Learning target: I can use evidence from a passage to explain how sealed-vessel experiments showed that matter is conserved, and to tell the difference between the law and the atomic explanation of it.
Read the passage once straight through. Read it a second time with a pencil, numbering the paragraphs 1 to 7 and circling every number and every name. Answer the questions in complete sentences. When a question asks for evidence, quote or closely paraphrase the passage and give the paragraph number. The experiments described here are historical; do not try to repeat them.
You will need: This worksheet, Pencil, Scrap paper for two-column notes
[1]Put a log in a fireplace and come back in the morning. The log is gone. In its place is a small heap of gray ash that weighs far less than the wood did. For centuries, people looked at that ash and drew the obvious conclusion: fire destroys matter. Part of the log, they thought, had stopped existing. It was a reasonable idea. It was also wrong, and showing why took careful weighing.
[2]The first clues came from metals, which seemed to do the opposite of wood. In the 1600s, chemists noticed that tin or lead heated in air for a long time turned into a crumbly powder that weighed more than the metal had. The English scientist Robert Boyle reported this in the 1670s. He heated tin in a sealed glass vessel, then opened the vessel and weighed the powder. It was heavier. Boyle suggested that tiny 'particles of fire' had passed through the glass and attached themselves to the metal. That explanation fit the facts he had. It missed one measurement.
[3]The missing measurement was made a century later by the French chemist Antoine Lavoisier. In the 1770s he repeated the experiment with a change that sounds small. He sealed tin inside a glass vessel, weighed the whole sealed vessel, and only then heated it. The tin turned to powder inside. He weighed the sealed vessel again. The reading had not changed. Then he broke the seal. Air hissed in, and the vessel became heavier. The amount it gained was the same as the amount the powder had gained compared with the original tin.
[4]Lavoisier saw what that meant. Nothing had passed through the glass. The tin had taken something from the air sealed inside with it, and when the vessel was opened, fresh air rushed in to replace what the tin had used up. The mass of the powder was the mass of the tin plus the mass of the part of the air it had captured. Today we know that part is oxygen and the powder is tin oxide. The work was a team effort. Lavoisier's wife, Marie-Anne Paulze Lavoisier, kept records, translated scientific papers written in English, and drew the detailed illustrations of the equipment.
[5]Precise weighing was at the center of everything Lavoisier did. He used some of the best balances in Europe and insisted that a chemist account for every bit of matter going into an experiment and every bit coming out. In 1789 he published a textbook, the Elementary Treatise on Chemistry, that stated the principle plainly: in every change, the quantity of matter is the same before and after. We now call this the law of conservation of matter, or conservation of mass.
[6]A law describes a pattern; it does not explain it. The explanation came from the idea that matter is made of atoms. In a chemical reaction, atoms do not appear, vanish, or turn into other kinds of atoms. They only change partners: bonds between atoms break, and new bonds form. Since every atom keeps its own mass, rearranging atoms cannot change the total. The burning log is the same story as the tin, run in reverse. Carbon and hydrogen atoms in the wood bond with oxygen atoms from the air and leave the fireplace as invisible gases, carbon dioxide and water vapor. The ash is only what did not become a gas. Add the mass of the gases to the mass of the ash and you get the mass of the wood plus the mass of the oxygen it used.
[7]So the rule for reading a balance is simple. In a closed system, where no matter can enter or leave, the mass does not change during a reaction. In an open system, the reading can fall when a gas escapes or rise when a gas joins in, and every change on the dial means matter crossed the boundary. It did not stop existing, and it did not come from nowhere. The law holds for all ordinary chemical and physical changes; changes inside an atom's nucleus follow different rules and belong to a later course. More than two centuries after Lavoisier, the sealed vessel is still the fairest test.
On scrap paper, make two columns: 'Who and when' and 'What the weighing showed.' Fill a row for Boyle (1670s), Lavoisier sealed (1770s), Lavoisier opened, and the 1789 textbook. Most questions can be answered by finding the right row and rereading that paragraph. For q6 and q7, add a third row labeled 'atoms' and write one sentence from paragraph 6 that mentions bonds.
Boyle's explanation was wrong, but he was not careless. Write a paragraph defending Boyle: list what he measured correctly, explain what single measurement would have overturned his fire-particle idea, and then explain why a scientist today would still call his work valuable. Finish with one sentence about what this episode shows about how a scientific law gets established: is it one experiment, or something else?