Anita prepared a fresh solution of sodium hydroxide in a test tube. She dropped a small piece of aluminium foil into it and then brought a burning matchstick near the mouth of the test tube. She hears a pop sound. The pop sound indicates the presence of

 When a small piece of aluminium foil is dropped in the sodium hydroxide solution, chemical reaction takes place in which precipitate of sodium aluminate and hydrogen gas evolved from test tube and then brought a burning matchstick near the mouth of the test tube hear a pop sound. The pop sound indicates the presences of hydrogen gas.