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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