Video Transcript
When a wooden log burns on a fire,
a lot of carbon dioxide is produced as well as heat and light. Which of the following statements
about the ash that remains after the wood has burnt is correct? (A) The ash has more mass than the
original log. (B) The ash has less mass than the
original log. Or (C) the ash has the same mass as
the original log.
In this question, we are trying to
determine the relationship between the mass of the wood before it is burnt and the
mass of the remaining ash. We can imagine a log of wood. The wood is a starting material of
the chemical reaction. And it contains a lot of
carbon. We are told that the wood burns on
a fire. In order for a material to burn,
there must be oxygen present. Oxygen is composed of O2
molecules. The wood and the oxygen in the air
are the reactants of the chemical reaction. When the wood burns, the carbon
chemically reacts with the oxygen in the air. While the log burns, heat and light
are given off. Heat and light are both different
forms of energy.
Energy does not have mass. So we know that giving off heat and
light will not affect the mass. We also know that a lot of carbon
dioxide is given off as well. Carbon dioxide is made up of CO2
molecules. Carbon dioxide is a product from
this reaction. As the wood continues to burn, more
and more carbon dioxide is produced and less and less carbon remains in the
wood. When the wood is done burning, ash
remains. Ash is a brittle solid that is
often black or gray in color. Ash is the carbon from the wood
that did not react. This reaction, like all chemical
reactions, will satisfy the law of conservation of mass.
The law of conservation of mass
states that mass is neither created nor destroyed during a chemical reaction. Because of this, we know that the
mass of the carbon before and after the reaction must be the same. Most of the carbon in the wood
becomes a part of the carbon dioxide gas. Only a small amount of carbon
remains as ash. So, the ash will have less mass
than the original log since much of the matter from the original log became carbon
dioxide.
We can see that the statement that
correctly describes the mass of the ash is answer choice (B). Therefore, the statement that
correctly describes the ash that remains after the wood has burnt is answer choice
(B). The ash has less mass than the
original log.
