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TSR Wiki > Study Help > Subjects and Revision > Revision Notes > Chemistry > Acids and Bases

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• When a substance dissolves in water, it forms an aqueous solution, which may be acidic, alkaline or neutral.
• Metal oxides dissolve to form alkaline solutions.
• Non-metal oxides dissolve to form acidic solutions.
• The pH scale is used to show how acidic or alkaline a substance is. It runs from 0 (being highly acidic) to 14 (being highly alkaline).
• Indicators are used to show whether a solution is acidic, alkaline or neutral by the way their colours change. Some common indicators are: Universal Indicator, Litmus, Methyl Orange and Phenolphthalein.
• A base is a substance that neutralises an acid. An alkali is a soluble base. (All alkalis are bases, but not all bases are alkalis)
• All acids contain hydrogen ions, H+ in solution.
• All alkalis contain hydroxide ions, OH– in solution.
• An acid is a proton donor, a base is a proton acceptor.
• An acid reacts with an alkali to produce a salt and water. This type of reaction is called a neutralisation reaction.

The hydrogen ions react with the hydroxide ions to form water:

Contents 1 Some general reactions of acids 2 Strong and Weak Acids 3 Salt Preparation 4 Table of Solubility 5 Testing for ions in solution 6 Flame Test — used for solid samples 7 Comments

Some general reactions of acids

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2. 
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N.B. Metals below hydrogen in the reactivity series (copper, silver, gold) will not react with acids.

• Reactions involving hydrochloric acid produce chlorides.
• Reactions involving sulphuric acid produce sulphates.
• Reactions involving nitric acid produce nitrates.

Strong and Weak Acids

• A strong acid or alkali is one which is completely ionised in water. E.g. hydrochloric acid (HCl) or sodium hydroxide (NaOH)
• A weak acid or alkali is one which is partially ionised in water. E.g. ethanoic acid (CH₃COOH) or ammonia (NH₃)

Salt Preparation

The following methods may be used to prepare a soluble salt:

1. Acid + Base, Acid + Metal, Acid + Carbonate

Brief Method: Add excess solid (base, metal or carbonate) to some acid. Filter off the solid. Evaporate the remaining solution to half volume and leave to crystallise for a few days.

2. Acid + Alkali

Brief Method: Measure accurately, using a pipette, some alkali (e.g. 25cm³). Add an indicator (usually phenolphthalein). Add acid from a burette until the indicator changes colour. Record your result. Repeat, more accurately (drop by drop near end) until two results are within 0.1cm³ of each other. Then repeat without indicator adding the same volume of acid as your accurate result. Evaporate the resulting solution to half volume and leave to crystallise for a few days. This method is known as titration.

• To prepare an insoluble salt: Mix two solutions of soluble salts. The metal ions “swap partners”, and the insoluble salt is precipitated.

Table of Solubility

Type of Compound	Solubility
Group I compounds and all ammonium compounds	Soluble
Nitrates	Soluble
Chlorides	Soluble (except lead chloride and silver chloride)
Sulphates	Soluble (except calcium sulphate, lead sulphate and barium sulphate)
Carbonates	Insoluble (except group I and ammonium carbonates)

Testing for ions in solution

Negative Ion	Test used	Result of test
Carbonate (CO32–)	Add acid	CO2 evolved—test using limewater.
Sulphate (SO42–)	Add barium nitrate or chloride	White precipitate of barium sulphate formed
Chloride (Cl–)	Add silver nitrate	White precipitate of silver chloride formed
Bromide (Br–)	Add silver nitrate	Cream precipitate of silver bromide formed
Iodide (I–)	Add silver nitrate	Pale Yellow precipitate of silver iodide formed

Positive Ion	Test used	Result of test
Copper (Cu2+)	Add Sodium Hydroxide solution	Blue precipitate of copper hydroxide formed
Iron (II) (Fe2+)	Add Sodium Hydroxide solution	Brown precipitate of iron(II) hydroxide formed
Iron (III) (Fe3+)	Add Sodium Hydroxide solution	Green precipitate of iron(III) hydroxide formed

Flame Test — used for solid samples

Metal Ion	Colour of flame
Sodium (Na+)	Orange/yellow
Lithium (Li+)	Scarlet
Potassium (K+)	Lilac
Copper (Cu2+)	Blue/green
Calcium (Ca2+)	Brick Red
Barium (Ba2+)	Apple Green

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