Acids, Bases and Salts – Core Vocabulary

A set of key vocabulary flashcards covering definitions, classifications, properties, and preparation methods related to acids, bases, salts, pH, indicators, and related chemical concepts.

Acid (Arrhenius)

A compound that, when dissolved in water, produces hydronium ions (H₃O⁺) as the only positive ions.

Base (Arrhenius)

A compound that reacts with hydronium ions to form salt and water; typically a metal oxide or hydroxide.

Alkali

A water-soluble base that furnishes hydroxyl ions (OH⁻) as the only negative ions in solution.

Hydronium Ion

The ion H₃O⁺ formed when a proton (H⁺) bonds to a water molecule; responsible for acidity in aqueous solutions.

Hydroxyl Ion

The ion OH⁻ produced by dissociation of alkalis in water; responsible for alkalinity.

Organic Acid

An acid derived from plants or animals, e.g., citric or acetic acid.

Inorganic (Mineral) Acid

An acid obtained from minerals, e.g., HCl, H₂SO₄, HNO₃.

Hydracid

An acid containing hydrogen and a non-metal other than oxygen, e.g., HCl, HBr.

Oxyacid

An acid containing hydrogen, another element, and oxygen, e.g., H₂SO₄, HNO₃.

Strong Acid

An acid that almost completely dissociates in water, giving a high concentration of H₃O⁺ ions.

Weak Acid

An acid that dissociates only partially in water, yielding a low concentration of H₃O⁺ ions.

Concentrated Acid

An aqueous acid solution with a relatively high percentage of acid (usually >1 mol dm⁻³).

Dilute Acid

An aqueous acid solution with a relatively low percentage of acid (<1 mol dm⁻³).

Strong Alkali

An alkali that almost completely dissociates in water to give a high concentration of OH⁻ ions; e.g., NaOH, KOH.

Weak Alkali

An alkali that dissociates only partially in water, producing few OH⁻ ions; e.g., NH₄OH, Ca(OH)₂.

Concentrated Alkali

An aqueous alkali solution containing a high percentage of base.

Dilute Alkali

An aqueous alkali solution containing a low percentage of base.

Basicity of an Acid

The number of hydrogen ions that one molecule of an acid can furnish or replace in solution.

Acidity of a Base

The number of hydroxyl ions one molecule of a base can furnish or replace in solution.

Monobasic Acid

Acid with basicity = 1; provides one replaceable hydrogen ion, e.g., HCl.

Dibasic Acid

Acid with basicity = 2; provides two replaceable hydrogen ions, e.g., H₂SO₄.

Tribasic Acid

Acid with basicity = 3; provides three replaceable hydrogen ions, e.g., H₃PO₄.

Monoacidic Base

Base with acidity = 1; yields one OH⁻ per molecule, e.g., NaOH.

Diacidic Base

Base with acidity = 2; yields two OH⁻ per molecule, e.g., Ca(OH)₂.

Triacidic Base

Base with acidity = 3; yields three OH⁻ per molecule, e.g., Al(OH)₃.

pH

The negative logarithm (base 10) of hydrogen-ion concentration: pH = –log[H⁺].

pH Scale

A scale from 0-14 expressing acidity (pH

Indicator

A weak organic acid or base that changes colour with pH, used to detect acidity or alkalinity.

Litmus

Natural indicator from lichens; turns red in acid, blue in alkali.

Universal Indicator

A blend of dyes or pH paper giving a range of colours to show approximate pH 0-14.

Phenolphthalein

Indicator colourless in acid/neutral, pink in alkaline solutions (pH ~ 8.2-10).

Methyl Orange

Indicator red-pink in acid and yellow in alkaline; colour change around pH 3-4.

Arrhenius Theory

Defines acids as H⁺ producers and bases as OH⁻ producers in aqueous solution.

Lowry–Brønsted Theory

Describes acids as proton donors and bases as proton acceptors.

Neutralization (Ionic)

Reaction in which H⁺ (or H₃O⁺) from an acid combines with OH⁻ from a base to form water and salt.

Heat of Neutralization

The enthalpy change when one gram-equivalent of acid is completely neutralized by a base.

Acid Rain

Precipitation with pH<5.6 containing H₂SO₄, H₂SO₃, HNO₃, and HNO₂ formed from SOₓ and NOₓ pollutants.

Deliquescence

Property of some salts to absorb moisture from air and dissolve in the absorbed water, forming a solution.

Efflorescence

Loss of water of crystallization by hydrated salts on exposure to air, leaving a powdery residue.

Water of Crystallisation

Fixed number of water molecules chemically bound within a crystal lattice of hydrated salts.

Salt (General)

An ionic compound formed by partial or complete replacement of an acid’s hydrogen ion by a metal or NH₄⁺ ion.

Acid Salt

Salt formed by partial replacement of acid hydrogen, retains replaceable H⁺ and shows acidic properties, e.g., NaHSO₄.

Normal Salt

Salt in which all replaceable H⁺ of the acid are fully substituted by metal or NH₄⁺, e.g., Na₂SO₄.

Basic Salt

Salt produced by partial replacement of hydroxyl groups in a polybasic base by an acid radical, e.g., Cu(OH)Cl.

Double Salt

Crystalline compound containing two simple salts combined in a fixed ratio, e.g., K₂SO₄·Al₂(SO₄)₃·24H₂O (alum).

Mixed Salt

Salt containing two different cations or anions, e.g., NaKCO₃ (sodium potassium carbonate).

Complex Salt

Salt that yields a complex ion and a simple ion in solution, e.g., K₂[HgI₄].

Direct Combination (Salt Prep)

Method where elements or simple compounds react directly to form a salt, e.g., Fe + Cl₂ → FeCl₃.

Displacement (Salt Prep)

More active metal displaces a less active one from its salt solution, forming a new salt.

Precipitation (Double Decomposition)

Formation of an insoluble salt by exchange of ions between two soluble salts in solution.

Neutralization of Insoluble Base

Preparing a salt by reacting a solid, sparingly soluble base with a dilute acid, e.g., CuO + H₂SO₄ → CuSO₄.

Neutralization of an Alkali (Titration)

Making a salt by titrating a soluble base with a dilute acid until the indicator shows neutrality.

Acid + Carbonate Reaction

Dilute acid reacts with carbonates or bicarbonates to form salt, water, and carbon dioxide gas.

Soluble Salt

Salt that readily dissolves in water; e.g., all nitrates, Na⁺, K⁺, NH₄⁺ salts.

Insoluble Salt

Salt that does not dissolve appreciably in water; e.g., AgCl, PbSO₄, BaSO₄.

Hydronium-Ion Formation

Proton from an acid attaches to the lone pair on water’s oxygen, forming H₃O⁺ via a coordinate bond.

Hydroxyl-Ion Formation

Water donates a proton to a base such as NH₃, leaving OH⁻ in solution.

Coordinate Covalent Bond

Bond in which both shared electrons originate from the same atom; occurs in H₃O⁺ and NH₄⁺ formation.

Gypsum

Hydrated calcium sulphate, CaSO₄·2H₂O; examples of water of crystallisation and efflorescence.

Blue Vitriol

Hydrated copper(II) sulphate, CuSO₄·5H₂O; exhibits efflorescence and used to prepare CuSO₄ solutions.