Acids, Bases and Salts

An acid produces hydrogen ions (${\text{H}}^{+}$) in water, while a base is anything that can neutralise an acid by accepting ${\text{H}}^{+}$. An alkali is simply a base that dissolves in water. A salt is the ionic compound formed when the ${\text{H}}^{+}$ of an acid is replaced by a metal ion or ammonium ion, and its name always comes from the acid (chloride, sulfate, nitrate) plus the metal used. Acids turn blue litmus red and taste sour; alkalis turn red litmus blue, feel soapy, and taste bitter — but taste and touch are descriptive knowledge only, never real lab tests, since both acids and alkalis are corrosive.

Every neutralisation reaction that produces a salt falls into one of four families: acid + metal, acid + metal oxide, acid + metal hydroxide, or acid + metal carbonate. Only metals more reactive than hydrogen (never copper, silver or gold) react directly with acids to release hydrogen gas; the other three families all produce water, and the carbonate reaction additionally releases carbon dioxide gas. Constructing these equations correctly means getting three things right: correct formulas, correct balancing, and correct state symbols matching the actual stage of the reaction described.

Soluble salts like copper(II) sulfate or sodium chloride are made in solution and then crystallised out, while insoluble salts like lead(II) sulfate form immediately by precipitation and just need filtering. Whenever the base, carbonate or metal is insoluble, use the excess-solid method: add it in excess to the acid so every drop of acid reacts, then filter off the leftover solid. Evaporate the filtrate only to the saturation point — test with a glass rod, and stop heating once crystals form on it, because evaporating fully on the flame ruins crystal shape or decomposes the salt.

Ammonium nitrate and ammonium sulfate, made by neutralising ammonia with acid, are the most common nitrogen fertilisers and are prepared exactly by the neutralisation methods above. Antacid tablets use weak bases like magnesium hydroxide or calcium carbonate to neutralise excess stomach acid (HCl) and relieve indigestion. Industrially, alkalis break down fats and grease in soap-making and drain cleaners through saponification, while strong acids clean metal surfaces (pickling) before galvanising or painting.

Criterion B and C questions often give crystal-yield data across different conditions and ask which variable mattered most — the key is separating factors that change reaction rate (stirring, particle size) from factors that change the amount of salt actually recovered (temperature, concentration, time). Citing the biggest number without explaining the underlying chemistry (rate vs solubility/yield) earns no reasoning marks. 'Evaluate' questions, like choosing between two solvents for production, always require both sides of the trade-off stated plus a final justified decision — a one-sided answer caps your mark.