Two or more reactants combine to form a single product.

General Form: $A + B \to AB$
Examples:
- Burning of coal: $C(s) + O_2(g) \to CO_2(g)$
- Formation of water: $2H_2(g) + O_2(g) \to 2H_2O(l)$
- Formation of slaked lime: $CaO(s) + H_2O(l) \to Ca(OH)_2(aq) + \text{Heat}$ (Exothermic)

A single reactant breaks down into two or more simpler products. These reactions often require energy (heat, light, electricity).

General Form: $AB \to A + B$
Types:
- Thermal Decomposition: By heating.
  Example: $CaCO_3(s) \xrightarrow{Heat} CaO(s) + CO_2(g)$ (Limestone to quicklime)
- Electrolytic Decomposition (Electrolysis): By passing electricity.
  Example: $2H_2O(l) \xrightarrow{Electricity} 2H_2(g) + O_2(g)$
- Photolytic Decomposition (Photolysis): By light energy.
  Example: $2AgCl(s) \xrightarrow{Sunlight} 2Ag(s) + Cl_2(g)$ (Used in black and white photography)

A more reactive element displaces a less reactive element from its compound.

General Form: $A + BC \to AC + B$
Reactivity Series: A list of metals in order of decreasing reactivity (e.g., K > Na > Ca > Mg > Al > Zn > Fe > Pb > H > Cu > Ag > Au). A metal higher in the series can displace a metal lower in the series.
Examples:
- Iron displacing copper: $Fe(s) + CuSO_4(aq) \to FeSO_4(aq) + Cu(s)$ (Iron nail turns brownish, blue copper sulfate solution fades)
- Zinc displacing hydrogen: $Zn(s) + 2HCl(aq) \to ZnCl_2(aq) + H_2(g)$

Two compounds exchange their ions to form two new compounds. These reactions often result in the formation of a precipitate, water, or a gas.

General Form: $AB + CD \to AD + CB$
Examples:
- Precipitation Reaction: Formation of an insoluble product (precipitate).
  $BaCl_2(aq) + Na_2SO_4(aq) \to BaSO_4(s) + 2NaCl(aq)$ (White precipitate of barium sulfate)
- Neutralization Reaction: Reaction between an acid and a base to form salt and water.
  $HCl(aq) + NaOH(aq) \to NaCl(aq) + H_2O(l)$

These reactions involve the transfer of oxygen or hydrogen, or electrons.

Oxidation:
- Gain of oxygen.
- Loss of hydrogen.
- Loss of electrons (more advanced concept, not primary for Class 10).
Reduction:
- Loss of oxygen.
- Gain of hydrogen.
- Gain of electrons (more advanced concept).
Redox Reaction: A reaction where one substance is oxidized while another is reduced simultaneously.
Oxidizing Agent: The substance that oxidizes another substance and gets reduced itself.
Reducing Agent: The substance that reduces another substance and gets oxidized itself.
Example: $CuO(s) + H_2(g) \xrightarrow{Heat} Cu(s) + H_2O(l)$
- $CuO$ is reduced (loses oxygen). It is the oxidizing agent.
- $H_2$ is oxidized (gains oxygen). It is the reducing agent.