Page 254 - Chemistry ICSE Class X

240 ICSE Chemistry – 10

What are the physical properties of nitric acid
ƒ Colour and odour. Pure nitric acid is colourless fuming liquid with
pungent odour. Impure acid or pure acid on standing develops
yellow colour due to the presence of dissolved oxides of nitrogen
(mainly NO ).
2
ƒ Solubility. It is completely soluble in water and forms a constant
boiling mixture (b.p. 120.5°C) with water containing 68.5 (by mass)
of nitric acid.
ƒ Density. Pure acid has a density of 1.54 g/mL. The constant boiling
mixture has a density of 1.4 g/mL at 20°C.
ƒ Boiling and freezing points. Anhydrous nitric acid boils at 355.6 K
(83.6°C) and freezes to a white solid at 231.4 K (–41.7°C).
ƒ Action on skin. It has a corrosive action on the skin and causes yellow
blisters.

What are the chemical properties of nitric acid
ƒ Thermal stability. On heating, HNO decomposes to give NO .
EŝƚƌŝĐ ĂĐŝĚ ƐƚŽƌĞĚ ŝŶ ŐůĂƐƐ ďŽƩůĞ ƚƵƌŶƐ 3 2
'
LJĞůůŽǁ ŽǀĞƌ Ă ƉĞƌŝŽĚ ŽĨ ƟŵĞ͘ dŚŝƐ ŝƐ 4HNO o 2H O + 4NO (g) + O (g)
3
2
2
2
ĚƵĞ ƚŽ ƚŚĞ ĚĞĐŽŵƉŽƐŝƟŽŶ ŽĨ ŶŝƚƌŝĐ
ĂĐŝĚ ƚŽ ŐŝǀĞ EK ŝŶ ƚŚĞ ƉƌĞƐĞŶĐĞ ŽĨ ƒ Acidic properties. Nitric acid is a strong monobasic acid. In
2
ƐƵŶůŝŐŚƚ͘ moderately dilute solution (| 0.1 M), it is dissociated to an extent of
about 93% in accordance with the reaction.
+
–
HNO (aq) + H O(l) U H O (aq) + NO (aq)
3
3
2
3
nitric acid nitrate ion
Being an acid,
Š it turns blue litmus to red.
Š it turns methyl orange to red.
Š it has no action on phenolphthalein.
Some typical reactions of nitric acid are described below:
ƒ With alkalis. It reacts with alkalis to form nitrates:
NaOH + HNO o NaNO 3 + H O
3
2
sodium hydroxide sodium nitrate
CaO + 2HNO o Ca(NO ) + H O
3
3 2
2
quicklime calcium nitrate
ƒ With carbonates and hydrogencarbonates. It decomposes carbonates
and hydrogencarbonates to give brisk effervescence of carbon
dioxide.
CaCO 3 + 2HNO o Ca(NO ) + CO (g) + H O
2
2
3
3 2
calcium carbonate
Na CO 3 + HNO o NaHCO + NaNO 3
2
3
3
sodium carbonate
NaHCO 3 + HNO o NaNO 3 + CO 2 + H O
3
2
sodium hydrogencarbonate
ƒ Oxidising properties. Nitric acid can be easily decomposed to give
out oxygen, so it acts as a strong oxidising agent. The concentrated
acid is usually reduced to nitrogen dioxide (NO ) while the dilute
2
acid gives nitric oxide (NO).
'
2HNO o H O + 2NO + [O]
2
2
3
(conc.) nascent oxygen
'
2HNO o H O + 2NO + 3[O]
2
3
(dilute) nascent oxygen