1. B. (NH4)2Cr2O7(s)  2 N2(g) + Cr2O3(s) + 4 H2O(g)
QUESTION:
Consider the following observations of a chemical change: A small amount of orange-red solid is heated in a test tube. A little explosion occurred and a green powder was obtained. The mass of the green solid was found to be less than that of the original orange-red solid. Which of the following chemical equations is consistent with the observations above?
A. Mg(s) + O2(g)  2 MgO(s)
B. (NH4)2Cr2O7(s)  2 N2(g) + Cr2O3(s) + 4 H2O(g)
C. 2 HgO(s)  2 Hg(l) + O2(g)
D. Pb(NO3)2(aq) + 2 NaCl(aq)  PbCl2(s) + 2 NaNO3(aq)
Consider the following observations of a chemical change: A small amount of orange-red solid is heated in a test tube. A little explosion occurred and a green powder was obtained. The mass of the green solid was found to be less than that of the original orange-red solid. Which of the following chemical equations is consistent with the observations above? ?
DONT READ THE CHOICES, JUST PAUSE
According to the observations, the starting material is a solid.
HIGHLIGHT “orange-red solid” in question
So we need at least one solid reactant. One of the formulas on the the left-hand side of our chemical equation should have a solid label.
We see that in choices A, B, and C, but not in choice D.
HIGHLIGHT (s) in Mg(s), (NH4)2Cr2O7(s), and HgO(s)
The s in parentheses stands for solid.
In choice D, both reactants are in aqueous solution.
HIGHLIGHT (aq) in Pb(NO3)2(aq) and NaCl(aq)
This means that the substances are dissolved in water.
CROSS OUT choice D.
One of the products is also a solid,
HIGHLIGHT “green solid” in the question
Therefore, we can rule out choice C. The products in choice C are a liquid...
HIGHLIGHT (l) label next to Hg
and a gas.
HIGHLIGHT (g) label next to O2.
CROSS OUT choice C
The observations tell us that the solid product has less mass than the original solid reactant.
HIGHLIGHT “The mass of the green solid was found to be less than that of the original orange-red solid.”
Therefore, we can eliminate choice A.
Since solid magnesium oxide
HIGHLIHGT MgO(s)
The chemical equation given here implies more mass. The mass of magnesium oxide would be expected to be equal to the combined mass of the reactants magnesium and oxygen.
CROSS OUT choice A
This is an example of a combination or synthesis reaction
CALLOUT “combination or synthesis”
Where two or more reactants combine to form just one product. We expect the product mass to be equal the combined mass of the reactants. We therefore expect it to also have more mass than any one of the reactants.
Therefore, the correct answer must be choice B.
Let’s examine choice B. We can see here that there is only one reactant... solid ammonium dichromate
HIGHLIGHT (NH4)2Cr2O7(s) (orange red color)
and there are three products... nitrogen gas and water vapor, which will escape from the test tube since it is a gas
HIGHLIGHT N2(g) and H2O(g)
and Cr2O3, which is called chromium oxide, which will remain in the test tube.
HIGHLIGHT Cr2O3(s) (sage green color)
We would, therefore, expect the chromium oxide to have less mass than the original solid.
This is an example of a decomposition reaction.
CALLOUT “decomposition reaction”
A decomposition reaction is one where you only have one reactant, and you end up with two or more products. Here we have one reactant...
ammonium dichromate... and three products... nitrogen, chromium oxide, and water vapor...
In any reaction, the total mass of the products equals that of the original reactant.
Therefore, for a decomposition reaction, any one of the products must have mass less than that of the original reactant.
As final note... You can also figure out the answer to this question if you are familiar with the properties of the substances given in the chemical equations;
or by looking them up. You will find that ammonium dichromate is, in fact, orange-red in color and chromium oxide is green. This reaction is commonly
used in chemical demonstrations; the explosion resembles that of an erupting volcano.
CLICK PAUSE END RECORDING

2. Video ID: © 2008, Project VALUE (Video Assessment Library for Undergraduate Education), Department of Physical Sciences Nicholls State University Author: Glenn V. Lo Narrator: Funded by Louisiana Board of Regents Contract No. LA-DL-SELECT-13-07/08