Physical change is a change in which the substance changes form but keeps its same chemical composition (reversible).
*Changes of state are considered to be physical changes. Liquid water and ice (frozen water) are both the same substance, water.
*If you fold a piece of paper it is a physical change. You have changed the form of the paper but you have not changed the fact that it is paper.
*If you heat an iron bar until it glows red hot, it is still chemically the same iron. The iron has not changed into something else.
*If you dissolve salt in water you have not changed the materials chemically. You still have salt and you still have water. This can be shown if you choose to separate the mixture by distillation or the simple evaporation of the water. The salt would be the residue and the water would be the distillate.
A chemical change is a change in which something new is formed (irreversible). The starting materials change into an entirely different substance or substances. This new substance has a different chemical composition than the starting materials. Examples of chemical changes would be the reaction of iron with air (rusting} or the reaction of a metal and acid.
Certain observations will indicate that a chemical change has occurred. These are:
*The reaction produced a change in temperature. The temperature could go up (gets hotter) or the temperature could go down (gets colder). Note: reactions that produce heat are known as exothermic reactions whereas reactions that absorb heat are known as endothermic reactions.
*Formation of gas bubbles.
*Formation of a solid (precipitate).
*A change in color. You may start with two colorless solutions but when they are mixed you might see a bright purple color.
*Formation of a different odor. The starting materials may not smell at all but as you mix these materials you may end up with a bad odor or a pleasant one.