Hydrogen (H), a colourless, odourless, tasteless, flammable gaseous substance that is the simplest member of the family of chemical elements. The hydrogen atom has a nucleus consisting of a proton bearing one unit of positive electrical charge; an electron, bearing one unit of negative electrical charge, is also associated with this nucleus. Under ordinary conditions, hydrogen gas is a loose aggregation of hydrogen molecules, each consisting of a pair of atoms, a diatomic molecule, H2. The earliest known important chemical property of hydrogen is that it burns with oxygen to form water, H2O; indeed, the name hydrogen is derived from Greek words meaning “maker of water.”

Hydrogen is correct because Water is decomposed into Hydrogen and Oxygen..and formula of water is H20.. Hydrogen and Oxygen are decomposed in the ratio 2:1..

Explanation:

Electrolysis of water:
Electrolysis of water is the process of splitting water molecules into their constituent elements, hydrogen and oxygen, using an electric current. This process takes place in an electrolytic cell, which consists of two electrodes (one positive and one negative) immersed in water.

Gas produced during electrolysis of water:
During the electrolysis of water, hydrogen gas (H2) is produced at the negative electrode (cathode), while oxygen gas (O2) is produced at the positive electrode (anode).

Volume of gas collected:
According to the question, the volume of gas collected in one of the test tubes is double that of the other. This means that the volume of hydrogen gas collected is double that of the oxygen gas collected.

Explanation:
During electrolysis, water molecules (H2O) dissociate into hydrogen ions (H+) and hydroxide ions (OH-). At the cathode, the hydrogen ions gain electrons and are reduced to form hydrogen gas (H2). At the anode, the hydroxide ions lose electrons and are oxidized to form oxygen gas (O2).

The volume of gas collected at each electrode depends on the stoichiometry of the reaction and Faraday's laws of electrolysis. According to Faraday's laws, the amount of substance produced at an electrode is directly proportional to the quantity of electricity passed through the electrolyte.

Since the electrolysis of water involves the formation of two moles of hydrogen gas for every mole of oxygen gas, the volume of hydrogen gas produced will be twice that of the oxygen gas produced. Therefore, in this case, the volume of hydrogen gas collected will be double that of the oxygen gas collected.

Conclusion:
Hence, the correct answer is option B) Hydrogen. The volume of gas collected in one of the test tubes in the electrolysis of water is double that of the other, and the gas collected is hydrogen.