Matter does not stay the same. It may undergo physical or chemical changes.
A physical change is a change that can be seen or felt, but that does not involve the break up of the particles in the reaction. During a physical change, the form of matter may change, but not its identity.
During a physical change, the arrangement of particles may change but the mass, number of atoms and number of molecules will stay the same.
Physical changes involve small changes in energy and are easily reversible.
A chemical change occurs when one or more substances change into other materials. A chemical reaction involves the formation of new substances with different properties. For example, hydrogen and oxygen react to form water
A chemical change may involve a decomposition or synthesis reaction. During a chemical change, the mass and number of atoms is conserved, but the number of molecules is not always the same.
Chemical reactions involve large changes in energy. Chemical reactions are not easily reversible.
The law of conservation of mass states that the total mass of all the substances taking part in a chemical reaction is conserved and the number of atoms of each element in the reaction does not change when a new product is formed.
The law of constant composition states that in any particular compound, all samples of that compound will be made up of the same elements in the same proportion or ratio.
Gay-Lussac’s Law states that in a chemical reaction between gases, the relative volumes of the gases in the reaction are present in a ratio of small whole numbers if all the gases are at the same temperature and pressure.