Grade 9

Oxides, acids, bases, salts

Classify common inorganic compounds and use simple reaction patterns to predict products. Learn how acids and bases react and how indicators reveal pH.

1 Lower secondary 2 Upper secondary 3 Undergraduate 4 Advanced 5 Research

IntuitionIntuition: particles and change

Rust is an oxide; vinegar contains an acid; drain cleaner may contain a base; table salt is a salt. These names describe chemical families, not simply taste or use.

Two pathways: Na → Na₂O → NaOH (a base) and S → SO₃ → H₂SO₄ (an acid). Neutralisation joins them into a salt and water.

SchoolSchool: models and rules

Definition: Key idea

An oxide is a binary compound of oxygen with another element. In the school classification, acids furnish H⁺ in water, bases furnish OH⁻, and salts contain cations and anions other than those of water.

Many metal oxides are basic: for example, calcium oxide reacts with water to form calcium hydroxide. Many non-metal oxides are acidic: carbon dioxide dissolves to form a weakly acidic solution.

Four families of compounds
Class	Formula pattern	Examples
Oxide	element + oxygen	CaO, CO₂, Fe₂O₃
Acid	H + acid residue	HCl, H₂SO₄, HNO₃
Base	metal + OH group	NaOH, Ca(OH)₂, KOH
Salt	metal cation + acid-residue anion	NaCl, CaCO₃, CuSO₄

\ce{HCl + NaOH -> NaCl + H2O}

\ce{SO3 + H2O -> H2SO4}

Example: Worked example

An acid solution and sodium hydroxide react. Name the products for hydrochloric acid.

Solution

Neutralisation gives sodium chloride and water: HCl + NaOH → NaCl + H₂O. One H⁺ and one OH⁻ form one water molecule.

Example: Second example

SO₃ dissolves in water. What forms?

Solution

The acidic oxide reacts with water to give sulfuric acid: SO₃ + H₂O → H₂SO₄.

In neutralisation, an acid and a base react to form water and a salt. The exact salt depends on the ions present; balance the chemical equation and check charge as well as atom counts.

Patterns to remember: active metal + oxygen → basic oxide; basic oxide + water → base; acid + base → salt + water; metal oxide + acid → salt + water. Test each prediction by writing formulas correctly.

Example: Calculation / application example

Write the balanced equation when magnesium oxide dissolves in dilute sulfuric acid.

Solution

MgO (basic oxide) + H₂SO₄ (acid) → MgSO₄ (salt) + H₂O (water). Both Mg and SO₄ carry charge ±2, so the ratio is 1:1.