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Elements and Compounds

Year 8 Science 2013
 A pure substance made up of only one
type of atom
 it cannot be broken down into simpler
substances by chemical reaction
 A pure substance that contains two or
more elements combined in a fixed
 It can be broken down into its elements
by chemical reactions
Conservation of Matter
 The total mass of the reactants in a
chemical reaction is always equal to the
total mass of the products
 Matter cannot be created or destroyed
Making and breaking compounds
 Sometimes atoms change the way they mix with each
other. We call this a chemical reaction.
 Sometimes a chemical reaction creates by-products
such as heat or light
 A chemical reaction can occur when:
 Lots of atoms or molecules join together to make one
new compound.
 A compound breaks down into individual atoms.
 A few compounds mix up to make many new
Examples of chemical reactions
Sodium Bicarbonate (Baking Soda ) + Hydrochloric Acid 
Sodium (Salt) + Water + Carbon Dioxide
NaHCO3 + HCl  NaCl + H2O + CO2
Magnesium + Oxygen  Magnesium Oxide
Mg2 + O2  2MgO
Water  Hydrogen + Oxygen
2H2O  2H2 + O2
 An atom that has a positive or negative
 Caused by the loss or gain of electrons
 Metals tend to lose electrons and nonmetals tend to gain electrons
 An atom that has gained an electron and
an atom that has lost one are attracted to
each other (like two ends of a magnet)