
Learning About the Law of Conservation of Mass
Jun 9, 2021
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The law of conservation of mass describes the phenomenon that scientists have observed that mass stays constant throughout a chemical reaction. When the law of conservation of mass was discovered by Antoine Lavoisier in 1789, it was revolutionary because it didn’t match what people observed. Mass appeared to disappear during a chemical reaction. It wasn’t until Lavoisier was able to create a closed system for his experiments that he realized the disappearing mass was actually solids being converted into invisible gases.

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What is Mass?
Mass is a measurement of how much matter is in an object. Scientists usually measure mass in grams or kilograms.
An object’s mass depends on its number of atoms and the mass of the individual atoms. For example, each carbon atom has a mass of 12 units because it has 6 protons and 6 neutrons. Gold atoms have a mass of 197 units because it has 79 protons and 118 neutrons.
You can see that the big dog has more mass than the little dog because the big dog weighs more. While mass and weight are not the same, but they are related.

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Mass Cannot Be Created or Destroyed (for the most part)
The law of conservation of mass says that mass cannot be created or destroyed. As a result, the mass in a closed system will always be the same. A closed system means matter can’t enter or leave. Earth is almost a closed system, but matter does enter and leave it. Around 60 tons of space rocks hit Earth every day. Most of it is tiny dust grains that burn up in our atmosphere.
Even though mass is added to the Earth every day, our planet isn’t getting bigger because it is also losing mass. The Earth leaks around 100 tons of its atmosphere into space every day. Don’t worry about running out of air to breathe though. The Earth’s atmosphere is 5.5 quadrillion tons. It will take over 150 billion years for the leak to deplete the atmosphere.

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An exception to the rule that mass cannot be created or destroyed can be found in the Sun. When hydrogen atoms fuse to form a helium atom, a tiny fraction of mass is lost. The lost mass is converted into the massive amount of energy we experience as sunlight. Even this mass isn’t destroyed, but it is converted into energy.
Closed Systems and the Law of Conservation of Mass
Scientists can create closed systems to observe chemical reactions. During the reaction, the amount of mass within the closed system does not change, but it may take new forms.
Within a chemical reaction, two or more substances called reactants are transformed into new substances called products. For example, combining hydrogen gas and oxygen gas (reactants) forms water (product). If you start with 89 grams of oxygen and 11 grams of hydrogen, you will get 100 grams of water. The mass doesn’t change, but the substance does.
The law of conservation of mass states that the mass at the beginning of a reaction always equals the mass at the end of the reaction.

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Antoine Lavoisier and the Law of Conservation of Mass
The law of conservation of mass has been around since at least 1789 when Antoine Lavoisier wrote about it in his book, Elements of Chemistry. Lavoisier, and other scientists of the time, were transforming chemistry into a respected science.

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The invention of sealed vacuum jars allowed scientists like Lavoisier to perform experiments within closed systems. In one experiment, Lavoisier used a powerful hand lens to heat a diamond trapped inside a sealed vacuum jar. The heated diamond reacted with oxygen inside the jar and formed carbon dioxide. Lavoisier measured the mass of the jar to prove that while the diamond disappeared, the mass inside it remained the same.
Sometimes it looks like things disappear. For example, when you burn a candle, the candle gets smaller. However, the total mass of the system stays the same because smoke and gas form.
It can be difficult to see the conservation of mass in your daily life because we do not live in closed systems, but it still exists. When you see something disappear, a new gas is likely forming.
The Law of Conservation of Mass Digital Science Unit
If you want to really learn about the law of conservation of mass, you need to do more than read about it once. Your brain needs lots of opportunities to think about new information to learn it. My digital science unit on the law of conservation of mass teaches about it and includes review activities to help you understand and remember what you learn. You can check out the digital science unit at Teachers Pay Teachers. Click the image below!