Preparation of a pure salts involves correct laboratory techniques and selecting a proper method.
Salts are mainly two types:
i. Soluble salts
ii. Insoluble salts
Comparison of Soluble and insoluble salts
Group
|
Soluble salts
|
Insoluble salts
|
Carbonates
|
Na, K
|
Most are insoluble
|
Chlorides
|
Most are soluble
|
Pb, Ag
|
Nitrates
|
All salts are soluble
|
|
Sulphates
|
Most are soluble
|
BaSO4, PbSO4, Ag2SO4 and SrSO4
|
Method of preparing soluble salts
One method out of four methods can be used depending on the availability of the metal containing compound/ metal itself and the reactivity of the metal/ metal compound1. Metal+ acid
2. Metal carbonate+acid
3. Metal oxide+ acid
4. Alkali+ acid
method 1:
If the metal is available for the preparation of the salt, if the acid and the metal reaction is not explosive (e.g: Na, K with acid) and if the metal is reacting with the acid (e.g: Mg, Zn --- metals below H in the reactivity series such as Cu, Ag does not react with acids) this method can be used.
Reaction taking place when an acid react with a metal is
Acid+ metal -> Metal salt + Hydrogen
e.g: HCl(aq) + Zn(s) -> ZnCl2 (aq) + H2 (g)
step 1: Put the acid in a beaker and warm (warming will increase the rate of the reaction)
step 2: Add the metal in excess (if the metal is in excess , the excess of the metal can be filtered out. If the acid was in excess remaining acid cannot be removed as it is soluble resulting in getting impure salt)
step 3: wait until the reaction is over ( This reaction produces Hydrogen gas bubbles , when the bubbling of hydrogen gas is over we know the reaction is over)
step 4: filter out the excess metal
step 5: gently evaporate the filtrate to obtain the salt (evaporating dish can be used for this. cooling can be performed using a water bath. However direct heating is not advised as the salt can evaporate along with water.
Slow cooling produces Large Crystals
Fast Cooling produces small crystals
Method 2:
similar method is used to prepare the salt if the metal carbonate is available. to know the point at which the reaction between the metal carbonate and the acid is over, should wait until the bubbling of carbon dioxide gas is over ( acid + metal carbonate -> metal salt+ Carbondioxide)Method 3:
similar method is used to prepare the salt, to know the point when reaction is over litmus paper can be used. when the acid comes neutral by the metal oxide, it will no longer turn blue litmus red.Method 4:
if the acid and the alkali are the starting materials, titration method has to be used to know the neutralization point as both the substances are soluble (e.g: Using HCl and NaOH to produce NaCl)
step 1: take a known volume (20 ml) of the acid into a titration flask using a pippete
step 2: add few drops of a suitable acid base indicator such as phenopthalin
step 3: perform the titration and find the end point
step 4: Now as we know the volumes of acid and the alkali completely neutralize each other, add volumes of acid and the alkali which completely react with each other in a separate beaker and prepare the salt.
step 5: evaporate and obtain the salt
Method of preparing Insoluble salts
Insoluble salts will remain as a precipitate in the aqueous solution. In order to prepare the insoluble salt, soluble components of the salt has to be mixed in aqueous solution.
e.g : BaSO4 is a insoluble salt it can be produced by mixing a soluble barium compound (BaCl2 is soluble ) and a soluble Sulphate containing compound (H2SO4 is soluble)
When these two are mixed the insoluble BaSO4 precipitate is produced. It can be filtered, washed and dried to obtain the pure salt.
Next about properties and uses of ammonia
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BaSO4
comparison of soluble insoluble salts
GCSE Chemistry
insoluble
large crystals
method
of
preparing
Salts
small crystals
soluble
titration method
