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SC8 - Acids and Alkalis (SC8b - Looking at acids (A concentrated solution…
SC8 - Acids and Alkalis
SC8a - Acids, alkalis and indicators
All aqueous solutions are either acidic, alkaline or neutral
Indicators and pH
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Litmus = blue in alkali, red in acid
Methyl orange = yellow in alkali, red in acid
Phenolphthalein = pin in alkali, colourless in acid
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SC8b - Looking at acids
A concentrated solution contains a lot of dissolved solute per unit volume, while a dilute solution contains only a small amount of solute
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if the concentration of hydrogen ions is increased by a factor of 10, the pH decreases by 1 and vice versa
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Acids with low pH values are strong acids. The molecules dissociate (break up) completely into ions when they dissolve in water and produce a high concentration of hydrogen ions
Acids with high pH values are weak acids. The molecules do not dissociate completely into ions in the solution
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SC8c - Bases and salts
Bases are substances that neutralise acids to form a salt and water only, all metal oxides are bases
State symbols = (s) solid, (l) liquid, (g) gas, (aq) aqueous - dissolved in water
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During neutralisation, hydrogen ions in the acid combine with the oxide ions to form water. This removes the hydrogen ions, so the pH increases (more neutral)
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Preparing soluble salts
The reaction between an acid and an insoluble metal oxide produces samples of different soluble salts
An excess of the base is always added, to make sure that the acid is used up and has been fully reacted
The mixture is then filtered to remove the residue from the filtrate, leaving only the salt and water
A solid sample of the salt is then obtained by allowing the water to evaporate so that crystallisation can occur
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SC8g - Solubility
A precipitation reaction is one in which soluble substances in solutions cause an insoluble precipitate to form
Soluble in water
All common sodium, potassium and ammonium salts
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Sodium, potassium and ammonium carbonate
Sodium, potassium and ammonium hydroxides
Insoluble in water
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Lead, barium, calcium sulfates
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All salts are ionic. When two solutions containing soluble salts react together, the ions from the salts swap
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