LESSON 8
What Goes Around Comes Around
Conservation of Matter
Delete
Think About It
The element copper can be mixed with other substances to make a colorful assortment
of compounds. What are these compounds? How can you demonstrate that they all
contain copper?
What happens to elements in a chemical change
To answer this question, you will explore

Translating the Copper Cycle

Tracking an Element

Conservation of Matter
Exploring the Topic
Translating the Copper Cycle
Chemical names and chemical formulas are powerful tools you can use to keep track of
matter. In fact, you can use them to figure out what you made at various steps in the
Lab: The Copper Cycle.
Copper nitrate, Cu(NO3)2(s)
There are several approaches you can take to figure out what was made at each step of
the cycle. First, you can compare the appearance of the compounds that you obtained
in each step with compounds that you have seen before. For example, in an earlier
class you examined samples of copper compounds that looked like these photos.
Copper sulfate, CuSO4(s)
Visual observation of compounds can give you some clues, but it is not enough to make
a definite identification. For example, it is hard to tell the difference between copper
sulfate and copper nitrate through observation because they are both blue. In fact,
several copper compounds are blue.
Another approach is to examine the chemical names and formulas of the substances
that were mixed together. The new substances are formed from parts of the starting
materials. For example, when sodium hydroxide, NaOH, is added to the beaker in the
second step of the cycle, a compound called copper hydroxide, Cu(OH)2, is produced.
Since a blue solid is formed and hydroxide, OH−, is one of the starting ingredients, you
can deduce that copper hydroxide is the product.
Adding Nitric Acid to Copper
Start at the beginning of the copper cycle and see if you can figure out what was
created after the first step by translating each step into chemical symbols and formulas.
Step 1: Nitric acid is added to copper powder. A clear blue solution and a brown gas
are formed.
We know the chemical names that go along with the first half of this step. Nitric acid is
added to copper powder resulting in a blue solution and a brown gas. Using chemical
formulas, you can write this as:
So far in class, you’ve seen two blue liquids. One was copper sulfate and the other was
copper nitrate.
Nitric acid, HNO3(aq), was added to the copper powder, so you can deduce that the blue
solution that formed was copper nitrate, Cu(NO3)2(aq), and not copper sulfate,
CuSO4(aq).
How about the brown gas? The brown gas must contain some combination of H, N, O,
or Cu because these are the only starting ingredients. Copper, Cu, does not form
gaseous compounds, but the other three elements do combine to form several different
gases. The chemical formulas and colors of these gases can be found in reference books
and are listed in the table below.
Aqueous copper nitrate, Cu(NO3)2(aq)
The only brown gas in this table is nitrogen dioxide, NO2(g). The completed chemical
sentence for Step 1 is:
This reaction also produces one more compound: water, H2O. You would not have
noticed this because it is clear and colorless. So the final chemical sentence for Step 1
is:
Notice that all the elements in the starting ingredients also appear in the products. No
elements are created or destroyed.
You can deduce the products for the other steps in the copper cycle in a similar way.
ENVIRONMENTAL
CONNECTION
Nitrogen dioxide, NO2(g), is a part of smog. It causes the red-brown color in the skies
above cities with large amounts of air pollution. Nitrogen dioxide irritates the eyes,
nose, throat, and respiratory tract. Continued exposure can cause bronchitis.
Tracking an Element
Once you have figured out the products of Steps 2, 3, and 4 of the copper cycle, you
can track the journey of copper through the cycle. The illustration below shows the
copper compounds that form at each step of the cycle.
The symbol Cu is found at each stage of the cycle. Thus, the element copper is
somehow combined in each of these compounds. And of course, the solid that forms at
the end of the experiment is elemental copper, Cu(s).
What you have observed with copper is true of other elements as well. For example,
nickel can be taken through a similar cycle, where various substances are added to
nickel powder, Ni(s). Just as with the copper cycle, you end up with nickel in the end.
Conservation of Matter
The copper cycle experiment brought you full circle, back to where you started. You
took a sample of the element copper and added substances to it. After several steps,
you ended up with copper powder once again. No matter what was done to the copper,
the copper was always there in some form. In other words, it was not created or
destroyed by the chemical transformations. Over many centuries, scientists have
gathered evidence that matter can never be destroyed or created through chemical
transformation. There is so much evidence that this is considered a scientific law.
The law of conservation of mass states that mass cannot be gained or lost in a
chemical reaction. In other words, matter cannot be created or destroyed.
It is possible to prove that no copper was gained or lost during the copper cycle
experiment by measuring the mass of the copper powder at the beginning and again at
the end. If you did the experiment perfectly, you would end up with exactly the same
amount of copper powder that you started with. However, in real life the mass of the
copper at the end of this experiment is a bit less than the mass of the copper at the
beginning due to several factors. Little amounts of copper are lost along the way,
because of spills, measurement errors, and sticking to the filter paper or beaker. These
small errors are hard to avoid. In addition, some copper compounds remain in the
discarded solutions. Nevertheless, mass is still conserved; even if the copper isn’t
visible in the beaker at the end, it still exists somewhere.
Law of Conservation of Mass
Matter cannot be created or destroyed.
Key Terms
law of conservation of mass
Lesson Summary
What happens to elements in a chemical change
Chemical names and formulas are used to keep track of elements and compounds as
they undergo chemical or physical changes. When elemental copper is tracked through
a series of reactions, the symbol Cu shows up in the formulas of the new compounds
that are made. Elemental copper can be combined with other substances to form new
compounds, but it is not destroyed by the chemical transformations. This concept is
known as the law of conservation of mass, which states that matter cannot be created
or destroyed.
HISTORY
CONNECTION
Pennies were last made of solid copper in 1836. Pennies made from 1962 to 1982 are
95% copper and 5% zinc. These pennies have a density of 8.6 g/cm3, which is just
slightly less than the density of copper, 9.0 g/cm3. Since 1982, pennies have been
made mostly of zinc with a copper coating. These pennies have a density of 7.2 g/cm3,
which is very close to the density of pure zinc, 7.1 g/cm3.
Exercises
Reading Questions
1.
How can chemical names and symbols help you figure out what copper compound you
made in each step of the copper cycle? Give an example.
2.
Explain the law conservation of mass in your own words.
Reason and Apply
3.
Lab Report Write a lab report for the Lab: The Copper Cycle. In your report, include
the title of the experiment, purpose, procedure, observations, and conclusions.
4.
Explain how the copper cycle experiment supports the claim that copper is an
element—a basic building block of matter.
5.
Nickel sulfate, NiSO4(aq), is a green solution. Nickel chloride, NiCl2(aq), is a yellow
solution. And hydrochloric acid, HCl(aq), is a clear, colorless solution. If you add nickel,
Ni(s), to hydrochloric acid, HCl(aq), what color solution do you expect to form? Explain
your reasoning.
6.
In the final step of the copper cycle, zinc, Zn(s), is added to copper sulfate, CuSO4(aq).
Elemental copper appears as a solid. Explain what you think happens to the elemental
zinc.
7.
Matter cannot be created or destroyed. List at least two long-term impacts that this
concept has for us on this planet.
Unit 1:Alchemy:Lesson 8