Study of Acids, Bases and Salts

The One Thing

The behaviour of acids, bases, and salts is explained primarily by the ions produced in aqueous solution. Acids and bases undergo characteristic reactions, including neutralisation, while salt preparation depends on reactant solubility, product solubility, and the formation of precipitates.

Reactions, Processes and Experiments

Key Terms

• Acid: A substance that produces hydrogen ions, H+, in aqueous solution; for example, hydrochloric acid produces H+ and Cl− ions in water.
• Base: A substance that reacts with acids and may produce hydroxide ions, OH−, in water; examples include sodium hydroxide and calcium hydroxide.
• Alkali: A base that dissolves in water to form an alkaline solution, such as sodium hydroxide or ammonium hydroxide.
• Salt: An ionic compound formed when the replaceable hydrogen of an acid is partly or completely replaced by a metal ion or ammonium ion.
• Indicator: A substance that changes colour in acidic and alkaline media, helping to identify the nature of a solution.
• Litmus: A natural indicator that turns blue litmus red in an acid and red litmus blue in an alkali.
• Methyl orange: An indicator that is red in acid, yellow in alkali, and orange near its transition range.
• Phenolphthalein: An indicator that is colourless in acid and pink in an alkaline solution.
• Universal indicator: A mixture of indicators that shows different colours over a wide pH range.
• pH: A scale used to express the acidity or alkalinity of a solution; values below 7 are acidic, 7 is neutral, and values above 7 are alkaline at about room temperature.
• Neutralisation: The reaction between an acid and a base to form salt and water.
• Strong acid or base: An acid or base that ionises almost completely in water, such as hydrochloric acid or sodium hydroxide.
• Weak acid or base: An acid or base that ionises only partially in water, such as ethanoic acid or ammonium hydroxide.
• Basicity of an acid: The number of replaceable hydrogen ions present in one molecule of an acid; hydrochloric acid is monobasic and sulfuric acid is dibasic.
• Acidity of a base: The number of hydroxide ions or replaceable hydroxyl groups supplied by one molecule of a base; sodium hydroxide is monoacidic and calcium hydroxide is diacidic.
• Normal salt: A salt in which all replaceable hydrogen ions of the acid have been replaced, such as sodium sulfate, Na2SO4.
• Acid salt: A salt formed by partial replacement of the ionisable hydrogen of a polybasic acid, such as sodium hydrogen sulfate, NaHSO4.
• Basic salt: A salt containing hydroxide groups because neutralisation of a base is incomplete, such as basic lead carbonate.
• Hydrated salt: A salt containing a fixed number of water molecules in its crystal structure, such as CuSO4·5H2O.
• Efflorescence: The loss of some or all water of crystallisation by a hydrated salt when exposed to dry air.
• Deliquescence: The absorption of moisture from air by a substance until it dissolves in the absorbed water.
• Hygroscopic substance: A substance that absorbs moisture from air but does not necessarily dissolve in it.
• Crystallisation: A method of obtaining pure crystals of a salt by concentrating its solution and cooling it.
• Precipitation: The formation of an insoluble solid when two suitable aqueous solutions react.
• Concentrated solution: A solution containing a relatively large amount of solute.
• Dilute solution: A solution containing a relatively small amount of solute.
• Acidic oxide: Generally a non-metal oxide that reacts with bases.
• Basic oxide: Generally a metal oxide that reacts with acids.
• Neutral oxide: An oxide that generally shows neither acidic nor basic behaviour.

Easily Confused

• Strong and weak versus concentrated and dilute: Strong and weak describe the extent of ionisation, whereas concentrated and dilute describe the amount of solute present; a strong acid can be dilute, and a weak acid can be concentrated.
• Efflorescence versus deliquescence: Efflorescence is the loss of water of crystallisation by a hydrated salt, whereas deliquescence is the absorption of atmospheric moisture until the substance dissolves.
• Deliquescent versus hygroscopic: A deliquescent substance absorbs enough moisture to dissolve, whereas a hygroscopic substance absorbs moisture but does not necessarily dissolve.
• Acid salt versus normal salt: An acid salt retains some ionisable hydrogen from a polybasic acid, whereas a normal salt has all replaceable hydrogen ions replaced.
• Neutralisation versus precipitation: Neutralisation forms salt and water from an acid and base, whereas precipitation forms an insoluble solid from suitable aqueous solutions.
• Acidic oxide versus basic oxide: Acidic oxides generally react with bases, whereas basic oxides generally react with acids.
• Monoacidic versus diacidic bases: A monoacidic base supplies one hydroxide ion or replaceable hydroxyl group per molecule, whereas a diacidic base supplies two.
• Monobasic versus dibasic acids: A monobasic acid has one replaceable hydrogen ion per molecule, whereas a dibasic acid has two.

What Gets Asked

• Writing and balancing equations: Questions may require equations such as HCl + H2O → H3O+ + Cl−, Zn + 2HCl → ZnCl2 + H2, or CuO + H2SO4 → CuSO4 + H2O. Marks are lost through incorrect balancing or omission of relevant state symbols and conditions.
• Identifying reaction types from observations: Effervescence in an acid–metal reaction indicates hydrogen, while effervescence in an acid–carbonate or acid–hydrogen carbonate reaction indicates carbon dioxide; a precipitate indicates precipitation.
• Using indicators and pH: Students may be asked for indicator colours or pH classifications. Blue litmus turns red in acid, red litmus turns blue in alkali, methyl orange is red in acid and yellow in alkali, and phenolphthalein is colourless in acid and pink in alkali.
• Distinguishing strength from concentration: The specific error is treating a concentrated acid as necessarily strong or a dilute acid as necessarily weak. Strength concerns ionisation; concentration concerns the amount of solute.
• Selecting a salt-preparation method: The method depends on solubility. Titration is used for a soluble acid and soluble base, excess solid is filtered when an insoluble reactant is used, and precipitation is used for an insoluble salt.
• Explaining hydrated copper(II) sulfate: Students may be asked to describe heating CuSO4·5H2O. The blue crystals lose water of crystallisation and become white anhydrous CuSO4; a salt solution should not normally be evaporated completely to dryness when crystals are required.