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Chapter 11 Chemical Changes, imageimage, red litmus paper remains red…
Chapter 11
Chemical Changes
classification of chemical changes
a reaction in which
two or more substances chemically
combine to form
one or more new substances
for example, carbon monoxide combines with oxygen to form carbon dioxide
carbon monoxide + oxygen → carbon dioxide
a reaction in which
a substance
breaks down into
two or more new substances
for example, when calcium carbonate is heated, it breaks down into carbon dioxide and calcium oxide
calcium carbonate → carbon dioxide + calcium oxide
what are the different types of chemical changes?
combustion
⤷ is a chemical reaction in which a substance is heated in the presence of oxygen to form new substance(s)
heat
and
light
is usually given out during
combustion
charcoal (carbon)
in a barbecue pit burns in
oxygen
to form
carbon dioxide gas
carbon + oxygen → carbon dioxide
thermal decomposition
⤷ is a chemical reaction in which a substance breaks down into two or more simpler substances when heated
calcium carbonate → calcium oxide + carbon dioxide
oxidation
⤷ is a chemical reaction in which a substance gains oxygen
rusting is an example of oxidation
iron + oxygen → rust
cellular respiration is another example
glucose + oxygen → carbon dioxide + water
neutralisation
⤷ is a chemical reaction when an acid is mixed with an alkali
acid + alkali → salt + water
the word 'salt' is a general term for a compound containing an element that is metal.
representing chemical reactions
word equations
shows the substances that are involved in chemical reactions. they include:
reactant(s)
used
product(s)
formed
condition(s)
under which the
reaction occurs
rearrangement of atoms in a chemical reaction
during a
chemical
reaction,
atoms
of the
reactants
rearrange themselves and combine
chemically
to form
products
atoms of the
products
are always obtained from the
reactants
chemical and physical changes
physical
no new substances
are formed
changes can
usually be reversed
using
separation techniques
e.g. freezing and boiling
chemical
one or more new substances
are formed
changes are usually
not easily reversed,
difficult to get back reactants
chemical reactions
chemical reaction between acids and metals
acid + metal → salt + hydrogen gas
hydrochloric acid + magnesium → magnesium chloride + hydrogen
how to test for the presence of hydrogen gas?
use a
lighted splint.
it will
extinguish
with a
'pop' sound
in the presence of hydrogen gas.
chemical reaction between acid and carbonate
acid + carbonate → salt + carbon dioxide gas + water
hydrochloric acid + magnesium carbonate → magnesium chloride + carbon dioxide + water
how to test for the presence of carbon dioxide gas?
use
limewater.
white precipitate
will be observed if carbon dioxide gas
bubbled through
the limewater.
pH scale
⤷ is used to measure how acidic or alkaline a solution is
the values of the pH scale range from
0 to 14
solutions
less
than
pH 7
is
acidic
solutions
more
than
pH 7
is
alkaline
solutions
with a pH value of 7
is
neutral
Universal Indicator
also measures pH values
red litmus paper
remains red
turns blue
remains red
blue litmus paper
turns red
remains blue
remains blue
VS
acidic
alkaline
neutral
tip! associate the word "acidic" with the colour red and the word "alkaline" with the colour blue :)
