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Chemical Reactions
Mrs. J. Ebejer Grech B.Ed.(Hons.)
1
What is a chemical reaction?
A chemical reaction is a reaction between 2 or more substances, where
the original substances change completely during the reaction, and at the
end new products are formed.
The substances that are present at the beginning of the reaction are
called _______________.
The substances that are present at the end of the reaction are called the
_____________.
Example: If we take a paper, the reactant is the paper. If we burn the
paper the reaction is burning. At the end of the reaction i.e. when the
paper completely burns, the product is ash. Ash is completely different
from the initial paper. Therefore, because there was a complete change
between the reactant and the product, the reaction is called a chemical
reaction.
A physical change is a change where the reactant doesn’t form a
completely new product.
For example, if a piece of paper is cut up into small pieces it still is paper.
This would be a physical change in the shape and size of the paper.
Chemical Change Physical Change
• The reaction cannot be reversed • The reaction can be reversed
• A new product is formed • A new product is not formed
Chemical Reactions
Mrs. J. Ebejer Grech B.Ed.(Hons.)
2
More Examples:
1. Consider freezing some water. The reactant is the ____________
and the product is the ____________. Can ice be converted back to
water? ____ Therefore is this reaction a physical or chemical
reaction? __________
2. If we mix some salt in some water, the reactants are the
__________ and the ______________ and the product is
_______________. Can we separate the two of them again?
_________ How? _________________________________
________________________________________________
_______________________________________________. So is
this a chemical reaction or a physical reaction? _______________
3. If we mix some flour, sugar, margarine, eggs and bake them into a
cake, what are the reactants? _____________________________
Can we separate them back to the original reactants after that the
cake is baked? _____________. So baking a cake is a chemical
reaction or a physical reaction? _________________.
More examples of chemical and physical reactions.
Chemical Reactions
Mrs. J. Ebejer Grech B.Ed.(Hons.)
3
Experiment:
Aim: To burn magnesium ribbon.
Apparatus: magnesium ribbon, tongs, Bunsen burner, matches, safety
glasses, watch glass.
Method:
1. Cut a piece of magnesium ribbon and hold it in a tongs.
2. Observe its colour and shape
3. Burn over a lighted Bunsen burner
4. Observe the flame
5. Observe the reaction.
Results:
Initial colour _______________________________
Shape ____________________________________
Colour of flame _____________________________
Final colour ________________________________
Shape ____________________________________
Precautions:
1. Wear protective safety glasses to protect your eyes
2. wear a lapcoat or apron to protect your uniform
3. do not touch the fire
4. do not smell the burning magnesium
5. do not bring the tongs towards your body while holding hot
magnesium.
Conclusion: Was there a complete change between the reactant and the
product?
______________________________________________________
Do you think we can reverse the reaction?
_______________________________________________________
Chemical Reactions
Mrs. J. Ebejer Grech B.Ed.(Hons.)
4
Do you think this was a physical or a chemical reaction? Why?
______________________________________________________
Equation: magnesium + oxygen � magnesium oxide.
Diagram:
Perform some more experiments and conclude which are the chemical
reactions and which are the physical reactions.
Chemical Reactions
Mrs. J. Ebejer Grech B.Ed.(Hons.)
5
Answer: Tick the correct answer
Physical change Chemical Change
Baking a cake
Striking a match
Making ice cubes
Dissolving salt in water
Burning some toast
Baking clay
Physical properties:
1) Has no colour or smell.
2) Has no effect on moist litmus paper or moist universal indicator paper
- it is neutral.
3) Burns with a characteristic 'pop'.
4) Hydrogen burns in air with a pale blue, almost invisible flame.
Specific test.
Hydrogen gas is recognized by the 'pop' when it burns. The 'pop' is the
sound of a small explosion, hydrogen is extremely flammable!
Experiment:
Aim: To Produce Hydrogen:
Apparatus: a piece of magnesium ribbon, tongs, boiling tube, hydrochloric
acid, another boiling tube, lighted splint, matches, safety glasses, apron /
lapcoat, cardboard paper.
Method:
1. Pour some hydrochloric acid in a boiling tube.
2. Insert a piece of magnesium ribbon in the acid using a tongs
3. Immediately cover the boiling tube with another inverted empty
boiling tube to collect any gas that is released.
4. Observe
5. After some time, cover the inverted boiling tube with your fingers
or a piece of cardboard and invert it.
Chemical Reactions
Mrs. J. Ebejer Grech B.Ed.(Hons.)
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6. Introduce a lighted splint into the second boiling tube without
letting any gas escaping.
7. Notice any sound.
Precautions:
1. Make sure you don’t touch the acid with bare hands
2. Wear safety glasses to protect your eyes
3. Be careful you don’t burn your hands while lighting the splint
4. Make sure the gas doesn’t escape from the inverted funnel
Results:
What happens to the magnesium?
