Acids

ACIDS

Acid is any substance that in water solution tastes sour, changes the colour of certain indicators (e.g., reddens blue litmus paper), reacts with some metals (e.g., iron) to liberate hydrogen and reacts with bases to form salts. They give H+ ions in aqueous solution.

Eg. HCl (Hydrochloric Acid), H2SO4 (Sulphuric acid), HNO3(Nitric Acid), CH3COOH (Acetic Acid), sulfonic acid, and phenol groups.

Strong Acids:   Those acids which dissociate into ions completely are called strong acids eg. H2SO4, HCl, HBr. HI, HNO3, HClO4

Weak Acids: Those acids which do not dissociate into ions completely are called weak acids eg. Citric acid, acetic acid, tartaric acid, formic acid.

Acid Source
Acetic acid Vinegar
Ascorbic acid Guava, amla
Citric acid Lemon, orange and other citrus fruits
Lactic acid Sour milk, curd
Methanoic acid Ant sting, nettle sting
Oxalic acid Tomato
Tartaric acid Tamarind

Note: Acids that give rise to more H+ ions are said to be strong acids, and acids that give less H+ ions are said to be weak acids

A – Physical Properties

 Acids are sour to taste and are corrosive in nature Thus, one should handle acids with care.

 Most acids dissolve in water either at room temperature or on heating to form a clear solution. Vinegar is a 3-5% solution of acetic acid in water. Depending on the amount of water, acids can be either dilute or concentrated. If the amount of water is more in an acid, it is called dilute acid and if the amount of water is less, it is called concentrated acid.

 Acids show acidic properties only in the presence of water.

 Aqueous solutions of Acids conduct electricity.

B – Chemical Properties

1. Reaction with Metals: Metal displaces hydrogen from the acids. The metal combines with the remaining part of the acid and forms a compound called a salt.

Acid + Metal → Salt + Hydrogen gas

   Zn + 2HCl ⇨ ZnCl2+ H2
Zinc + Hydrochloric Acid ⇨ Zinc Chloride + Hydrogen Gas

  2Na + 2HCl ⇨ 2NaCl + H2
Sodium+ Hydrochloric Acid ⇨ Sodium Chloride + Hydrogen Gas

  Fe + 2HCl ⇨ FeCl2+ H2
Iron+ Hydrochloric Acid ⇨ Iron Chloride + Hydrogen Gas

  Zn + H2SO4⇨ ZnSO4 + H2

Zinc + Sulphuric Acid ⇨ Zinc sulphate + Hydrogen Gas
However, such reactions are not possible with all metals.

2. Reaction with Metal Carbonates:

Metal carbonate + Acid → Salt + Carbon dioxide + Water

   Na2CO3+ 2HCl ⇨ 2NaCl + CO2 + H2O
Sodium Carbonate  Sodium Chloride

  MgCO3+ 2HCl ⇨ MgCl2 + CO2 + H2O

Magnesium Carbonate Magnesium Chloride

  CaCO3+ 2HCl ⇨ CaCl2 + CO2 + H2O
Calcium Carbonate Calcium Chloride

CaCO3+ H2SO4 ⇨ CaSO4 + CO2 + H2O

Calcium Carbonate Calcium Sulphate

  Na2CO3+ H2SO4 ⇨ Na2SO4 + CO2 + H2O
Sodium Carbonate Sodium Sulphate

 Na2CO3+ 2HNO3⇨ NaNO3 + 2H2O + CO2
Sodium Carbonate Sodium Nitrate

3. Reaction with Metal Hydrogencarbonates:

Metal hydrogencarbonate + Acid → Salt + Carbon dioxide + Water

 NaHCO3+ HCl ⇨ NaCl + CO2 + H2O
Sodium Hydrogen Carbonate

  2NaHCO3+ H2SO4 ⇨ Na2SO4 + 2CO2 + 2H2O
Sodium Hydrogen Carbonate Sodium Sulphate

The gas evolved because of a reaction of an acid with metal carbonate or metal hydrogen carbonate turns lime water milky. This shows that the gas is carbon dioxide gas. This happens because of formation of white precipitate of calcium carbonate.

Ca(OH)2 + CO2 ⇨ CaCO3 + H2O
Calcium Hydroxide Calcium Carbonate[White]

CaCO3 + CO2 + H2O ⇨ Ca(HCO3)2
Calcium Hydrogen Carbonate

But when an excess of carbon dioxide is passed through lime water, it makes a milky colour of lime water disappear. This happens because of formation of calcium hydrogen carbonate. As calcium hydrogen carbonate is soluble in water, thus the milky colour of solution mixture disappears.

3. Reaction of Metallic Oxides: The general reaction between a metal oxide and an acid can be written as –

Metal oxide + Acid → Salt + Water

2HCl + CaO ⇨ CaCl2 + H2O
Calcium Chloride

H2SO4 + ZnO ⇨ ZnCl2 + H2O
Zinc Chloride

Al2O3 + 6HCl ⇨ 2AlCl3 + 3H2O
Aluminium Chloride

Since metallic oxides react with acids to give salts and water, similar to the reaction of a base with an acid, metallic oxides are said to be basic oxides.

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