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Co nservatio n of mass in chemical reactio ns
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Bunsen Burner
magnesium ribbon
Co nservatio n of mass in chemical reactio ns
Suitable for: 11–16 years
Curriculum and learning links:
Mass changes in chemical reactions
Learning objectives:
• State that combustion is an example of an oxidation reaction.
• Explain why metals gain mass in oxidation reactions.
• Suggest why some fuels appear to lose mass when combusted.
Activities:
Opening activity
• Demonstrate to the class some items burning – for example, a wooden splint, a piece of
paper, a candle.
• Ask students to suggest what they think will happen to the mass of the wood, paper and
candle when they are burnt.
• Ask them if this is always the case. Do they think that something could burn and get heavier?
Development activities
Burning Magnesium
• Ask students to burn varying lengths of
magnesium ribbon in crucibles with
lids on. This will prevent the loss of the
product (magnesium oxide), but the lids
should be lifted periodically using tongs to
ensure an adequate supply of oxygen.
• Ask students to record the mass of the
metal before and after burning. They should
continue to heat until the contents of the
crucible have stopped glowing white.
• If you don’t have access to crucibles, you
can burn the magnesium on tin lids or
metal bottle tops that have had the rubber
seal removed. However, the data will be
less accurate.
• Collect the data from all of the students
together onto a central spreadsheet. This
will allow a trend to be discussed as a
class. You could create a graph
electronically from these results, or ask
students to draw their own by hand.
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Conservation of mass in chemical reactions
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Reflect and review
• Ask students what conclusions they can draw from the data. What happened to the mass
of the magnesium when it burned?
• Watch Clip 25 and discuss what happened in Dr Yan’s experiment.
• Challenge students to explain why wood, paper and all hydrocarbons and carbohydrates
appear to lose mass during combustion.
Want to explore further?
• Discuss with students the fact that (allowing for experimental errors) the data should
follow a straight line of best fit, starting at the origin of the graph. This suggests that
magnesium atoms and oxygen atoms combine in a fixed ratio.
• More advanced students could complete predicted and percentage yield calculations
using relative atomic masses. Some students could explore early ideas about burning by
researching the Phlogiston theory.
You will need
• Clip 25 (3’06”)
• Wooden splint
• Paper
• Candle
• Heatproof mats
• Tripods
• Gauzes
• Tongs
• Bunsen burners
• Top pan balances that measure to
0.1g or 0.01g
• Magnesium ribbon
• Porcelain crucibles with lids
Or:
Metal bottle tops
Technician’s notes
• The magnesium ribbon can be cut to approximate lengths before the lesson.
Health and safety
).
all activities (including eye protection
for
ly
app
s
ine
del
gui
y
tor
ora
lab
l
• Norma
lar care
oxygen is highly exothermic. Particu
and
m
siu
gne
ma
en
we
bet
n
ctio
• The rea
should be taken to avoid burns.
led for a
les using tongs, once they have coo
cib
cru
the
ve
mo
y
onl
st
mu
ts
den
• Stu
few minutes.
magnesium when it is burning as
the
at
g
kin
loo
m
fro
ed
rag
cou
dis
• Students should be
.
teacher.
the bright light can cause eye damage
the students should be limited by the
by
d
use
m
siu
gne
ma
of
ps
stri
the
but
• The length of
heat small quantities of substances,
to
y
wa
d
goo
y
ver
a
e
vid
pro
s
top
• Metal bottle
ng a
t. Any residue can be burned off usi
firs
d
ove
rem
be
to
d
nee
l
wil
ls
sea
the rubber
hot Bunsen flame.
ment has
ensure that an adequate risk assess
to
r
che
tea
the
of
ility
sib
pon
res
the
• It is
been completed.
© BBC