Video Transcript
A student placed a small quantity
of zinc carbonate into a test tube. She weighed the test tube and zinc
carbonate and found their mass to be 55 grams. She then heated the test tube
strongly for five minutes. After the test tube had cooled
down, she reweighed it and the remaining powder inside. What is the new mass of the test
tube and zinc compound? (A) More than 55 grams, (B) less
than 55 grams, or (C) 55 grams.
The question tells us that the test
tube and the zinc carbonate originally weighed 55 grams. The test tube was then heated, and
after it had cooled down, it was reweighed. Zinc is a metal, so zinc carbonate
is a metal carbonate. We need to know what happens when a
metal carbonate is heated. Many types of compounds decompose
when heated. This is called thermal
decomposition, which can be defined as the breaking down of a substance when heated
to form two or more other substances. When a metal carbonate undergoes
thermal decomposition, it forms a metal oxide and carbon dioxide. The capital Δ symbol is used to
show that heat was involved.
The carbonate in the question is
zinc carbonate. So, when heated, it will form zinc
oxide and carbon dioxide. So we now know that the products of
the reaction are zinc oxide and carbon dioxide. But to work out what the measured
mass would be, we need to consider the law of conservation of mass. This states that the mass of a
closed system cannot change. So, if this reaction took place in
a sealed container, the mass of the starting material, zinc carbonate, would be the
same as the mass of the products, zinc oxide and carbon dioxide, in which case the
mass of zinc oxide and carbon dioxide produced combined with the mass of the test
tube would also equal 55 grams.
But this is not a closed system, as
the reaction takes place in a test tube and the question doesn’t specify that it’s
sealed. So, when the zinc carbonate is
heated, forming zinc oxide and carbon dioxide, the carbon dioxide is released into
the atmosphere, as it’s a gas. So, when the test tube is reweighed
at the end of the experiment, the carbon dioxide will not be included in the
mass. Instead, only the zinc oxide and
test tube are weighed. If the zinc oxide, carbon dioxide,
and test tube equals 55 grams, then only the zinc oxide and test tube must be less
than 55 grams. Since this is what is weighed, the
answer to the question “What is the new mass of the test tube and zinc compound?” is
(B) less than 55 grams.
