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Properties of Acids

  1. Acids have a sour taste.
  2.  Strong acids such as tetraoxosulphate(VI) acid, hydrochloric acid, trioxonitrate(V) acid, etc., are highly corrosive in nature and can burn the skin. Therefore, these acids should be handled with care  and should not be brought in contact with skin or clothes.
  3. Acids are readily soluble in water. Dissolution of acids in water is generally an exothermic process.

An acid should be diluted by adding acid to water and not water to the acid. If water is added to the acid, the heat produced may cause the mixture to splash out and cause burns. The glass container may also break due to excessive local heating.

  1. Acids conduct electricity in their aqueous solutions
  2. Acids impart specific colours to the acid-base indicators Acid-base indicators are the chemical compounds which have different colours in acidic and basic solutions. They can be used to test whether the given solution is acidic or basic. Some common acid-base indicators are given in Table 22.1

Table 22.1. Common Acid-Base Indicators

 

Indicator                                    Colour in Acidic                    Colour in Basic

Medium                                    Medium

Litmus solution                                   Red                                        Blue

Methyl orange                                     Pink                                         Yellow

Phenolphthalein                                  Colourless                               Pink

Methyl red                                          Red                                         Yellow

  1. react with dilute acid solutions and displace hydrogen from them. The metal combines with the remaining part of the acid to form salt.

Metal + Acid à Salt + Hydrogen gas

Zn + H2SO4 à ZnSO4 + H2

Mg + 2HCl   à MgCl + H2

2Al + 3H2SO4 à Al2(SO4)3 + 3H2

 

7. Reaction of Acids with Metal trioxocarbonate(IV ) and hydrogentrioxocarbonate(IV)  Metal trioxocarbonate(IV) and hydrogen trioxocarbonate( IV) react with acid to form corresponding salt and liberate CO2 gas with brisk effervescence.

Metal trioxocarbonate(IV) + Acid à7 Salt + Carbon(IV) oxide + Water

Metal hydrogentrioxocarbonate(IV) + Acid à7 Salt + Carbon(IV) oxide + Water

Na2CO(s) + 2HCl (aq) à 2NaCl (aq) + CO2 (g) + H2O (l)

NaHCO3 (s) + 2HCl (aq) à NaCl (aq) à NaCl (aq) + CO2 (g) + H2O (l)

CaCO3 (s) + 2HCl (aq)  à  CaCl2 (aq) + CO2(g) + H2O (l)

Carbon(IV) oxide is a colourless, odourless gas. When the gas is passed through lime water, milkiness appears due to formation of calcium carbonate.

When carbon(IV) oxide is passed through lime water in excess, the milkiness disappears due to formation of soluble calcium hydrogentrioxocarbonate(IV).

8. Reaction of Acids with Bases. Acids react with bases to from salt and water.

Acid + Base à Salt + Water

The reaction between acids and bases to form salt and water is known as neutralization reaction. For example,

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

H2SO4(aq) + 2NaOH (aq) à Na2SO4 (aq) + 2H2O (l)

HNO3(aq) + KOH (aq) à KNO3 (aq) + H2O(l)

Phenolphthalein has pink colour in basic medium. When sufficiently large quantity of HCl solution is added to the NaOH solution, the medium becomes acidic due to the presence of excess acid. In acidic medium, the phenolphthalein is colourless and hence the solution becomes colourless. When we again add NaOH solution, it neutralizes the excess acid and the solution again becomes alkaline. Therefore, the pink colour reappears.


Table 22.2. Acids Present in Some Natural Substances

 

Natural substance                                Acid

 

Citrus fruits (Oranges, lemons, etc.)               Citric acid

Apples                                                             Malic acid

Sour Milk (Curds)                                           Lactic acid

Vinegar                                                           Etbanoic acid

Tomatoes                                                         Oxalic acid

Rancid butter (spoiled butter)                         Butanoic acid

Urine                                                               Uric acid

Gastric juice                                                    Hydrochloric acid

Tamarind, grapes                                             Tartaric acid

Fats                                                                 Stearic acid

Sting of bees and ants                                     Methanoic acid

(Formic acid)

Nettle plant sting                                            Methanoic acid