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Changes is Matter
• Physical Changes • Chemical Changes
– Does not produce
– Might have
– Changes in states
of matter: solid,
– One or more
into new substances
– Have different
– Cooking is an
example of chemical
• Chemical changes are called chemical
• A chemical reaction is a process in which
atoms of one or more substances
rearrange to form one or more new
Signs of a Chemical Reaction
• Changes in the physical properties of
color, state of matter, and odor are all
signs that a chemical reaction might have
• A change is energy is another sign.
Signs of a Chemical Reaction
• Change in
• Change in Energy
– Warming or cooling
– Release of light
– Formation of bubbles
– Formation of a
* A precipitate is a solid that forms from a reaction between two liquids
What happens in a chemical
• Atoms rearrange to form new substances
– There are 2 types of substances: Elements and
– Substances have a fixed arrangement of atoms
– A drop of water each water molecule will have one
oxygen atom bonded to two hydrogen atoms
– If this arrangement changes the substance is no
longer water and a different substance has formed
– The new substance will have different chemical and
– This is what happens during a chemical reaction
• Bonds break and bonds form
– Atoms rearrange when chemical bonds
between the atoms break
– Particles are moving in all substances, even
– These particles collide with one another.
– If particles collide with enough energy, the
bonds between the atoms can break,
separate, rearrange, and form new bonds.
• A chemical equation is a description of a
reaction using element symbols and
• Element symbols represent elements
• Chemical formulas represent compounds
• Each element can exist as just one atom
– C = carbon
Cu = Copper
• Some elements exist in nature as diatomic
molecules – two atoms of the same
element bonded together.
– This is shown by a subscript 2
– H2 = hydrogen
O2 = oxygen
• A chemical formulas uses elements’
symbols and subscripts to describe the
number of atoms in a compound
• If an element does not have a subscript,
the compound contains only one atom of
Balanced: 2Na + Cl2 => 2NaCl
Writing Chemical Equations
• A chemical equation included both the
substances that react and the substances that
are formed in a chemical equation.
• The starting substances in a chemical reaction
are the reactants
• The substances produced by the chemical
reaction are products
• The arrow shows the direction of the reaction
• Reactants are written left of the arrow and
products are written to the right
Conservation of Mass
• French chemist Antoine Lavoisier found
that the total mass of the reactants always
equaled the total mass of the products.
• This is the law of conservation of mass,
which states that the total mass of
reactants before the reaction must equal
the total mass of products after the
Atoms are conserved
• Mass is conserved because atoms are
• Bonds break and new bonds form during a
• However, atoms are neither created, nor
Is an equation balanced?
• An equation is written so that the number
of atoms of each element is the same, or
balanced, on each side of the arrow
Conservation of mass
Conservation of atoms
Make it balanced!
4 H, 2 O
• Add Coefficients!
• A coefficient is a number placed in front of an element
symbol or chemical formula in an equation.
• Only coefficients can be changed when balancing an
• Changing subscripts changes the identities of the
substances that are in the reaction.
Balancing a Chemical Equation
1. Write the unbalanced equation.
2. Count atom of each element in the
reactants and in the products.
3. Add coefficients to balance the atoms.
4. Write the balanced chemical equation.