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chemical property chemical change dissolving
8.1 Chemical Properties and Changes
Ability to Change • In a chemical change, the properties that
give a substance its identity change.
8.1 Chemical Properties and Changes
• Properties of matter help identify objects.
• Properties are either physical or chemical.
What properties do elements have?
Ability to Change (cont.)
• A chemical property is the ability or inability of a substance to combine with or change into one or more new substances.
8.1 Chemical Properties and Changes
Chemical Properties • Examples of chemical properties include
burning or rusting.
• Some substances do not react.
8.1 Chemical Properties and Changes
Physical Properties • Physical properties can be observed
without changing the substance.
8.1 Chemical Properties and Changes
Chemical and Physical Changes • Chemical changes change one
substance into another substance.
• Usually chemical changes cannot be easily reversed.
8.1 Chemical Properties and Changes
Property Changes
Forming New Substances • All chemical changes produce substances
that are different from the starting substances.
– A compound is decomposed into its elements.
– Two elements join to form a compound.
8.1 Chemical Properties and Changes
Physical Changes
• A physical change is a change in which the properties of a substance change but the identity of the substance remains the same.
• Dissolving is a process in which substances mix evenly with one another.
8.1 Chemical Properties and Changes
law of conservation of mass reactants products diatomic molecule coefficient
8.2 Chemical Equations
Is matter conserved in chemical reactions? • Chemical equations show that in chemical
reactions, atoms rearrange, but no atoms are gained or lost.
8.2 Chemical Equations
Conservation of Mass • The law of conservation of mass states
that the total mass before a chemical reaction is equal to the total mass after the reaction.
8.2 Chemical Equations
Writing a Chemical Equation • Reactants are the starting materials in a
chemical reaction and are placed on the left side.
• Products are the ending materials in a chemical reaction and are placed on the right side.
8.2 Chemical Equations
Word Equations • “Tin + oxygen → tin oxide” reads as “tin
plus oxygen produces tin oxide.”
• Word equations can be long and do not show that mass is conserved.
8.2 Chemical Equations
Elements, Compounds, and Molecules • Instead of writing long word equations,
scientists use symbols and formulas.
– Symbols represent atoms.
– Formulas represent molecules.
– Molecules may be elements or compounds.
8.2 Chemical Equations
Elements, Compounds, and Molecules (cont.)
• Some elements are diatomic, that is, they contain two atoms.
8.2 Chemical Equations
• Molecules may also contain two different elements, such as hydrogen and oxygen in a water molecule.
Elements, Compounds, and Molecules (cont.)
8.2 Chemical Equations
Chemical Equations • Use the symbols and formulas instead
of words.
8.2 Chemical Equations
Balancing a Chemical Equation • Atoms are neither gained nor lost in a
reaction, so both sides of the equation must have the same number of atoms.
8.2 Chemical Equations
Counting Atoms • A subscript tells how
many atoms of an element are in one molecule.
8.2 Chemical Equations
• A coefficient tells how many atoms, molecules, or formula units are in a reaction.
Counting Atoms (cont.)
8.2 Chemical Equations
Writing Balanced Equations Step 1 Determine the correct symbols and formulas
for reactants and products.
Step 2 Write reactant symbols and formulas to the left of an arrow and product symbols and formulas to the right.
Step 3 Count the number of each kind of atom on both sides.
Step 4 Use coefficients to make the number of each kind of atom the same on both sides of the arrow.
Step 5 Check to see that each kind of atom balances.
8.2 Chemical Equations
Equations for Common Chemical Reactions Reaction of methane:
CH4 + O2 → CO2 + H2O balance hydrogen CH4 + O2 → CO2 + 2H2O balance oxygen
CH4 + 2O2 → CO2 + 2H2O
8.2 Chemical Equations
Equations for Common Chemical Reactions (cont.)
Baking soda and vinegar:
NaHCO3 + HC2H3O2 → CO2 + H2O + NaC2H3O2
8.2 Chemical Equations
Using Parentheses with Formulas • When counting atoms inside parentheses,
the subscript multiplies all atoms inside.
Ca + H2O → Ca(OH)2 + H2
Ca + 2H2O → Ca(OH)2 + H2
8.2 Chemical Equations
