7 Acids, bases and salts Flashcards
Hydrated substance
A substance that is chemically combined with water and an anhydrous substance as a substance containing no water
Water of crystallisation
The water molecules present in hydrated crystals, including CuSO45H2O and CoCl 26H2O
Salt
An ionic compound containing cations and anions, formed by the reaction of acids.
Reactions producing salt
acid + metal → salt + hydrogen
acid + metal oxide (insoluble base) → salt + water
acid + metal carbonate → salt + carbon dioxide + water
acid + base → salt + water
Ammonium salt + base → Salt + ammonia(g) + water
Anions sourced from acids
Acid: Anion: Name of anion: Example of salt
hydrochloric acid: Cl–: chloride: sodium chloride
sulfuric acid: SO42–: sulfate: copper sulfate
nitric acid: NO3–: nitrate: calcium nitrate
The method of salt preparation is the same for all three reactions as the metal, metal oxide or metal carbonate are all solids, using the following method
- Warm some acid gently (to speed up the reaction)
- Add to the acid an excess of either the metal, metal oxide or metal carbonate. This ensures all the acid has been neutralised . For the reactions involving metals or metal carbonates you may need to leave aside until the evolution of gas has stopped.
- Filter the mixture to remove the excess solid (Figure 2).
- Transfer to an evaporating dish and evaporate some of the water off over a Bunsen burner until crystals start to form.
- Leave to cool.
- Filter to get the crystals.
- Dry the crystals on filter paper.
Indicator
A substance that changes colour depending if it is in the presence of acid or base.
Method for preparing a salt from an acid and an alkali
- Place a known volume of acid using a volumetric pipette into a conical flask.
- Add a few drops of the indicator.
- Add the alkali to the acid until the indicator changes colour.
- Record the volume of alkali added to the acid.
- Repeat without the indicator or add decolourising charcoal (if this is added, the solution will need to be heated and then filtered to remove the charcoal).
- Transfer to an evaporating dish and heat to evaporate off some of the water until crystals form.
- Leave to cool.
- Filter to get the crystals.
- Dry the crystals on filter paper.
Solubility rules
Soluble:
all ammonium, sodium and potassium salts
all nitrate salts
nearly all chloride, bromide and iodide salts
nearly all sulfates
Carbonates salts: ammonium, sodium and potassium
Hydroxide salts: ammonium, sodium and potassium, Calcium is partial
Insoluble:
silver chloride, silver bromide, silver iodide and lead chloride
barium sulfate, calcium sulfate and lead sulfate
most carbonates
most hydroxides
Preparing insoluble salts explained
When two solutions containing soluble salts are mixed, one of the salts forms as an insoluble solid known as a precipitate (abbreviated to ppt.) and the solution goes cloudy.
The other product is a soluble salt and this stays in solution.
Of the two soluble salts, one contains the cation and the other soluble salt contains the anion of the insoluble salt you are trying to prepare.
Preparing insoluble salts method
- Mix equal quantities of the two soluble salts
- Filter to remove the insoluble salt or precipitate (the soluble salt goes through the filter paper and is known as the filtrate )
- Wash the precipitate with water
- Leave to dry.
acid
A substance that acts as a source of hydrogen ions in solution or a proton donor.
Base
A substance that neutralises acids by combining with the hydroxide ions in them or proton acceptor.
The different colours of acids and bases in some common indicators.
Indicator Colour in acid Colour in base
litmus red blue
methyl orange red yellow
universal indicator paper strong – red / weak – orange/yellow strong – purple / weak – blue
thymolphthalein colourless blue
Strong acids
Strong acids are fully ionised or dissociated in aqueous solution.
hydrochloric acid is a strong acid,
HCl(aq) → H⁺(aq) + Cl⁻(aq)
