Page 3 - 3.LESSON NOTES-(PRESENCE OF IONS IN ACIDS & BASES))

2NaCl + H2SO4 → Na2SO4 + 2HCl
(conc) (dry gas)
HCl (dry) + Dry Blue litmus → No colour change.

HCl (dry) + Moist Blue litmus → Colour changes to Red.

1. The HCl gas liberated here is in it’s dehydrated (dry) form and does not
+
have H ions. So, it does not show any acidic behavior.
Hence the colour of dry blue litmus paper does not change.

2. In presence of moist blue litmus paper, the dry HCl gas dissolves in the
moisture present in litmus paper to form aqueous HCl and liberate H+
ion.
Hence the colour of the moist blue litmus paper changes to red.
In Bases- Example :
Let’s take the example of aqueous ammonium hydroxide (NH4OH)-

NH3 (dry) + Dry Red litmus paper → No colour change.
(ammonia)
NH3 (dry) + Moist Red litmus paper → Colour changes to Blue.

(ammonia)
NH3 (dry) + H2O → NH4OH(aq)
(ammonium) (base)

-
1. Dry ammonia does not have any (OH) ions and hence does not show any
basic properties.
Hence dry ammonia does not change the colour of dry red litmus paper.

2. Dry ammonia reacts with moisture present in moist red litmus paper, to
form Ammonium hydroxide, which contains (OH)- ions and shows basic
properties.
Hence the colour of moist red litmus paper changes to blue

All Alkalis are BASES but all bases are not ALKALIS

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