Is there any gas being produced? ____________
This is the word equation of this reaction:
Magnesium + hydrochloric acid magnesium chloride + hydrogen
Conclusion:
Magnesium reacts with hydrochloric acid and _____________ are
observed. _______ is released and when it was tested with a lighted
splint, it burnt with a _________. This sound proves that the gas
released from the reaction of magnesium and hydrochloric acid is ______
Metal + Acid � Salt + Hydrogen.
Chemical Reactions
Mrs. J. Ebejer Grech B.Ed.(Hons.)
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1. Fill in the blanks. The following words might help you.
light chemical used up heat irreversible new
Burning a match is a ________________ reaction. It is an
________________ change. The wood is ______________ and
__________ substances are made. Energy is given out as __________
and __________.
2. Fill in the blanks. The following words might help you
pop chemical Hydrogen acids corroded
Some metals react with __________ to make hydrogen. We test for
______________ using a lighted splint. A __________ means it is
hydrogen. We say that the acid has _____________ the metal, because
the metal is used up in the _________________ reaction.
Combustion is a reaction that occurs when burning an organic material in
air. For example burning a paper, or burning wood etc. . . This releases a
gas called carbon dioxide.
Physical properties of carbon dioxide:
- appearance – colourless , invisible gas
- present in air (0.03%) – responsible for greenhouse effect
- does not burn
Experiment: Producing carbon dioxide
1. heating a carbonate
Chemical Reactions
Mrs. J. Ebejer Grech B.Ed.(Hons.)
8
2. breathing out
3. reacting a carbonate with an acid
Experiment: Testing for carbon dioxide
Pass CO2 through a solution of lime water.
What happens? ___________________________
This gas is CO2
This is the test for carbon dioxide. Carbon dioxide turns lime water
milky.
Complete these sentences:
All carbonates react with ____________ to produce
____________________ gas. We can test this gas by bubbling it into
___________ which turns ___________.
Other reactions that release carbon dioxide gas.
This diagram shows what happens when yeast reacts with sugar solution.
Chemical Reactions
Mrs. J. Ebejer Grech B.Ed.(Hons.)
9
a. This process is called ________________________. Yeast use
sugars to produce energy. They also produce alcohol and carbon
dioxide.
b. Carbon dioxide is given off during this reaction.
c. The word equation for the above reaction is:
___________________________________________________
d. This reaction is used in the production of
- bread
- alcohol (wine, beer, etc)
Making cakes
Baking powder contains bicarbonate of soda and a weak acid. When the
powder gets wet, the acid and the bicarbonate of soda react together.
One of the products of the reaction is carbon dioxide gas. This helps
cake mixture to rise.
Acid Rain
Marble stone (or limestone) is calcium carbonate. Carbonates dissolves in
acids. What do you know about acid rain? What happens to limestone in
acid rain?
_______________________________________________________
Fizzy drinks
What happens when you open a bottle of
soft drink or fizzy water that has been
shaken? Do you know that the gas that
makes that ‘fizz’ is carbon dioxide? In fact
fizzy drinks are also called carbonated
drinks.
The greenhouse effect
The carbon dioxide in the air has the
same effect as the glass in a
greenhouse. The carbon dioxide
stops some of the heat energy from
the Earth escaping, and the Earth
stays warm. This is called the
Greenhouse effect or Global
warming. Scientists think that too
much carbon dioxide will increase
Chemical Reactions
Mrs. J. Ebejer Grech B.Ed.(Hons.)
10
the temperature of the air.
Answer these questions.
1. Write some sentences about making of bread and alcohol.
2. What is making the Earth’s temperature getting hotter?
3. Write down the word equation of fermentation. What is the
importance of this reaction?
The Fire Triangle
Fire is a chemical reaction that requires three main ingredients:
• fuel (carbon)
• oxygen
• heat
These three ingredients make up the fire triangle. If
any one is not present, a fire will not burn. Fuel
generally is available in ample quantities in the forest.
Fuel must contain carbon. It comes from living or dead plant materials
(organic matter). Trees and branches lying on the ground are a major
source of fuel in a forest. Such fuel can accumulate gradually as trees in
the stand die.
Oxygen is present in the air. As oxygen is used up by fire, it is
replenished quickly by wind. Heat is needed to start and maintain a fire.
Heat can be supplied by nature through lightning. People also supply a
heat source through misuse of matches, campfires, trash fires, and
cigarettes. Logging equipment, trains, and automobile exhaust systems
also can supply a heat source for fire. Once fire has started, it provides
its own heat source as it spreads.
Physical Properties
Oxygen is
• a colourless gas, without smell or taste,
• is slightly heavier than air,
Chemical Reactions
Mrs. J. Ebejer Grech B.Ed.(Hons.)
11
• is sparingly soluble in water,
It is vital for most living organisms. It is obtained from air. There is 21
per cent by volume or 23 per cent by weight oxygen in the atmosphere.
