Revision: Acids, Bases and Salts >> Acids and Bases Chemistry (English Medium) ICSE Class 10 CISCE

Acids are defined as compounds which contain one or more hydrogen atoms and when dissolved in water, produce hydronium ions (H₃O+) the only positively charged ions.

An acid is a compound which when dissolved in water produces hydronium ion(H₃O⁺), the only positively charged
ion and a negative ion.

The basicity of an acid is defined as the number of hydronium ions (H₃O⁺) that can be produced by the ionization of one molecule of that acid in aqueous solution.

A base is either a metallic oxide or a metallic hydroxide or ammonium hydroxide which reacts with hydronium ions of an acid to form a salt and water only.

An alkali is a basic hydroxide which when dissolved in water produces hydroxyl (OH^-) ions as the only negatively charged ions.

Ionization is the condition of being dissociated into ions by heat or radiation or chemical reactions or electrical discharge.

The universal indicator, is a mixture of indicator dyes that gives a spectrum of colours depending on how acidic or alkaline a solution is. Universal indicators give different colours at different concentrations of hydrogen ions in a solution.

The pH of a solution is the negative logarithm to the base 10 of the hydrogen ion concentration expressed in moles per litre.

• Organic acids come from plants, contain carbon, and are usually weak.
• Inorganic acids come from minerals, don’t have carbon (mostly), and are strong.
• Monobasic, dibasic, and tribasic acids give 1, 2, or 3 H₃O⁺ ions per molecule.
• Oxyacids have oxygen (like H₂SO₄); hydracids don’t (like HCl).
• Acid strength depends on ionisation; concentration means how much acid is in water.

• By Synthesis: Acids like HCl are made by combining hydrogen with non-metals.
• From Acidic Oxides: Acidic oxides + water → acids (e.g. CO₂ + H₂O → H₂CO₃).
• By Oxidation: Sulphur or phosphorus + HNO₃ → H₂SO₄ or H₃PO₄.
• By Displacement: Strong acid (like H₂SO₄) displaces weaker acids (like HCl) from their salts.

• Taste & Safety: Acids taste sour; mineral acids are corrosive and should never be tasted.
• Physical State: Acids can be solids (e.g., boric acid) or liquids (e.g., HCl, H₂SO₄); some are volatile, others are not.
• Skin Effect: Strong acids cause burns; conc. H₂SO₄ chars skin, HNO₃ turns it yellow, HCl gives amber color.
• Indicators: Acids turn blue litmus red; also change colors of methyl orange and red cabbage extract.
• Conductivity: Acids conduct electricity in aqueous solutions, acting as electrolytes.

• Acids + Metals → Salt + Hydrogen gas (H₂).
• Acids + Bases → Salt + Water (Neutralisation).
• Acids + Carbonates → CO₂ gas is released.
• Acids + Sulphites → SO₂ gas is released.
• Acids + Sulphides → H₂S gas is released.

• Carbon Dioxide Absorption: Strong alkalis like NaOH and KOH absorb CO₂ from the air to form carbonates and water.
• Neutralisation: Bases neutralise acids to form salt and water (e.g., Ca(OH)₂ + HCl → CaCl₂ + H₂O).
• Precipitation of Hydroxides: Bases form insoluble hydroxides with salts of heavy metals like Cu, Fe, and Zn.
• Amphoteric Hydroxides: Hydroxides of Zn, Al, and Pb dissolve in excess NaOH/KOH, forming soluble salts.
• Ammonia Release: Alkalies release ammonia gas when heated with ammonium salts (e.g., NH₄Cl + NaOH → NH₃).