Video Transcript
The following figure shows what
happens when sodium metal is reacted with some liquid water. Why does the reaction give off a
lot of gas? (A) The reaction produces a lot of
oxygen gas as the sodium replaces the oxygen in the water. (B) The sodium produces a lot of
nitrogen gas. (C) The reaction produces a lot of
sodium hydroxide that instantly turns into a gas. (D) The reaction produces a lot of
hydrogen gas as the sodium replaces the hydrogen in the water. (E) The reaction produces a lot of
steam as the water instantly turns into a vapor.
Let’s begin by carefully examining
the figure. We can see here that a piece of
solid sodium metal is reacting with liquid water. We are told that the reaction gives
off a lot of gas. In the figure, we can see some
bubbling, which indicates a gas is being released. We also notice the presence of
flames, which occur when the gas ignites. All of these signs in the figure
indicate that the reaction is very intense. Some substances are highly active
and can react with a lot of other chemicals. The diagram shown is called the
series of chemical activity. The most active substances are
close to the top, and the least active substances are near the bottom. We can identify that sodium is
highly active because it is close to the top of the series of chemical activity.
The series of chemical activity
also shows which metals can displace hydrogen from water molecules. And we can see from the series that
hydrogen is much less active than sodium. Metals that are above hydrogen on
the series of chemical activity can displace hydrogen from water molecules during a
chemical reaction. Let’s write a chemical equation to
represent the reaction between sodium metal and water. When sodium reacts with water, a
sodium hydroxide solution and hydrogen gas are produced. This reaction is a great example of
a single substitution reaction in which an active metal, in this case sodium,
replaces hydrogen in the water. Because sodium is such an active
metal, the reaction happens very quickly and vigorously and hydrogen gas is given
off very rapidly.
Therefore, the answer choice that
correctly describes the reason for gas evolution when sodium metal reacts with water
is answer choice (D). The reaction produces a lot of
hydrogen gas as the sodium replaces the hydrogen in the water.
