Chemical and Physical Changes Put this in your notes Chemical changes produce new substances that have different properties from the original substances. Ex: fire, light sticks, baking soda with vinegar releases gas Physical changes only affect only things such as size, shape, state of matter. Ex: crumpling paper, melting or freezing water

Chemistry Clip The Chemistry of Fire

Chemical Equations To describe what is going on in a reaction, we use chemical equations. Reactants are on the left side of the equation and are the chemicals that exist before the reaction. Products are the substances that form as a result. Ex: Hydrogen + Oxygen = water H2 + O 2 = 2 H 2 O (we’ll get back to that big two in front later)

Chemical Equations (continued) Conservation of Mass- A scientific law stating that the mass of the products of a chemical reaction must equal the reactants of the reaction.-Antoine Lavoisier In other words, what you start with and what you end with have to balance out. Atoms can’t just go POOF! and disappear.

Balancing It All Out H 2 + O 2 = H 2 O 1.Count the atoms in an equation. Do they all equal out? We get 2 hydrogens on both sides, but 2 on the reactant side and only 1 on the product side. 2. To fix this, we use coefficients, large numbers placed in front of a formula. You can NOT change subscripts to balance an equation! H 2 + O 2 = 2 H 2 O Is this correct?

H 2 + O 2 = 2 H 2 O 3.Re-examine the equation and count the atoms again. This time, we get 2 hydrogen on the first side, 4 on the second. We have 2 oxygen on the first side and two on the 2 nd. 4.We need to add another coefficient. Where does it need to go? 5.Add the next coefficient in front of the first hydrogen. What does the coefficient need to be? 2 H 2 + O 2 = 2 H 2 O Now does everything work?

Coefficients Things to remember: 1. Coefficients apply to the entire formula, while subscripts only to the element its next to. 2. Always go back and recount all your atoms after you add coefficients to be sure you are getting a balance.

Practice Problems Ag + H 2 S  Ag 2 S + H 2 Na + AlCl 3  NaCl + Al KBr + Cl 2  KCl + Br 2 H 2 O 2  H 2 O + O 2