Summary notes for CIE IGCSE Biology Topic 8: Acids, bases and salts
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Course
Chemistry
Institution
GCSE
Complete revision notes for Topic 8 of the CIE IGCSE Chemistry course: Acids, bases and salts. Explanations with diagrams for every specification point. These notes are written for candidates taking the Extended paper.
NOTE a base which is water-soluble is referred to as an alkali
Bases are usually oxides or hydroxides of metals
What makes a base basic?
When alkalis react, they gain electrons to form negative hydroxide ions (OH–).
The presence of the OH– ions is what makes the aqueous solution an alkali.
Bases (alkalis) are proton acceptors as they ionize in solution producing OH– ions which can
accept protons. These OH– ions make the aqueous solution alkaline.
Describe the meaning of weak and strong acids and bases
Acids and alkalis can be either strong or weak, depending on how many ions they produce
when dissolved in water
,Strong
Strong acids and bases ionize completely in water, producing solutions of very low pH for an
acid or very high pH for a base
Strong acids include HCl and H2SO4 and strong bases include the Group I hydroxides.
Weak
Weak acids and bases partially ionize in water and produce pH values which are closer to
the middle of the pH scale.
Weak acids include organic acids such as ethanoic acid, CH3COOH and weak bases include
aqueous ammonia.
Equilibrium
For both weak acids and bases, there is usually an equilibrium set-up between the
molecules and their ions once they have been added to water.
Example of a weak acid: propanoic acid
CH3CH2COOH ⇌ H+ + CH3CH2COO-
Example for a weak base: aqueous ammonia
NH3 + H2O ⇌ NH4+ + OH-
In both cases the equilibrium lies to the left, indicating a high concentration of intact
acid/base molecules, with a low concentration of ions in solution.
Concentration vs. strength
A concentrated solution of either an acid or a base is one that contains a high number of
acid or base molecules per dm3 of solution.
It does not necessarily mean that the acid or base is strong though, as it may be made from
a weak acid or base which does not dissociate completely.
For example, a dilute solution of HCl will be more acidic than a concentrated solution of
ethanoic acid, since most of the HCl molecules dissociate but very few of the CH 3COOH do.
Describe the characteristic properties of acids as reactions with metals, bases, carbonates
and effect on litmus and methyl orange
Acids and metals
Only metals above hydrogen in the reactivity series will react with dilute acids.
, When acids react with metals, they form a salt and hydrogen gas:
Acid + Metal → Salt + Hydrogen
Examples of reaction between acids and metals:
Acids and bases
Acids are substances that can neutralise a base, forming a salt and water.
Bases are substances which can neutralise an acid, forming a salt and water.
Metal oxides and metal hydroxides can act as bases.
When they react with acid, a neutralisation reaction occurs.
In all acid-base neutralisation reactions, salt and water are produced:
Acid + Base → Salt + Water
Examples of reaction between acids and bases:
Acids and metal carbonates
Acids will react with metal carbonates to form the corresponding metal salt, carbon dioxide
and water:
Acid + Metal Carbonate → Salt + Carbon Dioxide + Water
Examples of reaction between acids and carbonates:
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