Compunds Ch4.2 8th Pdf

Chapter 4 Section 2

Objectives  Explain how elements make up compounds

 Describe the properties of compounds  Explain how a compound can be broken down into its

elements  Give examples of common compounds

Compounds: Made of Elements  Most elements take part in chemical changes fairly

easily  Elements are rarely found alone in nature

 Elements combined chemically with other elements

form compounds

Compounds continued…  A particle of a compound is called a molecule

 Molecules of compounds form when atoms of 2 or

more elements join together

Ratio of Elements in a Compound  Elements join in specific ratios according to their

masses to form compounds  Ex: Water: H2O  The ratio of the mass of hydrogen to mass of oxygen is 1:8  This ratio is always the same  Check your periodic table and see on pgs. 338-339  Familiar compounds Fig. 1 pg. 94

Water (H20)

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Properties of Compounds  Each compound has its own physical and chemical

properties

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 Ex: hydrogen peroxide (H2 2)  Why are hydrogen peroxide bottles opaque (dark

brown)?

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Properties: Compounds vs. Elements  Properties of compounds differ from elements they are

formed from  Ex: Table salt (NaCl)  Sodium reacts violently with water  Chlorine is a poisonous gas  Combined chemically, they are safe to eat!  Dissolves in water without exploding

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Breaking Down Compounds  Some compounds can be broken down by chemical

changes into simpler compounds or elemental form  Ex: Carbonic acid (gives soda its “fizz”)  When the can is opened, carbonic acid breaks down into Carbon dioxide (C02) and water (H2O)  Co2 and H2O can be broken down further into the

elements (C, H, O)

Methods of Breaking Down Compounds  Add energy  Heat energy

 Electric current

Compounds in your World  In industry:  Aluminum (makes cans & airplanes)  

Not found alone in nature Aluminum Oxide (Al2O3) broken down

 Ammonia (make fertilizers) 

Combining elements nitrogen and hydrogen (NH3 )

Compounds in Nature  Proteins – found in all living things  Nitrogen

• Carbon dioxide (C02) – important roles in life • Carbon, Oxygen

Quick Quiz  Compounds cannot be broken down by any means

(True/False?)  Compounds can be broken down only by chemical

changes (True/False?)  Heating can break down some compounds

(True/False?)