Question

Justify combustion of hydrogen to water is chemical change

Answer 1

Hydrogen + Oxygen = WaterThe simple statement that water is made from hydrogen and oxygen doesn't give us a very clear picture of what really goes into the creation of a molecule of water. A quick look at the chemical equation for the formation of water tells us more.2H2 + O2 = 2H2OIt takes two molecules of the diatomic hydrogen gas, combined with one molecule of the diatomic oxygen gas to produce two molecules of water. In other words the ratio of hydrogen to oxygen is 2:1, the ratio of hydrogen to water is 1:1, and the ratio of oxygen to water is 1:2.There's something more though that doesn't show up in the equation. Energy. The formation of water from it's elements produces, in addition to water, a tremendous amount of energy, 572 kJ to be exact.2H2 + O2 = 2H2O + ENERGYThis is an example of an exothermic reaction, a reaction that produces energy. It is also an example of what is called a combustion reaction, where a substance (in this case hydrogen gas) is combined with oxygen. You are probably familiar with this reaction through two tragic examples of the unleashed energy of the combustion reaction of hydrogen, the Hindenburg, and the spaceshuttle Challenger.Hydrogen Fuel?Yes - hydrogen is a good, clean fuel, producing only water as a by-product. Unfortunately it produces so much energy that it can get out of control, resulting in an explosion. But let's forget about that explosive part for a minute and think about the possibilities - Hydrogen as a New Clean Fuel - it could be the end of the energy crisis - but where would we get the hydrogen?Can we create Hydrogen from Water?Oh Yes! It's the same chemical reaction, but run in reverse:2H2O + ENERGY = 2H2 + O2Notice now that the requirement is for energy to be ADDED TO the reactants. This is an example of an Endothermic reaction. This means that we could use Water as a Fuel! IF (and this is a big if) we could find an easy way to convert the water to hydrogen and oxygen, then the hydrogen could be used as a clean fuel.One way to convert Water to Hydrogen and Oxygen is through the process of Electrolysis - using electricity as the source of energy to drive the reaction. Let's take a look at what that might look like: 
IMAGE SOURCE: "Chemistry in Context" Wm C Brown Publishers, Dubuque Iowa, 2nd edition, A project of the American Chemical Society, ed: A. Truman Schwartz et al., 1997, Chapter 5 "The Wonder of Water"Isn't this rather circular?Using Energy to break water to form hydrogen to combine oxygen to form Energy - in this way is rather circular. In fact, because of the laws of thermodynamics, you can't break even in this exchange of energy. However, there exist better ways to disassemble water - namely using CATALYSIS. 
IMAGE SOURCE: "Chemistry in Context" Wm C Brown Publishers, Dubuque Iowa, 2nd edition, A project of the American Chemical Society, ed: A. Truman Schwartz et al., 1997, Chapter 5 "The Wonder of Water"What does a catalyst do?A catalyst is a chemical compound that acts to speed up a reaction, but in the process is not itself changed. Therefore the catalyst, at the end of the reaction, is free to act again to assist another reactant through the reaction.Catalysts work by lowering the energy barrier between the reactants and the products. In this case:2H2O + ENERGY = 2H2 + O2where it normally takes a tremendous amount of energy to convert reactants to products - the addition of a catalyst can decrease the amount of energy required and therefore speed the reaction up!2H2O + CATALYST+ energy = 2H2 + O2 + CATALYSTDoes this catalyst really exist?Sort of...... Have you ever wondered how a plant uses water and carbon dioxide to create glucose and oxygen? This too is an endothermic reaction, an energy producing reaction run in reverse. Normally we would think of using glucose as a fuel, through oxidation we could produce carbon dioxide, water and energy - In fact this is what OUR bodies do to provide us with the energy we need for maintaining all of our bodily functions including THINKING!Glucose (C6H12O6) + Oxygen (O2) = Water (H20) + Carbon Dioxide (CO2) + ENERGYTo run the reaction in reverse, the plant utilizes a catalyst - CHLOROPHYLL - and the energy from the SUN to aid in the decomposition of water. While the chlorophyllic reaction does produce diatomic oxygen gas, it does not produce the hydrogen in a gaseous form. The hydrogen released from the water is used for the formation of glucose.Could we use such a catalyst for converting Water and Sunlight into Fuel?

Answer 2

water molecule break down into Hydrogen and Oxygen....
Hence it can be said that it's a chemical change called eletrolysis.