Combined Oxygen also occurs
• in water,
• in vegetable and animal tissues,
• in nearly all rocks and
• in many minerals.
Oxygen occurs to a larger extent in the earth's crust than any other
element.
Reaction of oxygen with metals.
• The diagram above shows that when hot alkali metal is put into a
gas jar filled with oxygen gas, the alkali metal will burn with bright
flame.
• They form white oxide powders after reaction.
• These oxides dissolve in water to form strongly alkaline metal
hydroxide solutions with pH value 13-14.
Experiment: Producing oxygen
Put manganese dioxide (black powder) into hydrogen peroxide.
Chemical Reactions
Mrs. J. Ebejer Grech B.Ed.(Hons.)
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What happens?
Is there any gas being produced? ____________
This is the word equation of what is happening:
_______________________________________________________
Testing for oxygen: Experiment:
How do you know that the gas produced is Oxygen?
Put a lighted splint in the gas. What happens?
____________________________
Put a glowing splint in the gas. What happens?
___________________________
This is the test for oxygen.
BURNING CANDLES
What happens when a beaker is put over
a burning candle?
_______________________________
Why?
_______________________________
_______________________________
This is the word equation:
Candle wax + oxygen carbon dioxide + water + energy
Dangerous fires start when burning gets out of control. Fire produces
large amounts of energy as heat and light. Firefighters sometimes have a
cylinder of oxygen on their backs, so they can breathe in a burning
building.
Preventing a fire using the fire triangle
Chemical Reactions
Mrs. J. Ebejer Grech B.Ed.(Hons.)
13
What are you removing when you try to put off a fire by:
a. using water?
_________________________
b. using a fire blanket?
_________________________
c. switching off the electricity supply?
_________________________
d. cutting down trees?
_________________________
e. using a CO2 fire extinguisher?
_________________________
f. using a powder fire extinguisher?
_________________________
Answer the following questions:
1. The chemical name for burning is __________________ . When
things burn, they react with ________________ gas in the air.
The products formed are called ______. _____________ gas
relights a _____________ splint.
2. You have burned magnesium in air. Remember that air is a mixture
of gases; so what do you think will happen if we burn magnesium in
pure oxygen gas?
________________________________________________
Chemical Reactions
Mrs. J. Ebejer Grech B.Ed.(Hons.)
14
3. You are given three test tubes containing samples of hydrogen,
carbon dioxide and oxygen. What tests do you do to find out which
is which?
a. Hydrogen – ____________________________________
b. Carbon dioxide - _______________________________
c. Oxygen - _____________________________________
RUSTING
When a metal is attacked by water, air or acids in their environment, they
corrode. Corrosion results in the metal become weaker and brittle. The
corrosion of iron and steel is specifically called rusting due to the red-
brown substance called rust that forms in the presence of water and
oxygen. The experiment below shows that an iron nail only rusts if
both water and oxygen are present:
How to prevent rusting
1. Paint or grease
This prevents water or oxygen reaching the iron. However, this is only a
temporary step since paint can flake off and grease can be rubbed off
quite easily. Bikes are often painted of greased to prevent rust, since this
is the cheapest method of prevention.
2. Plastic
Chemical Reactions
Mrs. J. Ebejer Grech B.Ed.(Hons.)
15
Plastic is cheap and acts as a cover for the iron, for instance, it stops
water or oxygen reaching the metal surface. Garden chairs are often
made from iron coated in plastic.
3. Galvanising
This involves the iron been covered, usually in the form of a paint, by
zinc.
4. Chromium plating
Works for the same reason as galvanising. Car bumpers are often
chrome-plated.
Hydrogen filled balloons
Hydrogen was used to provide lift for the first balloons. The Germans
Zeppelin Company were masters in producing them. The airships were
filled with the lightest of all gases, hydrogen. Britain built two large
passenger airships but on October 4th , 1930, one of them crashed in
France. 48 people died. After that experience, air travel was left to the
Germans.
Adolf Hitler, the leader of Germany, used Germany’s biggest airship,
called the Hinderburg, as a symbol of supremacy. This airship could
carry 72 passengers but needed about 200 people to help land it. It was
about three football pitches long! However, in 1937 disaster struck on
its sixty third voyage. It crashed as it came in to land in USA after
crossing the Atlantic. People couldn’t believe their eyes s it burst into
flames on landing. In just 30 seconds all that was left was its skeleton.
The hydrogen gas burned with the oxygen in the air. 36 people died on
that day.
Modern airships are filled with a gas called helium, and they are a very
safe way to travel as helium does not burn at all!
Why did they use hydrogen to lift balloons?
____________________________________________________
Hydrogen is no longer used in balloons. Why?
____________________________________________________
Today Helium is used instead of Hydrogen. Why?
____________________________________________________
