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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 oneor 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 CH 4
C H H C H H H
Examples of Compounds Table sugar Carbon Dioxide Water Methane Covalent C 12 CH 4 H CO 2 H 2 O 22 O 11
Ionic Compounds Atoms gain or lose electrons to form ions.
All the the the positive ions attract all negative ions everywhere in 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 MgCl 2
Examples of Ionic Compounds
NaCl Sodium Chloride
CaCO 3 Na 2 SO 4 NaOH Calcium Carbonate Sodium Sulfite Sodium Hydroxide
Naming Compounds Every compound has a...
1.
Name : indicates the elements present in the compound 2.
Formula compound.
: indicates the symbols and ratio of each element present in the
Rules for Naming Compounds Ionic Compounds
See page 81-2
Covalent Compounds
See page 83 Complete practice problems pages 82 &83 on
Physical & Chemical Changes
Physical Changes: the appearance of may have changes holding the atoms molecules and ions have not been broken and no new bonds have been made.
a substance but the bonds together in
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 others are broken.
while
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: 1.
Color change 2.
3.
Heat, light, sound or consumed produced Gas bubbles released 4.
5.
A precipitate formed Difficult to reverse
Applications of Chemical Changes Harnessing combustion Combustion amounts of releases large 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”