Performance Benchmark P.12.A.7
Students know that, in chemical reactions, elements combine in predictable ratios, and
the numbers of atoms of each element do not change. I/S
Chemical reactions occur constantly in our daily lives. By understanding what the
equations for these reactions represent, we can put this information to practical use such
as getting energy to heat our homes, driving our cars and trucks, and cooking many of the
foods we eat. Some chemical reactions, such as digesting the food we eat, are so innate
that we really don’t even think about them until we want to change our diet to eat more
healthy foods or to lose or gain weight.
John Dalton, an English school teacher, proposed atomic theory in 1808. Dalton’s atomic
theory explained the law of conservation of mass, the law of definite proportions, and the
law of multiple proportions. Four of Dalton’s main ideas are stated below (from
http://www.chemheritage.org/EducationalServices/webquest/dalton.htm).
1. All matter is composed of tiny particles, called atoms.
2. Each element is made of a different kind of atom, and the atoms of different
elements have different masses.
3. Atoms are neither created nor destroyed in chemical reactions.
4. Atoms of different elements combine in number ratios, with more than one ratio
being possible for a given combination of elements.
To learn more about John Dalton and his atomic theory, go to
http://www.chemheritage.org/classroom/chemach/periodic/dalton.html
Atomic theory helps explain how elements combine in predictable ratios, and the
numbers of atoms of each element do not change. For an example, consider charcoal
briquettes that are burning on a grill to release heat for cooking. Charcoal is a form of
carbon. When it burns, the following reaction occurs.
Carbon reacts with oxygen to produce carbon dioxide
Expressing this in a chemical equation the reaction would be expressed as
C + O2  CO2
In terms of what is occurring with the atoms and their bonding, both broken and formed,
the reaction can be represented as shown below.
C + O=O  O=C=O
The ratio of carbon to oxygen atoms to form carbon dioxide is 1 carbon atom to 2 oxygen
atoms. Carbon dioxide is composed of 1 carbon atom and 2 oxygen atoms. This
equation is balanced. The atoms have been rearranged in their bonds, and the total of the
reacting atoms (1 carbon atom and 2 oxygen atoms) equals the total of the atoms in the
product (1 carbon atom and 2 oxygen atoms).
If we look at the burning of methane (CH4), the major component in natural gas, the
following chemical reaction occurs when methane burns and produces heat.
methane burns in oxygen to produce carbon dioxide and water
This reaction is represented in the chemical equation below as
CH4 + O2  CO2 + H2O
In this case, the equation is not balanced because there are different numbers of carbon,
hydrogen, and oxygen atoms on both sides of the equation. To represent the relative
numbers of each substance on the reactants and products side, the equation must be
balanced as
CH4 + 2 O2  CO2 + 2 H2O
When balancing chemical equations, the coefficients in front of each chemical formula
must be changed. This balanced equation has 1 atom of carbon, 4 atoms of hydrogen and
4 atoms of oxygen on both the reactants and products sides. No atoms of any element
were created nor destroyed. Bonds were broken and reformed. When this reaction
occurs, it will always be in the molecular ratio of 1 methane to 2 oxygen to 1 carbon
dioxide to 2 water. If someone burned double the molecules of methane, then 2 methane
would need 4 molecules of oxygen to produce the 2 molecules of carbon dioxide and 4
molecules of water. There would be the same total number of atoms of carbon,
hydrogen, and oxygen of reactants as products. This is predictable ratio.
Figure 1. Representation of atoms of carbon, hydrogen, and oxygen when methane burns in oxygen to
form carbon dioxide and water. The small dots on the water molecules represent unpaired electrons on
oxygen. (from: http://www.elmhurst.edu/~chm/vchembook/511natgascombust.html)
To learn more about stoichiometric relationships in combustion, go to
http://itl.chem.ufl.edu/2045/lectures/lec_4.html.
Performance Benchmark P.12.A.7
Students know that, in chemical reactions, elements combine in predictable ratios, and
the numbers of atoms of each element do not change. I/S
Common misconceptions associated with this benchmark:
1. Students incorrectly think that whatever the subscript is for an element on the
reactant side of the equation, the same subscript must be used for that element on
the product side.
However, the correct formulas for the reactants and products must be written based on
the names of each substance and/or oxidation numbers rather than just “dragging”
subscripts from reactant side of equation to product side.
For more explanation and practice in balancing equations, go to
http://www.unit5.org/christjs/chemical_equations.htm.
2. Students can get confused about whether to balance chemical equations by
changing coefficients (which is correct) or by changing subscripts (which is
incorrect) in formulas.
Once the formulas of reactants and products are written correctly, the equation must be
