Acids, Bases and Salts – Key Vocabulary

Vocabulary flashcards covering major terms, substances, reactions and concepts from the Chapter ‘Acids, Bases and Salts’. Each card provides a concise definition to aid quick revision.

Acid

A substance that releases hydrogen ions (H⁺) in aqueous solution, tastes sour, turns blue litmus red and has pH < 7.

Base

A substance that releases hydroxide ions (OH⁻) in aqueous solution, tastes bitter, turns red litmus blue and has pH > 7.

Alkali

A base that is soluble in water and produces OH⁻ ions in solution (e.g., NaOH, KOH).

Indicator

A dye or mixture of dyes that changes colour to show whether a solution is acidic or basic.

Litmus

Natural indicator from lichen; blue litmus turns red in acid, red litmus turns blue in base.

Phenolphthalein

Synthetic indicator that is colourless in acids and pink in bases.

Methyl Orange

Synthetic indicator that is red in acids and yellow in bases.

Olfactory Indicator

Substance whose odour changes in acidic or basic media (e.g., onion, vanilla, clove).

Neutralisation Reaction

Reaction in which an acid and a base react to form a salt and water.

Metal–Acid Reaction

Acids react with metals to produce a salt and hydrogen gas (e.g., Zn + H₂SO₄ → ZnSO₄ + H₂).

Hydrogen Gas Test

Evolved gas burns with a ‘pop’ sound when a lighted splint is brought near it.

Metal Carbonate + Acid

Produces salt, carbon dioxide and water; CO₂ turns lime water milky.

Lime Water Test

Carbon dioxide makes Ca(OH)₂ solution cloudy due to formation of CaCO₃.

Metallic Oxide

Oxide of a metal; generally basic and reacts with acids to form salt and water.

Non-metallic Oxide

Oxide of a non-metal; generally acidic and reacts with bases to form salt and water.

Hydronium Ion (H₃O⁺)

Form taken by hydrogen ion in aqueous solution; responsible for acidity.

Hydroxide Ion (OH⁻)

Ion produced by bases in water; responsible for basicity.

pH Scale

Numeric scale (0–14) indicating acidity or basicity; pH 7 neutral,

Universal Indicator

Mixture of indicators giving a spectrum of colours to estimate pH value.

Strong Acid

Acid that ionises completely in water, producing a high concentration of H⁺ ions (e.g., HCl, H₂SO₄).

Weak Acid

Acid that ionises partially in water, producing fewer H⁺ ions (e.g., CH₃COOH).

Strong Base

Base that dissociates completely in water, giving a high concentration of OH⁻ ions (e.g., NaOH).

Weak Base

Base that dissociates partially in water, giving fewer OH⁻ ions (e.g., NH₄OH).

Dilution

Process of adding water to decrease the concentration of H₃O⁺ or OH⁻; highly exothermic.

Exothermic Process

A reaction or process that releases heat to the surroundings (e.g., mixing acid with water).

Alkali Burns Warning

Concentrated acids and bases are corrosive; containers carry hazard symbols.

Acid Rain

Rainwater with pH < 5.6 due to dissolved acidic oxides; harms aquatic life and monuments.

Antacid

Mild base (e.g., Mg(OH)₂) used to neutralise excess stomach acid and relieve indigestion.

Tooth Decay pH

Enamel corrodes below pH 5.5; bacteria-produced acids lower mouth pH.

Salt

Ionic compound formed from acid–base reaction, consisting of cation from base and anion from acid.

Salt Family

Group of salts sharing the same cation or anion (e.g., NaCl and Na₂SO₄ are sodium salts).

Rock Salt

Naturally occurring brownish crystals of NaCl mined from ancient dried-up seas.

Chlor-Alkali Process

Electrolysis of brine producing NaOH, Cl₂ and H₂ gases.

Bleaching Powder

Ca(ClO)₂ produced by reacting Cl₂ with Ca(OH)₂; used for bleaching and water disinfection.

Baking Soda

Sodium hydrogencarbonate (NaHCO₃); mild basic salt used in baking, antacids, fire extinguishers.

Baking Powder

Mixture of NaHCO₃ and edible acid (e.g., tartaric acid); releases CO₂ to make cakes rise.

Washing Soda

Sodium carbonate decahydrate (Na₂CO₃·10H₂O); used in cleaning, glass manufacture, softening water.

Water of Crystallisation

Fixed number of water molecules bound in a crystal lattice (e.g., CuSO₄·5H₂O).

Plaster of Paris

Calcium sulphate hemihydrate (CaSO₄·½H₂O); sets to hard gypsum when mixed with water.

Gypsum

Calcium sulphate dihydrate (CaSO₄·2H₂O); heating forms Plaster of Paris.

Olfactory Change Example

Onion cloth strip loses smell in base but not in acid, demonstrating odour indicator behaviour.

Lime (CaO) Application

Farmers add quick lime or slaked lime to acidic soil to raise pH for optimal crop growth.