1) Take about 1.0 g of solid NaCl in a clean and dry test tube.
2) Add some concentrated sulphuric acid to the test tube.
1) A gas is coming out of the delivery tube.
2) If we test the gas with dry and wet blue litmus paper there is no change in colour.
Chemical equation :
1) We can conclude that dry HCl gas (hydrogen chloride) is not an acid.
2) Because we have noticed that there is no change in colour of dry litmus paper.
3) But HCl aqueous solution is an acid because wet blue litmus paper turned into red.