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An acid is a substance that furnishes hydrogen ions (H+) when dissolved in water.
For example, in its aqueous solution, hydrochloric HC1 (aq) dissociates as:
      HC1 (aq) → H+(aq) + Cl (aq)

An acid is a substance which
(i) is sour in taste
(ii) turns blue litmus paper into red
(iii) contains replaceable hydrogen
(iv) gives hydrogen ion (H+) in aqueous solution (Arrhenius theorem)
(v) can donate a proton (Bronsted & Lowry concept)
(vi) can accept electron (Lewis theorem)

Uses of acid :
1. As food :
      (a) Citric acid — Lemons or oranges (Citrus fruits)
      (b) Lactic acid — sour milk
      (c) Butyric acid — Rancid butter
      (d) Tarteric acid — Grapes
      (e) Acetic acid — Vinegar
      (f) Maleic acid — Apples
      (g) Carbonic acid— Soda water aerated drinks
      (h) Stearic acid — Fats
      (i) Oxalic and — Tomato, wood sorrel

2. Hydrochloric acid (HCl) is used in digestion
3. Nitric acid (HNO3) is used in the purification of gold & silver.
4. Cone. H2SO4 and HNO3 is used to wash iron for its galvanization.
5. Oxalic acid is used to remove rust spot.
6. Boric acid is a constituent of eye wash.
7. Formic acid is present in red ants.
8. Uric acid is present in urine of mammals

Strength of acids
» strong acid (completely ionised in water) - HCI, HNO3, H2SO4
» weak acid (partially ionised in water ) - CH3COOH, H2CO3, HCOOH

Classification of acids
» Hydra acids - NH3, H2S, HCI, HBr, HF
» Oxy acids - HNO3, H2SO4, HClO4, HIO4

Strong Acid

The acids that completely dissociate in water are called strong acids
Nitric acid completely dissociates in water.
      HNO3(aq) → H+(aq) + NO-3(aq)

There are only seven strong acids :
1. HCI - Hydrochloric Acid
2. HBr - Hydrobromic Acid
3. HI - Hydroiodic Acid
4. HCIO - Perchloric Acid
5. HCIO3 - Chloric Acid
6. H2SO4 - Sulphuric Acid
7. HNO3 - Nitric Acid

Weak Acid

The acids that dissociate partially in water are called weak acids. All organic acids like acetic acid and some inorganic acids are weak acids. Since their dissociation is only partial, it is depicted by double half arrows.
      HF(aq) ↔ H+(aq) + F-(aq)

The double arrows indicates here that :
(i) the aqueous solution of hydrofluoric acid not only contains H+(aq) and F-(aq) ions but also the dissociated acid HF(aq).
(ii) there is an equilibrium between the dissociated acid HF(aq) and the ions furnished by it, H+(aq) and F- (aq)

Examples :
(a) CH3COOH Ethanoic (acetic) acid,
(b) HF Hydrofluoric acid

Basicity of an acid

The number of removable hydrogen ions from an acid is called basicity of that acid.
Mono basic acid (one removable H+ ion) — HCl, HNO3
Dibasic acid (two removable H+ ion) — H2SO4, H2CO3 H3PO3
Tribasic acid (three removable H+ ion) — H3PO4

Acidic strength
I. HF < HC1 < HBr < HI
II. CH3COOH < H2SO4 < HNO3 < HCl

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