What are changes in matter?
Chemical changes
What are changes in matter?
There are three types of changes:
Physical changes (ice melting)
Chemical changes (wood burning)
Nuclear changes (uranium reacting in a nuclear
reactor)
Physical Changes
 Difference between characteristic properties and non-
characteristic properties:
 Characteristic properties help to identify a substance.
Example: melting point.
 Non-characteristic properties help to describe a
substance. Example: mass.
In a physical change, characteristic properties do not
change. Only the shape or the states of matter (solid,
liquid, gas) can be changed.
Definition
 A physical change alters neither the nature nor the
characteristic properties of matter. The atoms and
molecules of the substance do not change.
 Example: boiling water (only the state of matter changes
from liquid to gas).
Chemical Changes
 Characteristic properties will change in a chemical
change because a new substance will be formed. The
atoms will make different bonds which will lead to
different molecules.
 Definition: a chemical change alters the nature and
characteristic properties of matter. The bonds between
atoms are rearranged, and new molecules are formed.
Chemical Changes
 Example: wood burning  ash is formed = new
substance.
Reactants becoming Products
 When substances change and form new ones as bonds
are broken and rearranged, the change occurs
gradually.
Some signs that a chemical change
occurred
 A) the release of gas
 B) the emission or absorption of heat
 C) the emission of light
 D) a change in color
 E) the formation of a precipitate
How to write chemical changes or
chemical reactions
 CH4(g) +
 Methane
2O2(g) 
reacts with oxygen
CO2(g) + 2H2O(g)
to form carbon dioxide and water vapour
 Symbols used:
  = the arrow means “forms” or “yields”
 + = the plus sign on the reactants side means “reacts
with” and on the products sign means “and”.
 (g) = gas
 (s) = solid
 (l) = liquid
 (aq) = aqueous (dissolved in water)
The Law of Conservation of Mass
 Definition: the total mass of reactants is always equal
to the total mass of products.
 Nothing is lost, nothing is created, everything is
transformed.
 Paper burning: the mass of the gases released in the
combustion and the mass of the ashes is equal to the
mass of the paper in its original form.
 The bonds between the reactants change.
Visualizing a Chemical Reaction
2 Na
10 mole Na
___
+
Cl2
5 mole Cl2
___
2 NaCl
10
? mole NaCl
___
Balancing chemical equations
 To follow the law of conservation of matter, we must
balance chemical equations.
 Definition: balancing chemical equations consists in
placing a coefficient before each reactant and product
so that the number of atoms of each element on the
reactant side is equal to the number of atoms of each
element on the product side.
Meaning of Coefficients
2 atoms Na 1 molecule Cl2
2 molecules NaCl
2 Na + Cl2
2 g sodium + 1 g chlorine
2 mol sodium
(2 mol Na) x (23 g/mol)
2 NaCl
=
1 mol chlorine
(1 mol Cl2) x (71 g/mol)
46 g
71 g
117 g
2 g sodium chloride
2 mol sodium chloride
(2 mol NaCl) x (58.5 g/mol)
117 g
Meaning of Chemical Formula
Chemical
Symbol
H2O
Meaning
One molecule
of water:
Composition
Two H atoms and one O atom
2 H2O Two molecules
of water:
Four H atoms and two O atoms
H2O2 One molecule
of hydrogen
peroxide:
Two H atoms and two O atoms
Balancing Equations Rules
 Coefficients must be whole numbers.
 Coefficients must be as small as possible.
 New substances must never be added, nor existing
substances removed.
 Subscripts in chemical formulas must never be
changed.
 The final equation should always be checked by
counting the number of atoms of each element on
both sides.
Balancing Equations: same number of
atoms in reactants and products
 N2 + 3H2  2NH3
 Is this formula balanced?
 How many atoms of N2?
 4FeS2 + 11O2  2 Fe2O3 + 8 SO2
 Is this formula balanced?
 How many moles of O2?
 CH4 + O2  CO2 + H2O
 Is this formula balanced?
