Law of conservation of mass (Lavoisier)
Your teacher lights a candle on a balance in the science lab and, as time goes by, the reading drops. Then she burns a piece of steel wool and, on the other hand, it weighs more. Is matter created or destroyed in a chemical reaction? The answer, which took centuries to arrive, is that it is not: matter only changes place.
Lavoisier's experiment
Antoine Lavoisier worked in Paris in the late 18th century. Scientists' ideas change as new evidence appears, and his work is a good example. Before him, many chemists weighed the solids and liquids in a reaction, but they did not worry about the gases that entered or escaped. Lavoisier was the first to enclose reactions in closed containers and weigh absolutely everything, including the air.
Complete the sentence with the missing words: "In any chemical reaction, the total mass of the $\dots$ is equal to the total mass of the $\dots$: mass is neither created…
Complete the sentence with the missing words: "In any chemical reaction, the total mass of the $\dots$ is equal to the total mass of the $\dots$: mass is neither created nor $\dots$, it is only transformed."
Aoife weighs two solutions in a sealed beaker: before mixing them, the balance reads $185\ \text{g}$. When mixed, a white precipitate appears. The figure shows the…
Aoife weighs two solutions in a sealed beaker: before mixing them, the balance reads $185\ \text{g}$. When mixed, a white precipitate appears. The figure shows the sealed beaker on the balance, before and after mixing.
What will the balance read after the reaction? Justify your answer using Lavoisier's law.
Lavoisier heated mercury for twelve days in a retort (a sealed glass vessel) containing air. The mercury became covered with a red powder and its mass increased, while…
Lavoisier heated mercury for twelve days in a retort (a sealed glass vessel) containing air. The mercury became covered with a red powder and its mass increased, while the air inside lost part of its volume. The figure shows the sealed retort.
- a) Why did the mass of the mercury increase?
- b) Where did the gained mass come from?
- c) Explain why the total mass of the sealed retort did not change during the experiment.
When it seems that mass changes
The problem is almost always the same: if the reaction releases or uses up a gas and we do not weigh that gas, the balance misleads us.
We leave a burning candle on a balance, in the open air, and see that the reading gradually decreases, as shown in the figure. Does this mean that matter is being…
We leave a burning candle on a balance, in the open air, and see that the reading gradually decreases, as shown in the figure.
Does this mean that matter is being destroyed? Where does the mass go that the balance no longer detects?
When cooking with butane gas, the cylinder gradually loses mass. Does this mean that matter disappears? Explain where the mass lost by the cylinder goes.
When cooking with butane gas, the cylinder gradually loses mass. Does this mean that matter disappears? Explain where the mass lost by the cylinder goes.
Darragh bakes a cake: the raw batter weighs $800\ \text{g}$ and, when taken out of the oven, the cake weighs $750\ \text{g}$. What has happened to the missing…
Darragh bakes a cake: the raw batter weighs $800\ \text{g}$ and, when taken out of the oven, the cake weighs $750\ \text{g}$. What has happened to the missing $50\ \text{g}$? Has that matter been destroyed?
Calculating with Lavoisier's law
In the lab you break down $18.0\ \text{g}$ of liquid water using electricity and measure that $16.0\ \text{g}$ of oxygen gas have been obtained: can you predict the mass of hydrogen without weighing it?
Yes. The law lets you calculate a mass that you do not know, if you know the others. For the water above: the mass before is $18.0\ \text{g}$, so the products must also total $18.0\ \text{g}$. Hydrogen $= 18.0\ \text{g} - 16.0\ \text{g} = 2.0\ \text{g}$.
The same idea applies to living things. In a sealed spacecraft, the mass of the oxygen an astronaut breathes plus the food and water he takes in equals the mass of what he gives out as carbon dioxide, water vapour and waste. Mass is not created: the extra mass in what is exhaled comes from the food and body tissue that react with the oxygen.
As a word equation, this reaction is: carbon + oxygen → carbon dioxide. Look at the interactive: the number of carbon and oxygen atoms is the same on both sides of the arrow, because they are the same atoms before and after, only regrouped.
In a reaction, $12\ \text{g}$ of carbon combine exactly with $32\ \text{g}$ of oxygen, with nothing left over, as shown in the diagram. What mass of carbon dioxide is…
In a reaction, $12\ \text{g}$ of carbon combine exactly with $32\ \text{g}$ of oxygen, with nothing left over, as shown in the diagram.
What mass of carbon dioxide is formed?
When heating $10\ \text{g}$ of copper in the presence of air, $12.5\ \text{g}$ of copper oxide are formed, as shown in the diagram. Why does the oxide weigh more than…
When heating $10\ \text{g}$ of copper in the presence of air, $12.5\ \text{g}$ of copper oxide are formed, as shown in the diagram.
