Video Transcript
The chemical timeline below gives
examples of different chemical reactions that take place over a range of
timescales. Which of the following reactions is
likely to occur at point X? (A) Cooking food, (B) the reaction
of iron and hydrochloric acid, (C) striking a match, (D) the reaction of alkali
metals with water, or (E) the chemical weathering of rocks.
Looking at the provided diagram, we
can see that X corresponds to a reaction which is very slow. The diagram doesn’t provide any
information about how slow this reaction is. However, we do know that this
reaction must be slower than the rusting of iron. Let’s take a look at each of the
answer choices and see if we can relate the speed of these reactions to the rusting
of iron.
Option (A) is cooking food. While there are a number of
chemical reactions involved in cooking food and the time it takes depends on the
type of food being prepared, most food can be cooked in a matter of minutes or a few
hours. So cooking food is unlikely to be
slower than the rusting of iron. This means we can eliminate answer
choice (A).
Answer choice (B) is the reaction
of iron and hydrochloric acid. When hydrochloric acid is added to
iron, bubbles immediately start to form, indicating that a reaction is
occurring. The reaction between iron and
hydrochloric acid occurs quite quickly. So answer choice (B) cannot be the
answer to this question.
Answer choice (C) is striking a
match. When a match is struck, the match
immediately begins to burn, a sign of a chemical reaction. So striking a match is a very fast
reaction. This means that answer choice (C)
cannot be the answer to this question.
Answer choice (D) is the reaction
of alkali metals with water. When an alkali metal like sodium is
added to water, an extremely vigorous explosive reaction occurs. So the reaction of alkali metals
with water is very fast. This means we can eliminate answer
choice (D).
This leaves us with answer choice
(E) the chemical weathering of rocks. The chemical weathering of rocks
may be due to water or acid rain dissolving the minerals that make up the rocks or
oxygen in the air reacting with metals in the rocks to form metal oxides. In general, the chemical weathering
of rocks is a slow process that can take several years. As this process is very slow, the
reaction which is likely to occur at point X is answer choice (E) the chemical
weathering of rocks.
