Lesson 1.1 · Unit 1 · HS-PS1-1
What chemistry studies, and what a chemical claim has to survive
Chemistry is the study of matter and the changes it undergoes, which sounds broad enough to include everything. What narrows it is the standard of evidence. A chemical claim is one that survives being weighed, and for two hundred years the balance has been the instrument that settles arguments in this subject.
- Matter is anything with mass that occupies volume. Light, heat and sound are not matter. Air is.
- Mass and weight are different quantities. Mass is the amount of matter and does not change with location. Weight is the force gravity exerts on that mass. A balance compares masses, which is why it reads the same on a mountain.
- Mass is conserved in a chemical change. The total mass of a closed system is the same before and after. Atoms are rearranged, not created or destroyed.
- "Closed" is doing the work in that statement. Nearly every apparent violation is a system that exchanged matter with its surroundings, usually a gas escaping or a gas being taken in.
- A chemical claim must specify a measurement. "This reaction gives off heat" becomes checkable when it says how much, measured how, on what quantity.
- Chemistry works across an enormous range of scale. Every explanation in this course moves between what you can weigh in the room and what individual particles are doing, and being explicit about which level you are describing is most of the skill.
Where students lose marks: writing that mass "was lost" when a gas escaped. The mass left the container; it did not leave existence. Say where it went.
The problem. A strip of magnesium is weighed, burned in air, and the white ash that remains is weighed. The ash is heavier than the metal was. Does this refute the conservation of mass?
Step one: get the measurements down. The magnesium strip has a mass of 2.43 g. After burning, the white solid has a mass of 4.03 g. The solid gained \( 4.03 - 2.43 = 1.60 \) g.
Step two: state what a violation would require. Conservation of mass is a claim about a closed system. This experiment is open to the room, so before concluding anything the account has to include whatever crossed the boundary.
Step three: ask what could have entered. The strip was burning in air. Air is matter, and something in it is being consumed, because the reaction stops if the air is excluded. The candidate is oxygen.
Step four: check the number against that idea. If oxygen combined with the magnesium, the mass gained should be the mass of oxygen taken in, which is 1.60 g. That is a prediction, and it is testable.
Step five: test it by closing the system. Repeat the burn inside a sealed flask containing air. Weigh the sealed flask and contents before and after. The mass does not change. Nothing was created; the container simply stopped the exchange.
Step six: confirm the identity of what entered. Burn the same mass of magnesium in a sealed flask of pure oxygen and measure how much oxygen disappears. It is 1.60 g, matching the gain exactly.
Step seven: state the conclusion at the right strength. The ash is heavier because it contains the original magnesium plus oxygen from the air. Conservation of mass is not refuted; the open system was simply not the whole system. This is the pattern for every apparent violation you will meet.
Antoine Lavoisier, Elements of Chemistry, 1789, in Robert Kerr's 1790 English translation. Public domain. The translator's spelling is preserved as written.
We may lay it down as an incontestible axiom, that, in all the operations of art and nature, nothing is created; an equal quantity of matter exists both before and after the experiment; the quality and quantity of the elements remain precisely the same; and nothing takes place beyond changes and modifications in the combination of these elements.
Lavoisier is not asserting this as a philosophical principle. He reached it by weighing sealed vessels before and after reactions, which is the experiment in the worked example above, and it is why he rather than anyone earlier is credited with it.
Questions 1 to 5 are recall. Questions 6 to 10 ask for reasoning.
- Define matter.
Show the full solution
Anything that has mass and occupies volume
- State the difference between mass and weight.
Show the full solution
Mass is the quantity of matter and is independent of location; weight is the gravitational force on that mass and varies with location
- State the law of conservation of mass, including the condition it requires.
Show the full solution
In a closed system, the total mass before a chemical change equals the total mass after it
- A sealed flask contains 12.50 g of one reactant and 8.30 g of another. They react
completely. What is the mass of the contents afterward?
Show the full solution
The flask is sealed, so nothing crosses the boundary. \( 12.50 + 8.30 \). 20.80 g
- Give the usual reason an open reaction vessel appears to lose mass.
Show the full solution
A gaseous product escaped from the container
- A student heats 5.00 g of a green solid in an open crucible and recovers 3.20 g of
a black solid. Explain what happened to the missing 1.80 g and how you would test
your explanation.
Show the full solution
Nothing is missing in the sense of destroyed. The decomposition released a gas that left the open crucible, so the 1.80 g is now in the room air. The test is to repeat the heating in a sealed vessel and weigh the whole vessel before and after: if the total is unchanged, the mass left the crucible rather than existence. A second test collects the gas over water and weighs it, which both accounts for the 1.80 g and identifies what was released. A gas escaped; confirm by repeating in a closed system and weighing the whole vessel
- Wood burns to a small heap of ash that weighs far less than the log. Iron rusts
and gains mass. Explain why these are the same phenomenon.
Show the full solution
Both are open systems exchanging gas with the air, and the direction of the apparent change depends only on whether the gas leaves or joins. Wood is largely carbon and hydrogen; burning converts it to carbon dioxide and water vapor, which leave, so the solid that stays behind is lighter. Iron is a solid metal; rusting combines it with oxygen from the air to make a solid oxide, which stays, so the solid gets heavier. Weigh either in a sealed container and the total is constant. Both exchange gas with the air, one losing it and one gaining it
- Explain why "this substance is better for cleaning" is not a chemical claim, and
rewrite it as one.
Show the full solution
Better is a judgment, and no measurement returns one, because better depends on what is being cleaned, at what cost, and with what risk. Chemistry can supply the facts the judgment uses but cannot make it. A checkable version fixes what is being measured and on what: "does a 5 percent solution of this substance remove a greater mass of a standard grease deposit from a glass slide in five minutes than a 5 percent solution of the alternative, at the same temperature?" Every term there can be measured and the result could come out either way. It is a value judgment; fix a measurable outcome, a quantity and a comparison
- A student weighs a sealed bag of baking soda and vinegar, mixes them, observes
vigorous bubbling and reweighs. The mass is unchanged, but the bag is now inflated.
Explain both observations.
Show the full solution
The mass is unchanged because the bag is closed: the atoms present at the start are still present, rearranged into different substances. The inflation happens because one of those substances is a gas, and a gas occupies far more volume per unit mass than the solid and liquid it came from. Mass and volume are separate quantities, and a chemical change conserves the first while frequently altering the second dramatically. If the bag were opened, the gas would leave and the mass would then fall. Mass is conserved in the closed bag; volume is not conserved because a gas was produced
- Lavoisier's contemporaries had the idea of conservation before him. Explain why he
is nonetheless credited with establishing it.
Show the full solution
Stating a principle and establishing it are different achievements. Earlier writers offered conservation as a plausible philosophical position, with nothing that would show it false. Lavoisier made it a measured result by working in sealed vessels with a balance accurate enough to detect the differences at stake, and by doing so on reactions where the naive observation points the other way, such as metals gaining mass on heating. The principle became evidence-based rather than assumed, and it simultaneously destroyed the competing phlogiston account, which had to posit a substance with negative weight to survive. He measured it in closed systems rather than asserting it, on cases where the appearance contradicts it