Chemical Changes"A chemical change (or chemical reaction) is a change in which one kind of matter is changed into a different type of matter. Some other examples of chemical changes include the rusting of your car, setting your shoe on fire or digesting food. All of these materials combine chemically with another material, and cannot be separated by any physical means.
A physical change is a change in the form of matter but not in its identity. The dissolving of one thing into another thing is illustrative. For instance, dissolving sugar into water. The water and the sugar retain their chemical identities and can be separated by physical means. One more example – ice melting to water. Ice and water are both H20. The identity of the matter is not changed, just the state that it is in. Remember: a physical change is when no new chemical substance is formed.
As has already been mentioned, a chemical reaction occurs when two or more chemical substances are mixed together and change into new substances. For this to happen, the bonds between atoms and molecules must break up and then re-form in different ways. Because the bonds can be strong, energy (usually in the form of heat), is often needed to start a reaction.
The new substances (products) have different properties from those of the original substances (reactants). Remember: when a new chemical substance is produced, the change is known as a chemical change.
This does not mean that new elements have been made. In order to make new elements, the nuclear contents must change, but there are magnitudes of difference in the amounts of energy in ordinary chemical reactions compared to nuclear reactions. The alchemists, in their efforts to change less expensive metals to gold, did not have the fundamental understanding of what they were attempting to do to appreciate the difference.
What happens to matter when it undergoes chemical changes According to the law of conservation of mass, atoms are neither created nor destroyed, during any chemical reaction.