Properties of Acids

Properties of acids include: (i) Taste, (ii) Action on Indicators, (iii) Conduction of electricity and dissociation of acids, (iv) Reaction of Acids with Metals, (v) Reaction of Acids with Metal Carbonates and Hydrogen Carbonates, (vi) Reaction of Acids with Metal Oxides, (vii) Reaction of Acids with Bases, and (viii) Corrosive Nature.

1. Taste

The sour taste of many unripe fruits, lemon, vinegar and sour milk is caused by the acids present in them. Hence, acids have a sour taste. This is particularly true of dilute acids.

  1. Lemon juice: Citric acid and ascorbic acid (vitamin C)
  2. Vinegar: Ethanoic acid (commonly called acetic acid)
  3. Tamarind: Tartaric acid
  4. Sour milk: Lactic acid

2. Action on Indicators

Indicators show different colours in presence of acids and bases.

  1. Litmus: Red
  2. Phenolphthalein: Colourless
  3. Methyl orange: Red

3. Conduction of electricity and dissociation of acids

Solutions of acids in water (aqueous solutions) conduct electricity. Such solutions are commonly used in car and inverter batteries. When acids are dissolved in water they produce ions which help in conducting the electricity. This process is known as dissociation.

Acids produce hydrogen ions (H+) which are responsible for all their characteristic properties. These ions do not exist as H+ in the solution but combine with water molecules as:

H+ + H2O → H3O+

The H3O+ ions are called hydronium ions. These ions are also represented as H+(aq).

On the basis of the extent of dissociation occurring in their aqueous solutions, acids are classified as strong and weak acids.

Strong Acids

The acids which completely dissociate in water are called strong acids. Nitric acid completely dissociates in water.

HNO3(aq) → H+(aq) + NO3(aq)

There are only seven strong acids:

  1. HCl - Hydrochloric Acid
  2. HBr - Hydrobromic Acid
  3. HI - Hydroiodic Acid
  4. HClO4 - Perchloric Acid
  5. HClO3 - Chloric Acid
  6. H2SO4 - Sulphuric Acid
  7. HNO3 - Nitric Acid

Weak Acids

The acids which 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 that

  1. the aqueous solution of hydrofluoric acid not only contains H+ (aq) and F(aq) ions but also the undissociated acid HF(aq).
  2. there is an equilibrium between the undissociated acid HF(aq) and the ions furnished by it, H+(aq) and F(aq)

Examples:

  1. CH3COOH - Ethanoic (acetic) acid
  2. HF - Hydrofluoric acid
  3. HCN - Hydrocynic acid
  4. C6H5COOH - Benzoic acid

4. Reaction of Acids with Metals

Metal displaces hydrogen from acids and hydrogen gas is released. The metal combines with the remaining part of the acid and forms a compound called a salt.

Acid + Metal → Salt + Hydrogen gas

For example, the reaction between zinc and dil. sulphuric acid can be written as:

Zn + H2SO4 → ZnSO4 + H2

Dilute sulphuric acid reacts with zinc to produce hydrogen gas. The gas bubbles rise through the solution. When the burning match stick is brought near the mouth of the test tube the gas in the test tube burns with a pop sound. This confirms that the gas evolved is hydrogen gas.

5. Reaction of Acids with Metal Carbonates and Hydrogen Carbonates

Metal carbonate + Acid → Salt + Water + Carbon dioxide

Metal hydrogen carbonate + Acid → Salt + Water + Carbon dioxide

When dilute HCl is added to sodium carbonate or sodium hydrogen carbonate, carbon dioxide gas is evolved.

Na2CO(s) + 2HCl (aq) → 2NaCl (aq) + H2O (l) + CO(g) ↑

NaHCO(s) + HCl (aq) → NaCl (aq) + H2O (l) + CO2 (g) ↑

On passing the evolved carbon dioxide gas through lime water, Ca(OH)2, the later turns milky due to the formation of white precipitate of calcium carbonate.

Ca(OH)2 (aq) + CO2 (g) → CaCO3 (s) + H2O (l)

If excess of carbon dioxide gas is passed through lime water, the white precipitate of calcium carbonate disappears due to the formation of water soluble calcium hydrogen carbonate.

CaCO3 (s) + H2O (l) + CO2 (g) → Ca(HCO3)2 (aq)

6. Reaction of Acids with Metal Oxides

Metal oxide + Acid → Salt + Water

The reaction between copper oxide and dilute hydrochloric acid results in the formation of copper (II) chloride (cupric chloride) which is a salt of copper. This salt forms bluish green solution. The reaction is:

CuO (s) + 2HCl (aq) → CuCl2 (aq) + H2O (l)

Many other metal oxides like magnesium oxide (MgO) and calcium oxide (CaO) or quick lime also react with acid in a similar way. For example,

CaO (s) + 2HCl (aq) → CaCl2 (aq) + H2O (l)

7. Reaction of Acids with Bases

Acid + Base → Salt + Water

When dilute HCl is added to NaOH solution, the two react with each other. When sufficient HCl is added, the basic properties of NaOH and acidic properties of HCl disappear. The process is called neutralization. It results in the formation of salt and water.

The reaction between hydrochloric acid and sodium hydroxide forms sodium chloride and water.

HCl (aq) + NaOH (aq) → NaCl (aq) + H2O (l)

Similar reactions occur with other acids and bases. For example ,sulphuric acid and potassium hydroxide react to form potassium sulphate and water.

H2SO4 (aq) + 2KOH (aq) → K2SO4 (aq) + 2H2O (l)

8. Corrosive Nature

The ability of acids to attack various substances like metals, metal oxides and hydroxides is referred to as their corrosive nature. The term corrosion is used with reference to metals and refers to various deterioration processes (oxidation) they undergo due to their exposure to environment. Acids are corrosive in nature as they can attack variety of substances.

Strong is different from corrosive

Corrosive action of acids is not related to their strength. It is related to the negatively charged part of the acid. For example, hydrofluoric acid, (HF) is a weak acid. Yet, it is so corrosive that it attacks and dissolves even glass. The fluoride ion attacks the silicon atom in silica glass while the hydrogen ion attacks the oxygen of silica (SiO2) in the glass.

SiO2 + 4HF → SiF4 + 2H2O