In chemistry I was told that 2 litres of hydrogen reacts with 1 litres of oxygen to produce 2 litres of water.But here volume of water and hydrogen is same so where the volume of oxygen went which also took part in reaction.If I am wrong please correct me and also clear my doubt

in chemistry I was told that 2 litres of hydrogen reacts with 1 litres of oxygen to produce 2 litres of water.But here volume of water and hydrogen is same so where the volume of oxygen went which also took part in reaction.If I am wrong please correct me and also clear my doubt

Read about Avogadro's Law.
Essentially, the volume of a gas depends only on the number of the particles of which it is composed. The hydrogen and oxygen become water molecules and that determines the volume of the gas.
Your queston was quite a puzzle a couple of hundred years ago.

This question applies only to hydrogen, oxygen, and water all in a gaseous state all at the same pressures and temperatures. In addition to what DrBob described, the key idea is that the individual molecules that make up each of these gasses are quite small, and that most of the space in the conatiners is actually empty - the molecules actually only occupy less than 0.01% of the volume. So when two H2 molecules combine with one O2 molecule to produce two H2O molecules it's not as if the H20 molecule requires more room than the original H2 molecules, ven though there's an extra O atom that's been added in.