO₂. If the products do recombine on cooling, it’s called thermal dissociation — a reversible reaction, e.g. NH₃ + HCl ⇌ NH₄Cl.
How Different Substances Respond to Heat
Mass increases when a metal is heated in air and reacts with oxygen to form its oxide — e.g. copper turns from reddish-brown to black as it forms copper(II) oxide: 2Cu + O₂ → 2CuO.
Mass decreases when heating causes decomposition and a gas escapes:
- Calcium carbonate decomposes to calcium oxide and CO₂ (CaCO₃ → CaO + CO₂) — the released CO₂ turns limewater milky, which is the standard test for the gas. Lithium and magnesium carbonate behave the same way (Li₂CO₃ → Li₂O + CO₂; MgCO₃ → MgO + CO₂).
- Copper(II) carbonate decomposes from green to black copper(II) oxide: CuCO₃ → CuO + CO₂.
- Lead(II) nitrate crystals decompose releasing reddish-brown NO₂ gas and oxygen: 2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂.
- Lead(IV) oxide decomposes at around 600°C: 2PbO₂ → 2PbO + O₂.
- Hydrated salts — those with water of crystallisation loosely bound in their structure — lose that water on heating to become anhydrous, e.g. blue CuSO₄·5H₂O becomes white anhydrous CuSO₄. Na₂CO₃·10H₂O and CaSO₄·2H₂O are further examples.
- Sugar decomposes on heating, losing water and leaving carbon (charcoal) behind, turning from white to black: C₁₂H₂₂O₁₁ → 12C + 11H₂O.
No change in mass occurs for salts that are simply stable to heat and don’t decompose at all, such as NaCl, MgCl₂ and KCl (sodium carbonate, Na₂CO₃, is a further example).
A Few More Useful Terms
- Water of crystallisation: water molecules loosely bound within a substance’s structure.
- Anhydrous means without water of crystallisation (e.g. anhydrous calcium chloride); hydrated means it contains water of crystallisation (e.g. CuSO₄·5H₂O); dehydration is the removal of that water.
- Conductors allow heat or electricity to pass through them (all metals); insulators do not (most non-metals, plastics).
- Electrolytes are substances that, in solution or molten, allow electricity to pass through while decomposing in the process (e.g. sodium chloride); non-electrolytes do not conduct electricity in this way.
