Chemistry » Physical and Chemical Change » Law Of Constant Composition

Summary and Main Ideas


  • 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.

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