10th class Science Salts a neutral substance

10th class Science Salts a neutral substance

In chemistry, salts are ionic compounds that can result from the neutralization reaction of an acid and a base according to Arrhenius concept.

 They are composed of cations (positively charged ions) and anions (negatively charged ions) so that the product is electrically neutral (without a net charge).

These component ions can be inorganic such as chloride (Cl−), as well as organic such as acetate (CH3COO−) and monatomic ions such as fluoride (F−) as well as polyatomic ions such as sulfate (SO42−).

Classification of salts: Salts are classified as follows.

(i) Normal salt - The salt which does not contain any ionizable or replaceable hydrogen atom in its molecule is called a normal salt. It is produced by complete replacement of all the obtainable hydrogen ions by metallic or ammonium ions e.g. NaCl, K2SO4 . 

NaOH + HCl → NaCl + H2O ; 2 KOH + H2SO4 → K2SO4 + 2 H2O

(ii) Acid salts - The salt which contains one or more replaceable hydrogen ions in its molecule is called an acid salt. It is produced by the partial replacement of the obtainable hydrogen ions by metallic or ammonium ions. Acid salts ionise in water giving hydronium ions ( H3O) +. e.g. Na2HPO4 , KHSO4 

2 NaOH + H3PO4 → Na2HPO4 + 2 H2O ; KOH + H2SO4 → KHSO4 + H2O

An acid salt can react with a base to form a normal salt.

(iii) Basic salts – The salt which contains one or more replaceable hydroxide ions in its molecule is called a basic salt. It is produced by the partial replacement of the obtainable hydroxide ions by acidic radicals. Basic salts ionize in water to give OH – ions. e.g. Mg(OH)Cl, Pb(OH) NO3

Mg(OH)2 + HCl → Mg(OH) Cl + H2O ; Pb(OH)2 + HNO3 → Pb(OH)NO3 + H2O

A basic salt can react with an acid to form a normal salt. 

(iv) Mixed salt – A salt which contains more than one acidic or basic radicals, other than hydrogen or hydroxyl ion, in its molecule is called a mixed salt. e.g. Ca(OCl) Cl, NaKSO4 . Mixed salts are prepared by different methods.

Ca(OH)2 + Cl2 → Ca(OCl) Cl + H2O ; NaOH + KOH + H2SO4 → NaKSO4 + 2 H2O

(v) Double salt - A salt which is formed by the combination of two simple salts is called a double salt. Such a salt is formed when the two salts are slowly crystallized together from a mixture of their saturated salt solution. e.g. saturated solutions of KCl and

MgCl2 on crystallization form a double salt KCl,MgCl2,6H2O. Similarly, saturated solutions of FeSO4 and (NH4)2SO4 on crystallization form a double salt FeSO4 (NH4)2SO4. 6 H2O.

KCl + MgCl2. 6 H2O --- H2O ----→ KCl. MgCl2. 6 H2O

FeSO4 7 H2O + (NH4)2SO4 --- H2O-------→ FeSO4. (NH4)2SO4 . 6 H2O

(vi) Complex salt - The salt which contains a simple ion and a complex ion is called a complex salt. It is formed by the combination of two simple salts under proper experimental conditions. Examples of complex salts include [Co(NH3)6] Cl3 , K4 [Fe(CN)6].

CoCl3 + 6 NH3 → [Co(NH3)6] Cl3 ; Fe(CN)2 + 4 KCN → K4 [Fe(CN)6]

This salt dissociates in water to give a simple ion and a complex ion. The ion which contains both metallic and non-metallic ions together which do not dissociate in water is called a complex ion.

[Co(NH3)6] Cl3 → [Co(NH3)6] 3 + + 3 Cl - ; K4 [Fe(CN)6] → 4 K+ + [Fe(CN)6] 4 –

In aqueous medium, only the simple ion gives its characteristic tests.

Acidic salts and basic salts are defined in an alternative way also.

