Revision: Acids, Bases and Salts Science SSLC (English Medium) Class 9 Tamil Nadu Board of Secondary Education

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.

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

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.

Metals like gold and silver are not reactive with either HCl or HNO₃. But the mixture of these two acids can dissolve gold. This mixture is called Aquaregia. It is a mixture of hydrochloric acid and nitric acid prepared optimally in a molar ratio of 3:1. It is a yellow- orange fuming liquid. It is a highly corrosive liquid, able to attack gold and other resistant substances.

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

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.

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

pH scale : The scale over which a range of pH values from acidic to basic are arranged is called a pH scale.

The pH of a solution is defined as the negative logarithm to the base 10, of the concentration of H⁺ ions in solution in mol dm^–3.

pH is expressed mathematically as

pH = -log₁₀ [H⁺] or pH = -log₁₀ [H₃O⁺]

The pOH of a solution can be defined as the negative logarithm to the base 10, of the molar concentration of OH⁻ ions in solution.

pOH = -log₁₀[OH^-]

pH scale is a scale for measuring the hydrogen ion concentration in a solution.

It is the phenomenon by which substances absorb moisture from air, but only sufficiently so as to become wet.

Salt is a compound formed by the partial or total replacement of the ionisable hydrogen atoms of an acid by a metallic ion or an ammonium ion.

It is the fixed amount of water that is present in a crystal as an integral part of its constitution. Hydrated salts are salts having water of crystallisation.

It is the phenomenon by which hydrated salts, on exposure to dry air, lose their water of crystallization and crumble to powder.

The salt formed by the partial replacement of the replacable hydrogen atoms of an acid molecule by a metallic or ammonium ion is called acid salts.
For example: NaHSO₄ , Na₂HPO₄

Normal salts are the salts formed by the complete replacement of the ionizable hydrogen atoms of an acid by a metallic or ammonium ion.

For example: Sodium chloride (NaCl), Na₂SO₄.

Efflorescence is the property of some salts to lose wholly or partly their water of crystallisation when their crystals are exposed to dry air, even for a short time.

Certain water-soluble substances, when exposed to the atmosphere at ordinary temperature, absorb moisture from the atmospheric air to become moist, and ultimately dissolve in the absorbed water, forming a saturated solution. Such a substance is called a deliquescent substance, and the phenomenon is called deliquescence. 

Dehydrating agents are substances that can remove water molecules even from compounds.

Some salts, unite with a definite quantity of water, which is known as the water of crystallisation.

On heating, crystals of sodium chloride produce a crackling sound, called decrepitation. 

Certain substances remove moisture from other substances and are, called desiccants or desiccating agents or drying agents.

The phenomenon due to which salt formed by a weak acid and a strong base, or by a strong acid and a weak base, dissolves in water to give an acidic or an alkaline solution, is known as hydrolysis.

Some salts absorb moisture (water vapour) from the atmosphere without dissolving in it. Such substances are called hygroscopic substances and the phenomenon is called hygroscopy.

• 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.

• 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.

1. The pH scale (0–14) measures the concentration of H⁺ ions in a solution; values < 7 indicate acids, > 7 indicate bases, and 7 is neutral.
2. A universal indicator shows different colours at different pH levels, helping to determine the strength of an acid or base.
3. Strong acids/bases produce more H⁺ or OH⁻ ions in solution, while weak acids/bases produce fewer ions at the same concentration.

• Normal salts – All hydrogen ions of the acid are replaced (e.g., NaCl, Na₂SO₄).
• Acid salts – Formed when only some hydrogen ions are replaced (e.g., NaHSO₄).
• Basic salts – Formed when only some OH⁻ ions are replaced (e.g., Pb(OH)Cl).
• Double salts – Made by combining two salts that crystallise together (e.g., Mohr’s salt).
• Mixed & Complex salts –
  • Mixed: Have more than one acid/base radical (e.g., NaKCO₃)
  • Complex: Give a complex ion on dissociation (e.g., [Ag(CN)₂]⁻)

• Salts conduct electricity when dissolved in water or molten form.
• Some salts contain water (water of crystallisation) and lose it on heating.
• Salts can be acidic, basic, or neutral depending on their parent acid and base.
• Most salts are water-soluble, and solubility changes with temperature.