Topic 3 - Acids

Acids are proton (H+) donors.  In aqueous solution, acids release these H+ ions.  Bases are proton acceptors.  In aqueous solution, soluble bases are called alkalis and they release hydroxide (OH-) ions.  A salt is produced when the H+ ion of an acid is replaced by a metal ion or an ammonium ion.   Acids react with metals to form salts and the gas, hydorogen is given off.   Acids also react with metal oxides to form salts and water.  They react with alkalis in a similar way, and they react with metal carbonates to form salts, water and carbon dioxide. Ammonia behaves as a base.  It neutralises, for example, sulphuric acid to form the salt, ammonium sulphate.  Some salts have water molecules incorporated into their structure.  These salts are called hydrated salts. The water is known as water of crystallisation.

Preparing a salt by titration:

(1) A known volume of acid is pipetted into a conical flask and universal indicator added. The acid is titrated with the alkali from the burette.

(2) The acid is added until the indicator turns green, pH 7 neutral. This means all the acid has been neutralised to form the salt

(3) The volume of alkali needed for neutralisation is then noted, this is called the endpoint volume. (1)-(3) are repeated with both known volumes mixed together BUT without the contaminating universal indicator.

(4) The solution is transferred to an evaporating dish and heated to partially evaporate the water causing crystallisation or can be left to slowly evaporate - which tends to give bigger and better crystals.

(5) The residual liquid can be decanted away and the crystals can be carefully collected and dried by 'dabbing' with a filter paper OR the crystals can be collected by filtration (below) and dried (as above).


(i) You can put the acid in the burette and the alkali in the flask.

(ii) Parts (1) to (3) are known specifically as an acid-base (alkali) titration, and the general method is known as a volumetric titration by which it possible to find out exactly what volume ratios are needed for neutralisation. So knowing one concentration, you can calculate the other.

For more information on how to prepare salts by neutralisation with oxides and carbonates, look here.


Preparing a soluble salt from a base:

(1) The required volume of acid is measured out into the beaker with a measuring cylinder. The insoluble metal, oxide, hydroxide or carbonate is weighed out and the solid added in small portions to the acid in the beaker with stirring.

(2) The mixture may be heated to speed up the reaction. When no more of the solid dissolves it means ALL the acid is neutralised and there should be a little excess solid.

(3)  The hot solution (with care!) is filtered to remove the excess solid metal/oxide/carbonate, into an evaporating dish.

(4) The hot solution is left to cool and crystallise. Then collect and dry the crystals with a filter paper.


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