balanced by changing coefficients in front of the chemical formula. This makes the
coefficient apply to everything in the formula that comes immediately after. Balancing is
often done simply by inspection and trial and error when the equations are simple.
For more information about balancing equation misconceptions, go to
http://72.14.253.104/search?q=cache:21DO1yEqv8cJ:chemed.rice.edu/IEinCE/diagnostic
s/Fall98/AMT/AMTresults.html+misconceptions+in+balancing+equations&hl=en&ct=cl
nk&cd=4&gl=us.
3. Students often forget that gases have mass.
If students are burning magnesium in a loosely covered crucible in lab, they are often
surprised when they mass the crucible and find that the mass of the magnesium oxide ash
is greater than the mass of the magnesium. It seems counterintuitive to them because
they are often familiar with burning logs in a fire and having the mass of ash be much
less than the mass of the logs they burned. They forget that burning logs produce
gaseous carbon dioxide and water vapor, which disperses into the air. When the
magnesium burns, it is combining with oxygen from the air, and thus the magnesium
oxide should have a greater mass than the mass of only the magnesium. The total mass
of the reactants still equals the total mass of the products, and mass is conserved.
To see a demonstration of magnesium burning in the air and an explanation of what is
occurring, go to http://boyles.sdsmt.edu/magburn/magnesium_burning.htm or to
http://www.angelo.edu/faculty/kboudrea/demos/burning_magnesium/burning_magnesiu
m.htm.
For laboratory directions, go to
http://www.sciencepages.co.uk/keystage3/resources/magnesium%20ws.pdf.
4. Students try to balance equations by placing coefficients in the middle of a
formula.
Coefficients may be placed only before the entire formula. If the ratio of atoms of each
element in the formula were to change, then this would violate the law of definite
proportions, which states that the ratio of atoms of each element in a compound is
definite and constant.
For more information on balancing equations and some practice equations, go to
http://dbhs.wvusd.k12.ca.us/webdocs/Equations/Balance-Equation.html.
Performance Benchmark P.12.A.7
Students know that, in chemical reactions, elements combine in predictable ratios, and
the numbers of atoms of each element do not change.
Sample Test Questions
1. The reaction equation for the decomposition of water to hydrogen and oxygen is
H2O  H2 + O2. Which choice below shows the balanced equation?
a. H2O  H2 + O2
b. 2 H2O  2 H2 + O2
c. 2 H2O  2 H2 + 2 O
d. H2O  H2 + O
2. What happens to the atoms involved in a chemical reaction?
a. They change into atoms of different elements.
b. They change into energy.
c. They combine in predictable ratios.
d. The numbers of atoms of each element changes.
3. When methane burns in air to form carbon dioxide and water, the balanced
equation is CH4 + 2 O2  CO2 + 2 H2O. When 8 molecules of methane are
burned, how many molecules of water are produced?
a. 8 molecules water
b. 12 molecules water
c. 16 molecules water
d. 24 molecules water
4. When potassium chlorate decomposes to potassium chloride and oxygen, the
unbalanced equation for the reaction is: KClO3  KCl + O2. Which set of
coefficients (in order) will balance this equation?
a. 2,3,2
b. 2,4,3
c. 2,4,2
d. 2,2,3
5. Which statement is true about both physical and chemical changes?
a. Mass is conserved in both physical and chemical changes.
b. Mass is NOT conserved in either physical or chemical changes.
c. Mass is conserved in physical changes but not in chemical.
d. Mass is conserved in chemical changes but not in physical.
6. What happens when wood burns with the oxygen in air to form water and carbon
dioxide?
a. The mass of the products becomes less than the mass of the reactants.
b. Most of the mass of the wood is converted into energy.
c. More energy is absorbed than given off in the reaction.
d. The mass of the wood and oxygen reacted equals the mass of the products.
7. When hydrogen and chlorine react to product hydrogen chloride, the reaction
equation is H2 + Cl2 HCl. What is the balanced equation for this reaction?
a. H2 + Cl2 HCl
b. H2 + Cl2 H2Cl2
c. H2 + Cl2 2H2Cl
d. H2 + Cl2 2 HCl
8. When photosynthesis occurs, carbon dioxide and water produce glucose and
oxygen. The balanced equation is 6 CO2 + 6 H2O  C6H12O6 + 6 O2. How many
molecules of C6H12O6 are produced when 18 molecules O2 are produced?
a. 1 molecule C6H12O6
b. 2 molecules C6H12O6
c. 3 molecules C6H12O6
d. 6 molecules C6H12O6
Performance Benchmark P.12.A.7
Students know that, in chemical reactions, elements combine in predictable ratios, and
the numbers of atoms of each element do not change. I/S
Answers to Sample Test Questions
1.
2.
3.
4.
5.
6.
7.
8.
(b)
(c)
(c)
(d)
(a)
(d)
(d)
(c)
Performance Benchmark P.12.A.7
Students know that, in chemical reactions, elements combine in predictable ratios, and
the numbers of atoms of each element do not change. I/S
Intervention Strategies and Resources
The following list of intervention strategies and resources will facilitate student
understanding of this benchmark.
1. How to Balance Chemical Equations Tutorial
There are many websites that can help students understand how to balance chemical
equations.

