If you push the button, you will cause the passage of an electrical current through the voltmeter.
Water is made of hydrogen an oxygen, more accurately a water molecule consist of two hydrogen atoms and one oxygen atom. When the current pass through the voltmeter the water molecule is broken down easily in hydrogen and oxygen. This experiment allows to collect separately both elements in different vessels. The volume of hydrogen is twice as much than oxygen since there is two atoms of hydrogen for every atom of oxygen.
Electric current is able to produce a chemical reaction, in this case water breakdown.
«Descomponemos el agua» es un módulo de la Sala del Agua incluido dentro de la sección «Agua y ciencia»