Chemical and Physical Changes and Reactions 8th grade Science Chemical and Physical Changes and Reactions

Chemical Changes Chemical reactions are taking place all around you and even within you Chemical reaction-a change in which one or more substances are converted into new substances (due to rearrangement) Reactants-substances that react Products-new substances produced

Chemical Changes Chemical changes occur when a material changes from one substance to another Several factors can indicate a chemical change has occurred: 1. Color change 2. Temperature change 3. Production of a gas 4. Formation of a solid

Physical changes Physical changes: those changes that occur in nature involving a change in STATE only (phase changes) A substance may change states of matter as a result of a physical change, but will not become a new substance Ex: freezing, melting, evaporating, tearing, etc.

Conservation of Mass Lavoisier-famous scientist who discovered that masses of reactants and products were the same in an experiment SO, he concluded: Law of Conservation of Mass-the starting mass of reactants equals the final mass of the products OR Matter is neither created nor destroyed during chemical reactions

A metal will replace any less active metal Notice: copper, silver and gold-least active metals SO: that is why these elements often occur as deposits of the relatively pure element.

All chemical reactions release or absorb energy This energy can take many forms, such as heat, light, sound and electricity Chemical bonds are the source of this energy

When chemical reactions take place, some chemical bonds in the reactants must be broken Breaking these bonds takes energy Bond formation releases energy

Exothermic Reaction- when the energy given off in a reaction is primarily in the form of heat *Exothermic reactions provide most of the power used in homes and industries

Endothermic reaction- when the energy needed is in the form of heat (Endothermic can also refer to phase/physical changes as well)