Video Transcript
Solid calcium carbonate decomposes
upon heating to produce carbon dioxide and solid calcium oxide. Which of the following symbolic
equations with state symbols describes this chemical reaction? (A) CaCO3 gas reacts to form CaO
gas plus CO2 solid. (B) CaCO3 solid reacts to form CaO
solid plus CO2 gas. (C) Ca2CO3 solid reacts to form CaO
gas plus CO gas. (D) CaCO3 solid plus O2 gas react
to form CaO solid plus CO2 gas. (E) CaCO3 solid plus CO2 gas react
to form CaO solid.
The first sentence in this question
is a chemical statement. A chemical statement describes a
chemical reaction in words. On the other hand, the answer
choices are symbolic equations. Symbolic equations, also known as
chemical equations, use chemical symbols to represent a reaction. For example, we see CaCO3 in
several of the answer choices. Ca is the chemical symbol that
represents the element calcium. C is the chemical symbol that
represents carbon. And O is the chemical symbol that
represents oxygen. Three is a subscript, which tells
us how many of a particular type of atom is present. Altogether, CaCO3 is a chemical
formula.
Chemical formulas in symbolic
equations may also be followed by state symbols. The letter s, l, or g is written
inside parentheses to tell if the species in the reaction is a solid liquid or
gas. Our job in answering this question
is to take the provided chemical statement and translate it into a correct symbolic
equation. In the provided chemical statement,
we can identify the word “decomposes” as a keyword. Chemical species that appear to the
left of the keyword are the reactants of the chemical reaction. Only one species, calcium
carbonate, appears to the left of the keyword. This means that the chemical
reaction only has one reactant.
In examining answer choices (D) and
(E), we see a plus sign is used on the left side of each equation. This tells us that there are two
reactants. The correct symbolic equation can
only contain one reactant. So we can eliminate these two
answer choices because they contain too many reactants.
Let’s continue to read through the
chemical statement. “Produce” is another keyword
indicating that the reactant is forming products. To the right of this keyword, we
can find the words that represent the products. Carbon dioxide and solid calcium
oxide are separated by the word “and,” which means that there are two different
products produced in the reaction.
Let’s create a basic word equation
using the names of the reactants and products. To the left of the reaction arrow,
let’s write calcium carbonate. And to the right of the reaction
arrow, let’s write carbon dioxide plus calcium oxide. The chemical statement tells us the
states of two of the species involved in the reaction. Both calcium carbonate and calcium
oxide are solids. And even though it is not included
in the chemical statement, we would expect carbon dioxide to be a gas.
Let’s rewrite our basic word
equation using the correct state symbols. After calcium carbonate and calcium
oxide, we’ll write an “s” inside parentheses. And after carbon dioxide, we’ll
write a “g” inside parentheses.
Finally, we should be able to
recognize that the chemical formula for carbon dioxide is CO2. Let’s go ahead and replace the
words “carbon dioxide” with CO2 in our equation. We are able to eliminate choice (A)
because the state symbol next to calcium carbonate and calcium oxide should be an
“s” for solid. Also, the state symbol next to CO2
should be “g” for gas. Choice (A) is not a correct answer
choice because the symbolic equation contains incorrect state symbols.
We can also eliminate answer choice
(C) because the state symbol next to calcium oxide should be “s” for solid, not “g”
for gas. In addition to this incorrect state
symbol, the chemical formula for carbon dioxide is incorrect. In this equation, it is written as
CO, which is the chemical formula for carbon monoxide.
This leaves only answer choice (B),
which is our correct answer choice. Answer choice (B) is the correct
symbolic equation with state symbols that matches the chemical statement provided in
the question.