The Illinois Institute of Technology gives directions for students to use paper
squares representing atoms and balance equations by manipulating the paper
“atoms”. Go to http://www.iit.edu/~smile/ch8601.html to access this activity.

For an explanation of the logic of balancing chemical equations as well as
practice, go to http://dbhs.wvusd.k12.ca.us/webdocs/Equations/Meaning-ofEquation.html.

For several examples, explanations, and interactive practice in balancing chemical
equations, go to http://richardbowles.tripod.com/chemistry/balance.htm#part1.

For a simple explanation of conservation of mass and a diagrammatic
representation of a chemical reaction, go to
http://www.iun.edu/~cpanhd/C101webnotes/matter-and-energy/masscons.html.

ChemTutor has many pages of explanation and practice in balancing equations.
To access this site, go to http://www.chemtutor.com/react.htm#bal.

USC has an interactive site on which students can practice balancing equations
and then check their work at http://chemmac1.usc.edu/java/balance/balance.html.

SciLinks has activities to balance equations and represents different elements in
different colors to help students visualize each element. To access, go to
http://www.middleschoolscience.com/balance.html.

The beginning of this web site gives an explanation of chemical reactions and
offers many opportunities to practice balancing equations. Go to
http://nobel.scas.bcit.ca/chemed2005/tradingPost/WEPM-S3-409_Introduction_to_POGIL.pdf.
2. Hand-On Chemical Balancing Activities
There are some websites that give directions for laboratory activities that can have
students do hands on work to understand the law of conservation of mass and the
predictable nature of ratios of reactants and products.

The Illinois Institute of Technology gives a simple, yet excellent activity that will
show that gases have mass. This lab involves the reaction of Alka Seltzer tablets
with water and is found at http://www.iit.edu/~smile/ch9403.html.

Another experiment from this same source uses common items of hardware (nuts,
bolts, screws, etc) to help students visualize relative masses. To access this, go to
http://www.iit.edu/~smile/ch8621.html.

This website offers directions for a laboratory activity in which students react
baking soda with vinegar and look at relative masses. Go to
http://misterguch.brinkster.net/MLX039.doc.

Baking soda can also be reacted with hydrochloric acid (the acid used to balance
the pH of pools). For directions, go to
http://www2.ucdsb.on.ca/tiss/stretton/CHEM1/lab7.html.