Preparation of Salts

Salts are classified into two, based on their solubility in water – Soluble and Insoluble salts.

Soluble salts are prepared by crystallization while insoluble salts are prepared by precipitationPrecipitation is any liquid or frozen water that forms in the atmosphere and falls back to the Earth. It comes in many forms, like rain, sleet, and snow. More or double decomposition and by the combination of constituent elements. 

Two factors that influence the method of preparing salts are:

1. Its solubility in water.
2. Its stability to heat.

Preparation of Soluble Salts:

1. By the Action of Dilute Acid on Metals:

A salt can be prepared by the direct displacement of the hydrogen ion in an acid by a metal higher than hydrogen in the activity series e.g calcium, magnesium, zinc, and iron.

Procedure:

(a)(i) Add dilute H₂SO₄ to grounded zinc metal in a conical flask.

(ii) Shake well. After a while, bubbles of colourless odourless gas will be given off. The gas is hydrogen, a colourless solution of zinc tetraoxosulphate (VI) will be obtained in the flask.

(iii) Evaporate the solution to dryness. White solid crystals of zinc tetraoxosulphate (VI) are obtained.

(a) Zn_(s) + H₂SO_4(aq) \( \rightarrow \) ZNSO_4(aq) + H_2(g)

(b) Mg_(s) + 2HCl_(aq) \( \scriptsize \rightarrow \) MgCl_2(aq) + H_2(g)

2. By Neutralization:

Salt can be prepared by titrating an alkali against an appropriate acid. A suitable indicator is used to determine when the neutralization reaction is complete.

KOH_(aq) + HCl_(aq) \( \scriptsize \rightarrow \) KCl_(aq) + H₂O_(l)

3. By the Action of Dilute Acid on Insoluble Base:

All the soluble salts of zinc, magnesium, calcium and lead can be obtained by the use of the oxide of the metal and appropriate acid e.g

(i) PbO_(s) + 2HNO_3(aq) \( \rightarrow \)Pb(NO₃)_2(aq) + H₂O_(l)

(ii) ZnO_(s) + H₂SO_4(aq) \( \rightarrow \) ZnSO_4(aq) + H₂O_(l)

4. Action of Acid on a trioxocarbonate (IV):

When dilute acid (HCl) is added to any trioxocarbonate (IV) salt, effervescence occurs, i.e evolution of colourless, odourless gas which is carbon (IV) oxide. 

\( \scriptsize (i) \: \underset {dirty \: green}{ CuCO_{3(s)}} \: +\: 2HCl_{(aq)} \: \rightarrow \:\underset {light \: green}{CuCl_{2(aq)}} \: + \: H_2O_{(l)} \: + \: CO_{2(g)} \)

(ii) PbCO_3(s) + 2HNO_3(aq) \( \rightarrow \) Pb(NO₃)_2(aq) + H₂O_(l) + CO_2(g)

Demonstration Experiment:

Preparation of Copper (II) tetraoxosulphate (VI) crystals from dilute tetraoxosulphate (VI) acid and Copper (II) oxide.

Boil mixture of H₂SO₄ and CuO on cooling, filter to remove undissolved CuO. The filtrate will contain copper (IV) tetraoxosulphate (VI). Evaporate some of the water to get a saturated solution. When cooled, crystals of the salt will be obtained.

Put the salt obtained in a dessicator or filter paper to dry. A crystal of copper (II) tetraoxosulphate (VI) is obtained.

CuO_(s) + H₂SO_4(aq) \( \rightarrow \) CuSO_4(s) + H₂O_(l)

Preparation of Insoluble Salts:

Salts which are insoluble in water can be prepared in the laboratory by the following methods.

1. Double Decomposition or Precipitation:

In double decomposition, two soluble salts are involved. One containing metallic radical and the other acidic radical. The two soluble salts react to precipitate an insoluble salt.

2. Combination of Constituent Elements:

Some metallic salts like chlorides and sulphides can be prepared by direct combination of elements.

(i) Fe_(s) + S_(s) \( \rightarrow \)FeS_(s)

(ii) 2Fe_(s) + 3Cl_2(g) \( \rightarrow \) 2FeCl_3(s)

(iii) Hg_(s)  +  Cl_2(g)  \( \rightarrow \)  HgCl_2(s)