Next Generation Science Standards

Performance Expectation

Use mathematical representations to support the claim that atoms, and therefore mass, are conserved during a chemical reaction.

• Clarification Statement: Emphasis is on using mathematical ideas to communicate the proportional relationships between masses of atoms in the reactants and the products, and the translation of these relationships to the macroscopic scale using the mole as the conversion from the atomic to the macroscopic scale. Emphasis is on assessing students’ use of mathematical thinking and not on memorization and rote application of problem-solving techniques.
• Assessment Boundary: Assessment does not include complex chemical reactions.

"Nothing comes from nothing."

No, you didn't accidentally click your way to the Shmoop philosophy corner. That little nugget comes from ancient Greek philosophers (they always come up with the best stuff) and is the basis for what students will be working on in this performance expectation.

Students won't only be chemists; they will also play the role of mathematicians when they use mathematical representations to show that atoms and mass are conserved during a chemical reaction. And who knows, maybe you'll hear them arguing the philosophical merits of a balanced chemical equation.

Here are some activity ideas to help your students dig into some moles:

• To help students visualize the conservation of atoms, send them to this balancing chemical reactions simulator. As they practice balancing equations, they'll see the atoms and molecules involved and have a better grasp of why equations must be balanced.
• Provide students with manipulatives to represent the different atoms in a reaction (colored candies are always fun, but if you'd rather not deal with squirmy students, colored beads work just as well). Project a chemical reaction on the board and have students use their manipulatives to show the reaction. Discuss whether the equation is balanced and what they can do to balance it. Repeat the process with several different equations and have students check their work with a neighbor.
• Set up a series of simple reactions around the room. Have students work in small groups to perform the reaction and record the balanced chemical equation for the reaction. After they have rotated through each station, review the results as a class.
• Divide students into small groups and provide each group with a scenario involving a real-world example of how stoichiometry and conservation of atoms can be used. Some examples could be the amount of fertilizer needed for an acre of crops or the amount of rocket fuel needed to get a rocket to the moon. When they've finished, invite each group up to discuss their scenario and their mathematical solution.

Disciplinary Core Ideas

PS1.B – Chemical Reactions: The fact that atoms are conserved, together with knowledge of the chemical properties of the elements involved, can be used to describe and predict chemical reactions.

Students did battle with this idea in the standard HS-PS1-2, so hopefully they don't have amnesia. If they are feigning forgetfulness (let's face it, most of the time they can't remember a pencil so it's not totally unlikely), remind them that a chemical reaction is a closed system. This means nothing goes in and nothing comes out.

As far as how that relates to what they're trying to accomplish here, they need to know that all of the atoms that started the reaction as reactants can be found at the end of the reaction, in some form or another, as products.

Knowing that atoms don't perform any sort of witchcraft or sorcery during a chemical reaction, along with understanding each atom's characteristics, helps us to predict what will happen during a chemical reaction. Students will use a bit of stoichiometry and their new best friend, the mole, to extend this knowledge and explain how mass is also conserved during a chemical reaction.

Conservation of atoms during a reaction is a bit of a rerun for students, so they should be past the tears of frustration phase. The math part may get some of them, particularly if conversions aren't their forte. Remember to stick to simple reactions and keep the focus on actually understanding the mathematics behind what they're doing as opposed to making them memorize a thirty-six-step problem solving strategy.

Be sure to take things one step at a time and use manipulatives to help them visualize how the math makes sense. Pairing students into expert and novice pairings is another great way for students to deepen their understanding. You'll have a room full of stoichiometry studs in no time.

Science and Engineering Practices

Using Mathematics and Computational Thinking: Use mathematical representations of phenomena to support claims.

We hope you have your whiskers trimmed and your digging claws sharpened, because we're going underground to study the mole.

Wait, what? What do you mean "the other kind of mole"?

Oh, right, the chemistry mole. The unit we use to convert between atoms or molecules and grams. That was awkward…

In this performance expectation, students have been assigned to use a mathematical representation to show that atoms and mass are conserved during a chemical reaction. They will need to understand the proportional relationship between the mass of the products and the reactants on an atomic scale, as well as a macroscopic scale.

The best way for them to do this is to use the mole as a unit of conversion between the two. They can then use the mathematical data they've gathered to support their claims about conservation of mass.

Crosscutting Concepts

Energy and Matter: The total amount of energy and matter in closed systems is conserved.

Chemical reactions are closed systems. Nothing gets in and nothing gets out. Sort of like the popular clique in high school. Or a supermax prison.

The idea that chemical reactions are closed systems is going to come in handy for students here. Their job is to use math to support the claim that atoms and mass are conserved in a chemical reaction. So they're basically trying to prove something we already know, which should makes things slightly less frustrating for them.

Scientific Knowledge Assumes an Order and Consistency in Natural Systems: Science assumes the universe is a vast single system in which basic laws are consistent.

Understanding the wonders of the universe would get pretty annoying if gravity only existed on certain planets or if friction actually increased the speed of objects in certain galaxies.

Scientists don't like to be annoyed, so they assume that the universe is a single, exceptionally large, system. To decrease the level of annoyingness even further, scientists assume that the laws we've accepted here on Earth apply everywhere else in the universe.

So, if you start your reaction with two moles of hydrogen, you should end with two moles of hydrogen, whether you're performing your experiment on Earth, Jupiter, or some galaxy far, far away.