Acids, Bases and Salts

The One Thing

Acidic and basic behaviour arises primarily from ions formed in water: acids produce H₃O⁺ ions, whereas bases produce OH⁻ ions or accept H⁺ ions. Their reactions, pH values and applications follow from this ionic behaviour, including neutralisation to form salts and water.

Reactions, Processes and Experiments

Key Terms

• Acid: A substance that produces hydrogen ions, H⁺, in water and usually has a sour taste.
• Base: A substance that produces hydroxide ions, OH⁻, in water or accepts hydrogen ions.
• Alkali: A base that dissolves in water, such as sodium hydroxide or potassium hydroxide.
• Indicator: A substance that changes colour to show whether a solution is acidic or basic.
• Litmus: An indicator that turns blue litmus red in an acid and red litmus blue in a base.
• Olfactory indicator: A substance whose smell changes or disappears in acidic or basic conditions, such as onion or vanilla essence.
• Universal indicator: A mixture of indicators that shows different colours over a wide pH range.
• pH: A measure of the concentration of hydrogen ions in a solution; lower pH means greater acidity.
• Neutralisation: The reaction between an acid and a base that produces salt and water.
• Salt: An ionic compound formed when the hydrogen ion of an acid is replaced by a metal ion or ammonium ion.
• Strong acid: An acid that ionises almost completely in water, such as hydrochloric acid.
• Weak acid: An acid that ionises only partially in water, such as acetic acid.
• Water of crystallisation: A fixed number of water molecules chemically attached to one formula unit of a salt.
• Baking soda: Sodium hydrogen carbonate, NaHCO₃, used in baking, antacids and some fire extinguishers.
• Washing soda: Sodium carbonate decahydrate, Na₂CO₃·10H₂O, used for cleaning and removing permanent hardness of water.
• Bleaching powder: Calcium oxychloride, commonly represented as CaOCl₂, used for disinfecting water and bleaching.
• Plaster of Paris: Calcium sulphate hemihydrate, CaSO₄·1/2H₂O, which forms gypsum when mixed with water.

Easily Confused

• Acid and alkali: An acid produces H⁺ or H₃O⁺ ions in water; an alkali is a base that dissolves in water and produces OH⁻ ions.
• Strong acid and concentrated acid: A strong acid ionises almost completely; concentration refers to the amount of solute present in a given volume.
• Dry hydrogen chloride and hydrochloric acid: Dry hydrogen chloride gas does not show acidic behaviour with dry blue litmus, whereas hydrochloric acid ionises in water and shows acidic behaviour.
• Universal indicator and litmus: Universal indicator gives a range of colours across the pH scale; litmus indicates acidity or basicity using red and blue colour changes.
• Baking soda and washing soda: Baking soda is NaHCO₃; washing soda is Na₂CO₃·10H₂O.
• Plaster of Paris and gypsum: Plaster of Paris is CaSO₄·1/2H₂O; when mixed with water, it forms gypsum, CaSO₄·2H₂O.
• Bee stings and wasp stings: Bee stings inject an acidic substance and may be treated with baking soda; wasp stings are generally alkaline and may be treated with vinegar.
• Acidic, neutral and basic solutions: pH below 7 is acidic, pH equal to 7 is neutral, and pH above 7 is basic.

What Gets Asked

• Explain why acids conduct electricity in water and write the ionisation equation HCl + H2O → H3O+ + Cl−. Marks are lost if the role of water or the formation of H₃O⁺ ions is omitted.
• Identify products and observations for reactions of acids with metals, metal carbonates, metal hydrogen carbonates, metal oxides and bases. Marks are lost when hydrogen and carbon dioxide are confused, or when water is omitted from neutralisation equations.
• Interpret the pH scale, including the significance of pH values below, equal to and above 7, and the tenfold change in hydrogen ion concentration for each pH unit. Marks are lost by treating the scale as linear.
• Write or complete equations for the chlor-alkali process, heating baking soda, preparing bleaching powder and converting Plaster of Paris into gypsum. Marks are lost by omitting formula coefficients, water of crystallisation or gaseous products.
• Explain household, biological and agricultural applications, including antacids, toothpaste, treatment of acidic or basic soil, and the different treatments for bee and wasp stings. Marks are lost by assigning the wrong substance to the wrong type of sting or soil.
• Distinguish salts by composition, hydration state and preparation, including sodium chloride from seawater, washing soda as Na₂CO₃·10H₂O, bleaching powder as CaOCl₂ and Plaster of Paris as CaSO₄·1/2H₂O.