Describe how substances react with other substances Examples: Combustibility – Will a substance burn? Reactivity – Will a substance react with other substances and how easily? Metals in Groups 1 and 2 are very reactive Sodium (Na) is very reactive, especially with water Don't try this at home! Hydrogen (H) is also very reactive The Hindenburg Disaster - Oh, the humanity! Stability – Usually do not react with other substances Noble gases in Group 18 Helium (He) is now used in Blimps

Occurs when 2 or more substances are combined and changed into new substances with new properties Also called chemical reactions Reactants – the substances that change, or react Products – the new substances that are produced During a chemical reaction, bonds between atoms are broke and new bonds are formed. Law of Conservation of Matter (Mass) – Matter cannot be created, nor destroyed. It can only be transformed. So…..

….the starting masses of the reactants is always equal to the final mass of the products Example: Photosynthesis 6CO 2 + 6H 2 O  C 6 H 12 O 6 + 6O 2 6 molecules of carbon dioxide and 6 molecules of water are the reactants Sugar and 6 molecules of water are the products There are 6 carbon atoms, 12 hydrogen atoms, and 18 oxygen atoms on each side of the equation This means the equation is balanced A chemical equation must be balanced to satisfy the Law of Conservation of Matter (Mass)

Temperature changes Instant hand warmers, instant ice packs Light and sound are produced Fireworks Color changes Autumn leaves, iron rusting, mixing chemicals to make paint Odor changes Food spoiling Bubbles or gas form Alka-seltzer Precipitate forms Cave formations made of calcium carbonate Example of a precipitate

Chemical reactions always involve energy changes 2 Types of reactions Exothermic Reactions – Give off energy (heat) Exo = outside Examples: fireworks exploding, wood burning, iron oxide (rust) forming, hand warmers Endothermic Reactions – absorb energy (heat) Endo = inside Examples: instants cold packs, add salt to ice Activation energy – the smallest amount of energy needed to start a chemical reaction Depends on reactants and other conditions, like temperature

How fast a reaction occurs Affected by 5 main factors Concentration of reactant particles More particles = faster reaction Total surface area of the reactants More surface area = faster reaction Think alka-seltzer Temperature Warmer = faster & colder = slower Ovens & refrigerators Nature of the reactants Groups 1 & 2 are very reactive Adding certain substances to the reaction Inhibitor – slows reaction Catalyst – speeds reaction Enzymes – catalysts produced by living things – Think saliva Elephant Toothpaste – example of a reaction with a catalyst Elephant Toothpaste

Acids – compounds that release hydrogen ions (H + ) in water Lemons, oranges, grapefruits – citric acid Vinegar – acetic acid Sulfuric acid, hydrochloric acid Have a sour taste - Warning never taste unknown chemicals Strong acids are corrosive – they eat away or break down materials Hydrochloric acid in our stomachs helps us digest food

Bases – compounds that release hydroxide ions (OH - ) in water – also called alkaline Soap Baking Powder Sodium hydroxide Have a bitter taste – Warning never taste unknown chemicals Strong bases are corrosive – they eat away or break down materials Sodium hydroxide is in drain cleaners

Scale used to describe the strengths of acids and bases Measure of hydrogen ions in the solution Scale ranges from is neutral (water) 0-7 = acid (the lower the number, the stronger the acid) 7-14 = base (the higher the number, the stronger the base)

Substance that changes color in the presence of an acid or base Litmus paper pH meter