Blue litmus turns red when it comes in contact with an acid. So Test tube A containing HCl will change the colour. It will turn red from blue.
HCl is an acid and sodium carbonate is a base. When an acid and a base react they form salt and water. This is called a neutralisation reaction.
When 2 ml of dil HCl is added to 1 g of sodium carbonate a brisk effervescence is observed.
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