TSR Wiki > Study Help > Subjects and Revision > Revision Notes > Chemistry > Acids and Bases
- 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:
Some general reactions of acids
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 dilute 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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