Why does the oxide weigh more than the copper it comes from? What mass of oxygen has combined?
An ice cube of $30\ \text{g}$ melts completely inside a tightly covered glass. How much will the resulting liquid water weigh? Has a chemical reaction taken place?…
An ice cube of $30\ \text{g}$ melts completely inside a tightly covered glass. How much will the resulting liquid water weigh? Has a chemical reaction taken place? Reason your answers.
A magnesium ribbon of $6\ \text{g}$ burns in air, and $10\ \text{g}$ of powdered magnesium oxide are collected. a) What mass of oxygen from the air has combined with the…
A magnesium ribbon of $6\ \text{g}$ burns in air, and $10\ \text{g}$ of powdered magnesium oxide are collected.
- a) What mass of oxygen from the air has combined with the magnesium?
- b) If we burned $12\ \text{g}$ of magnesium with excess oxygen, what mass of oxide would we obtain?
Reactions in open and closed containers
To check Lavoisier's law in the laboratory you have to think the set-up through carefully: if any gas can escape uncontrolled, the check fails even though the law is true.
The same idea works with an Alka-Seltzer tablet in water. In an open bottle the reading drops as the carbon dioxide escapes; if you screw the cap on a closed bottle, the reading stays the same (open the cap slowly afterwards, as the pressure builds up). Compare the two bottles on the same balance.
Inside an airtight glass bell jar, there are $250\ \text{g}$ of air and a candle of $20\ \text{g}$. We light it, and after a while, the candle weighs $14\ \text{g}$. The…
Inside an airtight glass bell jar, there are $250\ \text{g}$ of air and a candle of $20\ \text{g}$. We light it, and after a while, the candle weighs $14\ \text{g}$. The figure shows the sealed bell jar.
- a) What is the total mass of the contents of the bell jar at that moment?
- b) By how much has the mass of the air mixed with the gases produced increased?
An open container contains $200\ \text{g}$ of hydrogen peroxide, which decomposes slowly, releasing oxygen gas. When the decomposition finishes, $184\ \text{g}$ of water…
An open container contains $200\ \text{g}$ of hydrogen peroxide, which decomposes slowly, releasing oxygen gas. When the decomposition finishes, $184\ \text{g}$ of water remains.
- a) What mass of oxygen has escaped from the container?
- b) Explain why this result does not contradict the law of conservation of mass.
Design an experiment using a bottle, a balloon, vinegar and bicarbonate of soda to demonstrate the law of conservation of mass even when a gas is given off in the…
Design an experiment using a bottle, a balloon, vinegar and bicarbonate of soda to demonstrate the law of conservation of mass even when a gas is given off in the reaction. Describe the setup step by step, predict what the balance will read before and after the reaction, and point out two experimental errors that could ruin the verification.
Checking the law with precision
In a real laboratory, balances and measurements are never perfect. When results do not match exactly, you have to distinguish between a small measurement error and a genuine contradiction of the law.
An iron nail of $28\ \text{g}$ is left outdoors, and when it has completely rusted, the iron oxide formed weighs $40\ \text{g}$. a) What mass of oxygen has been bound…
An iron nail of $28\ \text{g}$ is left outdoors, and when it has completely rusted, the iron oxide formed weighs $40\ \text{g}$.
- a) What mass of oxygen has been bound from the air?
- b) Express that mass of oxygen in milligrams.
In a lime kiln, $100\ \text{kg}$ of limestone are heated to produce $56\ \text{kg}$ of quicklime. a) A worker claims: "Mass has been lost, so Lavoisier was wrong."…
In a lime kiln, $100\ \text{kg}$ of limestone are heated to produce $56\ \text{kg}$ of quicklime.
- a) A worker claims: "Mass has been lost, so Lavoisier was wrong." Explain to him why this is not the case.
- b) Calculate the mass of carbon dioxide that has escaped through the chimney.
- c) How would you repeat the experiment to test the law of conservation of mass directly?
A group of students checks the conservation of mass in a precipitation reaction and notes down: mass before mixing, $164.8\ \text{g}$; mass after, $164.6\ \text{g}$. a)…
A group of students checks the conservation of mass in a precipitation reaction and notes down: mass before mixing, $164.8\ \text{g}$; mass after, $164.6\ \text{g}$.
- a) Do these data refute Lavoisier's law?
- b) Calculate the difference between both measurements and express it in milligrams.
- c) Give two possible experimental causes for this slight discrepancy.
Back to the beginning: the candle seems to lose mass because its products ($\mathrm{CO_2}$ and water vapour) escape into the air as invisible gases, whereas the steel wool weighs more when it burns because it traps oxygen atoms from the air, forming solid iron oxide; if you carry out both combustions in a closed container, the balance does not change at all.