1) Basic salts - Salts that hydrolyze to produce hydroxide ions when dissolved in water are called basic salts . These salts react with water to produce alkalinity in the solution.

Such salts are prepared by the reaction between a strong base and a weak acid. For example, sodium carbonate is a salt of strong base NaOH and weak acid H2CO3 . This salt hydrolyses in water to produce OH – ions. Hence though it does not contain OH – ion, sodium carbonate is a basic salt.

Na2CO3 + 2 H2O → 2 Na+ + 2 OH - + H2CO3

Other examples of such salts include KCN, CH3COONa, Na3PO4 .

2) Acidic salts - Salts that hydrolyze to produce H + ions when dissolved in water are called acidic salts. These salts react with water to produce acidity in the solution. Such salts are made by the reaction between a strong acid and weak base.

For example, ammonium chloride is a salt of strong acid HCl and a weak base NH4OH. This salt hydrolyzes in water to produce H + ions . Hence though it does not contain H + ion, ammonium chloride is an acidic salt.

NH4Cl + H2O → NH4OH + H + + Cl –

Other examples of such salts include FeCl3 , Al2(SO4)3, NH4NO3 .

3) Neutral salts – Salts that do not hydrolyze when dissolved in water are called neutral salts. All normal salts are neutral salts. Such salts are made by the reaction between a strong acid and a strong base. For example, sodium chloride is a salt of strong base NaOH and strong acid HCl. On dissolution in water, the base and acid fully ionize and produce equal number of H + and OH – ions. Hence this salt solution is neither acidic nor basic i.e. it is neutral.

NaOH + HCl → Na+ + OH - + H + + Cl –

Other examples of such salts include KCl , K2SO4, NaNO3 .

Uses of salts Many salts find use in day to day life.

1) Potassium permanganate and potash alum are used to purify water.

2) Magnesium sulphate ( Epsom salt ) is used in purgatives.

3) Baking soda is used in cooking so also as a mild antacid.

4) Common salt is added to add taste to our food.

5) Plaster of paris is used for making statues and for false ceiling.

6) Ammonium sulphate, ammonium nitrate and some phosphates are used as fertilizers.

7) Silver bromide is used in photography.

8) Calcium carbonate is used in making chalk powder.

Typical properties of some water soluble salts

(i) Water of crystallization - 

Several salts contain definite amount of water molecules loosely combined in their structure. This chemically combined water present in the salt accounts for the crystalline nature of the salt. Hence this water is called ‘water of crystallization’. Salts containing water of crystallization are called hydrated salts. The examples of hydrated salts include FeSO4. 7 H2O, CuSO4. 5 H2O, Na2CO3. 10 H2O.

(ii) Efflorescence - 

Certain hydrated crystals, when exposed to the atmosphere at ordinary temperature, lose their water of crystallization and are transformed into a powder. Such substances are called efflorescent substances and the property is called efflorescence. Effloresce is maximum when the temperature is high and the atmosphere is dry. The examples of efflorescent substances include Na2SO4. 10H2O, MgSO4.7 H2O.

(iii) Deliquescence - 

Certain substances, when exposed to the atmosphere at ordinary temperature, absorb moisture from the atmosphere or air . They become moist, lose their crystalline form and ultimately dissolve in the absorbed water forming a saturated solution. Such substances are called deliquescent substances and the property is called deliquescence. Deliquescence is maximum when temperature is low and atmosphere is humid. The examples of deliquescent substances include CaCl2. 6 H2O, Cu(NO3)2. 3 H2O, MgCl2.6 H2O.

(iv) Hygroscopy -

Certain substances, when exposed to the atmosphere at ordinary temperature, absorb moisture from the atmosphere without dissolving in it . Such substances are called hygroscopic substances and the property is called hygroscopy. Hygroscopic substances are generally anhydrous solids or liquids. They are used as drying agents for drying gases. The examples of hygroscopic substances include CaCl2, CaO, Silica gel, Ethyl alcohol.