How molecules are symbolized
Cl2
2Cl
2Cl2
• Molecules may also have brackets to indicate
numbers of atoms. E.g. Ca(OH)2
• Notice that the OH is a group
O Ca O
H
H
• The 2 refers to both H and O
• How many of each atom are in the following?
a) NaOH
b) Ca(OH)2 Na = 1, O = 1, H = 1
c) 3Ca(OH)2 Ca = 1, O = 2, H = 2
Ca = 3, O = 6, H = 6
Balancing equations: MgO
 Example: Magnesium + Oxygen
 Mg + O2  MgO
Mg + O O

• However, this is not balanced
• Left:
Mg = 1, O = 2
• Right: Mg = 1, O = 1
Mg O
Unbalanced and Balanced Equations
H
Cl
Cl
H
H
H
H
Cl
H2 + Cl2  HCl (unbalanced)
reactants
H
Cl
2
2
H
H
Cl
Cl
Cl
H2 + Cl2  2 HCl (balanced)
reactants
products
1
1
Cl
H
Cl
2
2
products
2
2
Types of Chemical Change
 Acid-base neutralization
 Combustion
 Cellular respiration and photosynthesis
Acid-Base Neutralization
 Definition: a chemical change involving the reaction of an
acid with a base, producing a salt (metallic ion and nonmetallic ion) and water.
 acid(aq) + base(aq)  salt(aq) + water(l)
 When a solution is neutralized, the concentration of H+ ions
is equal to that of OH- ions and the pH is 7.
 If there are more H+ ions than OH- ions, the neutralization
reaction is incomplete and the pH is less than 7.
 If there are more OH- ions than H+ ions, the neutralization
reaction is incomplete and the pH is more than 7.
Neutralization
HCl + NaOH → H2O + NaCl
acid
base
water
salt
Combustion
 To understand combustion, we must first know the
meaning of oxidation.
 Oxidation is a chemical reaction involving oxygen or a
substance with properties similar to those of oxygen.


Humidity accelerates the formation of rust = an oxidation of
iron.
Definition: Combustion is a form of oxidation
that releases a large amount of energy.
Combustion Requirements
 Three conditions for combustion to take place:
 1. there must be fuel
 2. there must be an oxidizing agent
 3. the ignition temperature must be reached
 The three conditions make up the triangle of fire.
 Combustion reaction =
Fuel + oxygen
𝑖𝑔𝑛𝑖𝑡𝑖𝑜𝑛 𝑡𝑒𝑚𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒
CO2 + H2O + energy
The triangle of fire
 A) oxidizing agent: aka oxidant, is a substance that can
cause a fuel to react. Oxygen is the most common
oxidant on Earth.
 B) A fuel is a substance that releases a large amount of
energy by reacting with an oxidizing agent. E.g. wood
and propane.
 C) the ignition temperature is the minimum
temperature at which the energy present is sufficient
to start combustion (they vary according to fuel types).
3 types of combustion
 1. Rapid combustion
 Lots of energy (mostly heat and light) is released within
a short period of time. E.g. log fire, candle
 2. Spontaneous combustion
 Fast combustion with no extra source of energy for the
ignition temperature. Forest fires = very hot summer
and very dry ground, wood can catch fire spontaneously.
 3. Slow combustion
 Very long period of time, energy released gradually into
the environment. E.g. metal corrosion, cellular
respiration.
Cellular Respiration and
Photosynthesis
 Cellular respiration takes place in the cells of most
living organisms.
 Definition: chemical change in which glucose and
oxygen are used to generate energy. The reaction also
produces carbon dioxide and water.
 Photosynthesis is the opposite reaction.
 Plant cells use solar energy to make their own glucose
and oxygen by using water and carbon dioxide.
 Definition: chemical change that produces glucose and
oxygen from solar energy, carbon dioxide and water.
Cellular respiration and
photosynthesis
 Organisms that carry out photosynthesis are called
producers because its product is used by other living
organisms to survive.
Nuclear Transformation
 In this type of change, the atom is changed at its core –
the nucleus.
 In uranium reactors, the nucleus is split and new
substances are formed (krypton and barium).