Grade 9 Science: Unit 1: Atoms, Elements, and Compounds Chapter 3: Elements combine to form compounds

Compounds A pure substance made of two or more kinds of elements combined in fixed proportions. Represented by a chemical formula. Chemical bonds hold them together. Are either ionic or covalent.

Model of HCl ( hydrochloric acid)

Covalent Compounds Atoms combine by sharing electrons to form molecules. Molecules: a group of atoms held together by sharing one or more pairs of electrons. Examples include carbon dioxide and water.

Formed from non-metals only. They do not conduct electricity. May be a solid, liquid, or gas at room temperature.

Methane CH4

H C H C H H H

Examples of Covalent Compounds Table sugar C12H22O11 Carbon Dioxide CO2 Water H2O Methane CH4

Ionic Compounds Atoms gain or lose electrons to form ions. All the positive ions attract all the negative ions everywhere in the same crystal. Formed from metals and non- metals.

All are solid at room temperature. High melting and boiling points. Will conduct electricity when melted or dissolved in water. Are also called “salts”.

Magnesium Chloride MgCl2

Examples of Ionic Compounds NaCl Sodium Chloride CaCO3 Calcium Carbonate Na2SO4 Sodium Sulfite NaOH Sodium Hydroxide

Naming Compounds Every compound has a... Name: indicates the elements present in the compound Formula: indicates the symbols and ratio of each element present in the compound.

Rules for Naming Compounds Ionic Compounds See page 81-2 Covalent Compounds See page 83 Complete practice problems on pages 82 &83

Physical & Chemical Changes Physical Changes: the appearance of a substance may have changes but the bonds holding the atoms together in molecules and ions have not been broken and no new bonds have been made.

Tend to be easy to reverse. Includes: all changes in state (melting, evaporation, condensation, freezing) Dissolving Cutting Tend to be easy to reverse.

Chemical Changes: Produce new substances with new properties; may or may not be noticeable. New bonds are formed while others are broken.

Chemical equations can be written for all chemical changes. For example the composition of water.

In a chemical reaction the mass of the reactants = the mass of the products. The elements are conserved but not the compounds. Ex. Corrosion, fruit ripening, combustion

Evidence of a Chemical Change: Color change Heat, light, sound produced or consumed Gas bubbles released A precipitate formed Difficult to reverse

Applications of Chemical Changes Harnessing combustion Combustion releases large amounts of energy which can be used to provide heat and light, electrical and mechanical energy.

Solving the corrosion problem (a process by which metals combine with oxygen; oxidation) Using chemical change for traditional products (example: tanning hides, making dyes and medicines from plants, and preserving food)

Core Lab Activity 3-3C pg. 92-3 Observing Changes in Matter

CORE STSE: “Plastics and Modern Life”