Many
science experiments rely on a chemical reaction to produce the energy and gas
required to create enough pressure for an explosion. These include the acid and
carbonate reactions typical of simple volcano experiments and the 'elephant
toothpaste' reactions where the decomposition of hydrogen peroxide into oxygen
and water is catalysed. When such reaction mixtures are enclosed or confined to
a restricted space, the pressure resulting from the build up of gaseous
products causes an explosion.
In
a chemical change, new products are formed. An example of this occurs when
vinegar is added to sodium carbonate in traditional 'volcano' reactions. The
new products formed in this case are carbon dioxide gas, water and sodium
acetate, according to the equation below:
acetic
acid (vinegar) + sodium carbonate ----> carbon dioxide + water + sodium
acetate
When
Mentos lollies are added to a carbonated drink such as Diet Coke, however,
there are no new products formed. The rapid production of bubbles is instead a
result of the dissolved carbon dioxide in the drink rapidly coming out of
solution and forming a gas. As the gas bubbles expand, the pressure inside the
bottle increases, resulting in a stream of foam shooting out through its narrow
opening (see figure 1).
Why Do Mentos Lollies Cause This
Explosive Reaction?
According
to Steve Spangler,
there are two main reasons for this rapid build up of carbon dioxide. Firstly,
the gums and proteins from the Mentos coating help to break the surface tension
of the water in the drink, which in turn allows the gas bubbles to escape more
readily. Secondly, the many small pits on the surface of the Mentos lollies act
as nucleation sites for carbon dioxide bubbles to gather in large numbers. When
these two factors combine, massive amounts of foam are produced in a short
space of time, creating explosive results.
Mentos and Diet Coke Experiment-
Materials and Teaching Method
In
this investigation students will compare the effects of plain (mint flavoured)
Mentos lollies with coloured ones. The difference between these two types of
lollies is their surface – the coloured ones are smooth and do not have the
pitted surface typical of the plain variety. Diet Coke is usually used because
it is more effective than ordinary Coke and results in a less 'sticky' clean
up.
The
following materials and equipment are required per group of around four
students:
•
2 x 1.25 litre bottles of Diet Coke
•
1 packet mint Mentos
•
1 packet flavoured Mentos
•
2 test tubes
•
2 pieces of card around 4 x 4 cm in size
•
digital camera (optional)
Students
should be instructed to copy down the following directions, which could be
followed by a teacher-led explanation.
•
Add the contents of each packet of Mentos to separate test
tubes.
•
Move outdoors and place the two bottles of Diet Coke next
to each other on a flat surface.
•
Unscrew the lids of each bottle and place the cardboard
over the openings of each vessel.
•
Invert the test tubes over the card. When ready,
simultaneously pull the cardboard out from under each tube. Move back quickly.
•
Compare the height of the fountain produced for each type
of Mentos. If possible, record this with a digital camera.
The
following questions could be written on the board after students write up the
experiment and their observations:
1.
Which type of Mentos – plain or coloured- produced the
biggest explosion?
2.
Can you suggest a reason for your observation?
3.
Explain why the experiment demonstrates a physical, rather
than a chemical change.
4.
What aspects of the experiment did you control (that is, keep
the same)?
Mentos and Diet Coke Experiment –
Follow Up Activities
Students
may wish to observe the effects of using Diet Coke compared to normal Coke or
other carbonated drinks (as in figure 2). In this case, plain Mentos lollies
should be used for each bottle. They could also experiment with other types of
candy to find out if similar nucleation processes occur in these cases.
References
Muir,
Hazel. "Science of
Mentos-Diet Coke Explosions Explained." newscientist.com, 2008.
Spangler,
Steve. "Mentos Diet Coke
Geyser." stevespanglerscience.com, 2010.
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