Video Transcript
The diagram below shows the preparation of nitric acid from sulfuric acid and a
nitrate salt. If pure nitric acid is colorless, why does the nitric acid prepared here have a
yellow color? (A) There is some of the sulfate product present in the nitric acid. (B) Nitric acid has a low pH and so is yellow in color. (C) Sulfur solid is produced during the reaction that dissolves the nitric acid. (D) It has been contaminated with NO2 from the decomposition of nitric acid. (E) There is leftover reactant in the nitric acid product.
To begin, it would be helpful to write a balanced chemical equation that represents
the preparation of nitric acid. We are told that sulfuric acid and a nitrate salt are used. These are the reactants of the chemical reaction. The chemical formula of sulfuric acid is H2SO4 aqueous.
Let’s use sodium nitrate as the nitrate salt, which has the chemical formula NaNO3
solid. These two reactants are mixed together in the flask on the left in the diagram and
gently heated. A double replacement reaction takes place, producing nitric acid, or HNO3, and the
salt, sodium bisulfate.
As the reaction occurs, the sodium bisulfate produced remains in the flask. However, the nitric acid produced does not. The nitric acid being produced in the reaction flask will boil at around 83 degrees
Celsius. The reaction mixture is heated below 100 degrees Celsius but is hot enough that the
nitric acid boils and leaves the reaction flask as a gas. HNO3 gas passes to the round bottom flask on the right where it is cooled with cold
water and kept in an ice bath. At this point, the nitric acid is in the liquid state. Let’s change the state symbol for HNO3 to liquid in our equation.
Now let’s pause to take a look at the answer choices. Based on the chemical equation that we wrote, we know that solid sulfur is not
produced during the reaction. So we can eliminate answer choice (C).
We also know that the sodium bisulfate produced remains in the reaction flask and
would therefore not be present in the flask where nitric acid is collected. So, we can also eliminate answer choice (A).
Since sodium bisulfate salt remains in the reaction flask, it’s pretty likely that
any unused sodium nitrate salt also remains in the flask. And since the boiling point of sulfuric acid is over 300 degrees Celsius, if any is
left over in the reaction flask, it would remain there. Knowing this allows us to eliminate answer choice (E).
Now let’s look at answer choice (B). Because nitric acid is a strong acid it has a low pH. Very pure nitric acid is normally colorless. However, its pH value does not explain why it sometimes appears yellow after
preparation. So, let’s eliminate answer choice (B).
This leaves only answer choice (D), which must be the correct answer. Let’s clear the incorrect answer choices from the screen to discuss why (D) is the
correct answer. The truth is if the nitric acid prepared is greater than 95 percent pure or the
reaction temperature is too high during preparation, nitric acid will decompose. One of the products of the decomposition of nitric acid is nitrogen dioxide gas,
which has a yellow-brown color.
Therefore, the reason that the nitric acid prepared has a yellow color in the diagram
is because it has been contaminated with NO2 from the decomposition of nitric
acid.
