conservation of mass in a reaction — HSC chemistry year 11, live

Conservation of mass in a reaction

In a chemical reaction atoms are rearranged into new substances, never created or destroyed, so the total mass of the products always equals the total mass of the reactants. When a reaction seems to lose mass on a balance, a gas has escaped from an open container; when it seems to gain mass, it has taken in a gas from the air. Sealing the reaction in a rigid closed container keeps every product on the pan, and collecting the gas in a separate run measures its volume, so the books balance. This is the real knowscape from knowhere, not a picture of one. Drag it. Watch what actually changes.

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Quantitative chemistry works like making toasties.

You can't count atoms, so this topic counts them sideways: mass, gas volume and concentration are all routes to moles. Reactions use moles in the balanced equation's whole-number ratio, and no atom is ever lost. Get to moles, follow the ratio, convert back.

You have 20 slices of bread and 8 of cheese. Each toastie takes two bread, one cheese.

Four toasties in, the ratio holds: every one uses exactly two bread to one cheese.

Cheese runs out at eight toasties. Four bread slices sit unused, and nothing went missing.

five things, not forty.

Atoms are only rearranged in a reaction, so the total mass of reactants equals the total mass of products: this is the law of conservation of mass.

Measure masses of solids and liquids on an electronic balance before and after; in a sealed flask every product stays on the pan and the reading does not change.

In an open container a gas product such as carbon dioxide escapes, so the balance reading falls by exactly the mass of gas lost.

Measure volumes of gases by collecting them in a gas syringe or an inverted measuring cylinder; at 25 °C and 100 kPa one mole of gas takes up 24.79 L.

Mass and gas volume tell the same story: 1.00 g of CaCO₃ gives 0.0100 mol of CO₂, which is 0.44 g lost from an open beaker or about 248 mL collected.

Burning magnesium in air seems to gain mass because oxygen atoms from the air join the product, not because new mass is made.

nothing here is a standalone fact.

knowhere maps every concept to what it rests on and what rests on it — 3 underneath this one, 1 built on top. Each one says why, in a sentence, not as an arrow on a diagram.

this conceptconservation of mass in a reactionMarble chips react with hydrochloric acid in a flask on a balance.

sits under itthe molebecause Linking the mass lost to the gas volume collected goes through moles.

sits under itmolar massbecause Turning 1.00 g of CaCO₃ into 0.44 g of CO₂ uses molar masses.

sits under itbalancing equations atom by atombecause Counting atoms on both sides of an equation is why the mass has to balance.

built on itsix reaction types and what they makebecause Predicting products relies on knowing atoms are only rearranged, never made or destroyed.

they don't read it. they run it.

Every HSC and VCE concept in knowhere is built like this one — mapped to your curriculum, never capped by it. Free for a week. No lock-in, cancel inside the week and pay nothing.

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Quantitative chemistry works like making toasties.

five things, not forty.

nothing here is a standalone fact.

3 more, same treatment.

they don't read it. they run it.