Chapter 3: Elements combine to form compounds Grade 9 Science:
Unit 1: Atoms, Elements, and Compounds
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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.
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Model of HCl ( hydrochloric acid)
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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.
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Formed from non-metals only. They do not conduct electricity.
May be a solid, liquid, or gas at room temperature.
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Methane CH 4
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C H CH H H H
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Examples of Covalent Compounds Table sugarC 12 H 22 O 11 Carbon
DioxideCO 2 WaterH 2 O MethaneCH 4
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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.
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All are solid at room temperature. High melting and boiling
points. Will conduct electricity when melted or dissolved in water.
Are also called salts.
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Magnesium Chloride MgCl 2
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Examples of Ionic Compounds NaClSodium Chloride CaCO 3 Calcium
Carbonate Na 2 SO 4 Sodium Sulfite NaOHSodium Hydroxide
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Naming Compounds Every compound has a... 1. Name: indicates the
elements present in the compound 2. Formula: indicates the symbols
and ratio of each element present in the compound.
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Rules for Naming Compounds Ionic Compounds See page 81-2
Covalent Compounds See page 83 Complete practice problems on pages
82 &83
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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.
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Includes: all changes in state (melting, evaporation,
condensation, freezing) Dissolving Cutting Tend to be easy to
reverse.
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Chemical Changes: Produce new substances with new properties;
may or may not be noticeable. New bonds are formed while others are
broken.
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Chemical equations can be written for all chemical changes. For
example the composition of water.
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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
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Evidence of a Chemical Change: 1. Color change 2. Heat, light,
sound produced or consumed 3. Gas bubbles released 4. A precipitate
formed 5. Difficult to reverse
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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.
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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)
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Core Lab Activity 3-3C pg. 92-3 Observing Changes in
Matter
