Unit A Section 3.0

Evidence of Chemical Change Recall that a physical change is altering a substance without changing its composition. Examples include changes in state/phase, size and shape. A chemical change is a result of a chemical reaction, where new substances are formed. All chemical reactions have the general form: The atoms (or ions) of the reactants are rearranged to form the products. The products have different properties than the reactants and energy flows into or out of the system

1) colour change One or more of the products has a different colour than the reactants. 2) formation of a gas If one of the products is a gas, bubbles will appear. 3) formation of a precipitate One of the products is only slightly soluble in water.

4) energy change This is often noticed as heat or light absorbed or released. A chemical reaction that releases energy is exothermic. A chemical reaction that absorbs energy is endothermic. 5) change in odour The products formed during chemical reactions may have a different odour, or no odour at all.

The Law of Conservation of Mass Recall that the law of conservation of mass states that the total number of atoms in the reactants is equal to the total number of atoms in the products. Bonds between atoms or ions are broken and new bonds are formed without the loss of any atoms or ions. Antoine Lavoisier (1743-1794) The atoms of the reactants are simply rearranged to form the products.

Homework: read pages 78 – 85 A3.1 Check and Reflect page 85 #’s 3 – 11