Chemical Changes 1 Review Chemical and Physical Changes

Chemical Changes 1

Review: Chemical and Physical Changes A change in matter in which no new substance is produced. It may involve a change of state, and it can usually be reversed. Examples: melting, freezing, dissolving 2

Chemical Changes A change in matter in which one or more new substances are produced. It is difficult to reverse. Clues: -Heat or light may be given off - Smell - Colour change - gas may be formed 03 Examples: burning, combustion 3

Physical and Chemical Properties PHYSICAL PROPERTIES - Colour - Melting Point - Boiling Point - Solubility - Hardness - Strength - Elasticity - Heat Conductivity - Electrical Conductivity - Ability to transmit light - Lustre ('shininess' or dullness) -Magnetic attraction - Ductility 4

CHEMICAL PROPERTIES - p. H - Reaction with oxygen (flammability or corrosion) - Reaction with water - Reaction with acids and bases - Reaction with metals 5

Let’s Try 6

What kind of change occurred you see in the picture? What kind of change is it? 7

Frying egg, what kind of changes? Making a cake, what kind of changes? Burning candle, what kind of changes? 8

Chemical Reaction All chemical reactions can be placed into one of six categories. Here they are, in no particular 1) Combustion: order: A combustion reaction is when oxygen combines with another compound to form water and carbon dioxide. These reactions are exothermic, meaning they produce heat. An example of this kind of reaction is the burning of napthalene: C 10 H 8 + 12 O 2 ---> 10 CO 2 + 4 H 2 O 9

2) Synthesis Reaction In a synthesis reaction two or more simple substances combine to form a more complex substance. Two or more reactants yielding one product is another way to identify a synthesis reaction. reactant + reactant -------> product 10

3) Decomposition In a decomposition reaction a more complex substance breaks down into its more simple parts. One reactant yields 2 or more products. Basically, synthesis and decomposition reactions are opposites. reactant -------> product + product 11

4) Single displacement: In a single replacement reaction a single uncombined element replaces another in a compound. Two reactants yield two products. reactant + reactant -----> product + product 12

5) Double displacement: In a double replacement reaction parts of two compounds switch places to form two new compounds. Two reactants yield two products. reactant + reactant -----> product + product 13

6) Acid-base: This is a special kind of double displacement reaction that takes place when an acid and base react with each other. The H+ ion in the acid reacts with the OH- ion in the base, causing the formation of water. Generally, the product of this reaction is some ionic salt and water: HA + BOH ---> H 2 O + BA One example of an acid-base reaction is the reaction of hydrobromic acid (HBr) with sodium hydroxide: HBr + Na. OH ---> Na. Br + H 2 O 14

Combustion Reactions Combustion reactions always involve molecular oxygen O 2. Anytime anything burns (in the usual sense), it is a combustion reaction. Combustion reactions are almost always exothermic (i. e. , they give off heat). For example when wood burns, it must do so in the presence of O 2 and a lot of heat is produced: Wood as well as many common items that combust are organic (i. e. , they are made up of carbon, hydrogen and oxygen). When organic molecules combust the reaction products are carbon dioxide and water (as well as heat). 15

For example consider the combustion of methanol (rubbing alcohol): Of course, not all combustion reactions release CO 2 and water, e. g. , the combustion of magnesium metal: 16