Acids, Bases & Making Salts
Acids and Alkalis
An acid is a substance that produces hydrogen ions (\(H^+\)) when dissolved in water. An alkali is a soluble base that produces hydroxide ions (\(OH^-\)) when dissolved in water. The pH scale runs from 0 (strongly acidic) through 7 (neutral) to 14 (strongly alkaline).
Common indicators help identify whether a solution is acidic, neutral, or alkaline. Litmus turns red in acid and blue in alkali. Methyl orange turns red in acid and yellow in alkali. Phenolphthalein is colourless in acid and pink in alkali.
Acids produce \(H^+\) ions in solution. Alkalis produce \(OH^-\) ions in solution. A neutral solution has pH 7. Below pH 7 is acidic; above pH 7 is alkaline.
Strong and Weak Acids
A strong acid is one that completely dissociates (breaks apart) into ions in water. Examples include hydrochloric acid (\(HCl\)), sulfuric acid (\(H_2SO_4\)), and nitric acid (\(HNO_3\)). A weak acid only partially dissociates — most of its molecules remain intact. Examples include ethanoic acid and citric acid.
The terms dilute and concentrated refer to how much acid is dissolved in a given volume of water. A dilute solution has a small amount of acid per unit volume; a concentrated solution has a large amount. The terms dilute/concentrated are about how much solute is present, while strong/weak are about how much the acid ionises.
The higher the concentration of \(H^+\) ions, the lower the pH. As the hydrogen ion concentration increases by a factor of 10, the pH decreases by 1.
Bases and Neutralisation
A base is any substance that reacts with an acid to produce a salt and water only. Alkalis are bases that dissolve in water. Neutralisation is the reaction between an acid and a base.
The ionic equation for an acid–alkali neutralisation is:
$$H^+(aq) + OH^-(aq) \rightarrow H_2O(l)$$
Acids react with different types of bases to produce salts:
Acid + metal \(\rightarrow\) salt + hydrogen (e.g. \(Mg + 2HCl \rightarrow MgCl_2 + H_2\))
Acid + metal oxide \(\rightarrow\) salt + water (e.g. \(CuO + H_2SO_4 \rightarrow CuSO_4 + H_2O\))
Acid + metal hydroxide \(\rightarrow\) salt + water (e.g. \(NaOH + HCl \rightarrow NaCl + H_2O\))
Acid + metal carbonate \(\rightarrow\) salt + water + carbon dioxide (e.g. \(CaCO_3 + 2HCl \rightarrow CaCl_2 + H_2O + CO_2\))
The name of the salt produced depends on the acid used: hydrochloric acid makes chlorides, sulfuric acid makes sulfates, and nitric acid makes nitrates. The metal in the salt comes from the base.
Hydrogen is tested using a burning splint — it burns with a squeaky pop. Carbon dioxide is tested by bubbling it through limewater (calcium hydroxide solution) — the limewater turns milky (cloudy).
Making Soluble Salts
Soluble salts can be made in two ways, depending on whether the reactant is soluble or insoluble.
Method 1: Using an insoluble reactant (e.g. a metal oxide or carbonate). Add excess of the insoluble reactant to the acid. The excess base ensures all the acid has reacted. Filter off the excess solid. Evaporate the remaining solution (or leave to crystallise) to obtain the salt crystals.
Method 2: Using a soluble base (alkali). A titration must be used. Measure the acid into a flask using a pipette, add a few drops of indicator, then add the alkali from a burette until the indicator shows neutralisation. Mix the acid and alkali in exactly the right proportions (without indicator this time), then evaporate to obtain salt crystals.
To prepare pure, dry hydrated copper sulfate crystals: (1) Warm dilute sulfuric acid in a beaker. (2) Add excess copper oxide powder and stir until no more dissolves. (3) Filter off the excess copper oxide. (4) Pour the blue copper sulfate solution into an evaporating basin. (5) Heat gently on a water bath until crystals start to form, then leave to cool and crystallise. (6) Pat dry with filter paper.
Solubility Rules and Precipitates
Solubility rules help you predict whether a salt is soluble or insoluble in water:
All sodium, potassium and ammonium salts are soluble. All nitrates are soluble. Most chlorides are soluble (except silver chloride and lead chloride). Most sulfates are soluble (except lead sulfate, barium sulfate and calcium sulfate). Most carbonates and hydroxides are insoluble (except those of sodium, potassium and ammonium).
When two solutions are mixed, if the ions can form an insoluble compound, a precipitate forms. For example, mixing silver nitrate solution with sodium chloride solution produces an insoluble white precipitate of silver chloride.
An insoluble salt can be made by mixing two solutions that contain the required ions. The precipitate is filtered off, washed with distilled water, and dried.