GCSE · Chemistry · Edexcel · Spec 1CH0

Conservation of mass in closed and open systems

Lift the lid on heated magnesium and the balance reading rises. Open a flask of calcium carbonate and acid and it falls. Is mass appearing and vanishing?

Chemistry · Mass on a balance

What does the balance reading do?

Pick a reaction, then choose a box: does the balance reading fall, rise or stay the same? When all four are sorted, switch the rule at the top and ask a bigger question.

Does the balance reading fall, rise or stay the same?

Still to sort

Reading falls (0)

Something has left the system.

Where the line is: A fall needs something to leave. If nothing can leave, the reading cannot fall.

Reading rises (0)

Something has entered the system.

Where the line is: A rise needs something to enter. It is the mirror image of a fall.

Reading stays the same (0)

Nothing has entered or left.

Where the line is: Nothing has been lost and nothing has been gained, so the balance still measures everything.

4 of 4 still to sort.

Four reactions, each sitting on a balance. Pick a reaction, then pick the box you think it belongs in. You'll see the reason every time.

Watch out: A changing reading is a clue about a gas entering or leaving. It is not evidence that mass has been created or destroyed.

Closed system or open system?

Closed systemvsOpen system

You need to be able to tell these apart before the readings make sense.

Focus

Can matter get in or out?

Closed system

No. Only energy can transfer between the system and its surroundings.

Open system

Yes. Substances can enter or leave the place where the reaction happens.

The insight

This is the difference that matters. If matter can cross the boundary, the balance may stop measuring the whole system.

What it looks like

Closed system

It has a plug or a lid.

Open system

It is open to the air.

Examples

Closed system

A flask with a plug or lid. A balloon fitted over a flask.

Open system

A burning match. A bonfire. Magnesium heated in a crucible with the lid lifted.

Same atoms, new partners

Tap each substance to see where its atoms came from, or where they went.

Tap a molecule to see what it does in the reaction.

Balanced, so the atoms add up

Tap each substance. The big number in front is a coefficient, and it multiplies every atom in the formula.

Tap a molecule to see what it does in the reaction.

Check your thinking

Why did the reading fall?

Calcium carbonate reacts with hydrochloric acid in an open flask on a balance. The reading goes down.

Which idea is closest to what you think right now?

The reading fell, so some of the mass was destroyed in the reaction.There are fewer products than reactants, so there's less mass at the end.The gas that formed doesn't have any mass, so it isn't part of the total.Carbon dioxide gas escaped. Gases have mass, so the total is the same, but not all of it is still on the balance.

How sure are you?

I'm sureI think soI'm guessing

The reading fell, so some of the mass was destroyed in the reaction.

This is a common misconception

It's an easy conclusion, but mass is conserved. The atoms are only rearranged, so a fall in the reading means something has left the system, not that mass has ceased to exist.

There are fewer products than reactants, so there's less mass at the end.

This is a common misconception

Counting substances doesn't tell you the mass. The number of products compared with the number of reactants is irrelevant to the total mass.

The gas that formed doesn't have any mass, so it isn't part of the total.

This is a common misconception

All gases have mass. The gas that formed is part of the total, which is why the total is unchanged.

Carbon dioxide gas escaped. Gases have mass, so the total is the same, but not all of it is still on the balance.

This matches the evidence

That's the one. The flask is open, so the gas left, and the balance only measures what's still on it. The total mass of reactants still equals the total mass of products.

A falling reading means something has left the system. It does not mean mass has disappeared.

Finding an unknown mass

Problem

3 g of hydrogen reacts with 24 g of oxygen to form water. What mass of water is produced?

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Follow the atoms, and when the balance disagrees, ask where the missing (or extra) atoms went.

What you need to know

Atoms are the building blocks of matter. In a reaction they are rearranged, never changed into a different type of atom.
Have a goYour friend insists that in one reaction a carbon atom turned into an oxygen atom. Are they right? Say why.
No. Atoms are rearranged, and no atom can change into a different type.
A reaction only rearranges the atoms that are already there, so one type of atom can never turn into another.
Reactants are the substances before a reaction, and products are the substances formed after it.
In a closed system, matter can't enter or leave and only energy can transfer. Think plug or lid.
An open system lets substances enter or leave: a burning match, a bonfire, magnesium heated with the lid lifted.
Mass is always conserved, open system or closed: the total mass of reactants equals the total mass of products.
If a gas product escapes from an open system, the balance reading falls, because all gases have mass.
Have a goFinish the sentence: if the balance reading falls in an open system, something has ____ the system.
left
The balance only weighs what is still on it, and an open system lets a gas leave. Mass is still conserved.
Gases can enter too: lifting the lid on heated magnesium lets oxygen in, so the reading rises.
With a closed system the reading stays the same, because nothing has been lost and nothing has been gained.
Balanced symbol equations have equal numbers of atoms of each element on both sides. Coefficients multiply every atom in a formula.
To find an unknown mass, use mass of reactants = mass of products and solve for the unknown.
Have a goIn a closed system, 10 g of one substance reacts with 15 g of another to make one product. What is the mass of the product?
25 g
Mass of reactants = mass of products, so 10 + 15 = 25. You add the masses; you don't compare the number of substances.

The big picture

Mass is never created or destroyed in a reaction, because the atoms are only rearranged. When a balance reading changes, something has entered or left the system, usually a gas going into or out of the air. You can also use the fact that the total mass of reactants equals the total mass of products to find an unknown mass.

Key points

1Atoms are only rearranged in a reaction, never changed into a different type of atom.

2Mass is conserved whether the system is open or closed: the total mass of reactants equals the total mass of products.

3A closed system has a plug or a lid, so the balance reading stays the same.

4In an open system a gas can escape (reading falls) or enter (reading rises), but mass is still conserved.

5Unknown mass: write the equation, put the masses underneath, turn the arrow into mass of reactants = mass of products, and solve.

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Worked example

Problem

In a closed system, 4 g of hydrogen reacts with some oxygen to form 36 g of water. What mass of oxygen reacted?

⚠ Watch out

Thinking a falling reading means mass was destroyed, or that fewer products means less mass. A gas has left, the number of products is irrelevant, and the total mass of reactants still equals the total mass of products.

🧠

Memory hook

Lift the lid and the magnesium gets heavier, because oxygen from the air has joined in. Different reading, same rule: follow the atoms.

✓

Check yourself

A friend says: 'The flask got lighter after the reaction, so mass was destroyed.' Write a two-sentence reply that uses the words open system, gas and conserved.

Flashcards

(14)

What are atoms, and what happens to them in a chemical reaction?

Atoms are the building blocks of all matter. In a reaction they are rearranged into the products, and no atom can be changed into a different type of atom.

Why can't a reaction make a particular product from just anything?

It is impossible to make a particular product without all the atoms that it needs.

What is the difference between a reactant and a product?

Reactants are the elements or compounds before a reaction takes place. Products are the elements or compounds formed after it.

What is a closed system?

An environment in which matter cannot enter or leave. Only energy can transfer. It has a plug or a lid.

What is an open system?

One in which substances can enter or leave where the reaction is taking place. Examples: a burning match, a bonfire, magnesium heated in a crucible with the lid opened.

What does conservation of mass say?

The total mass of the reactants equals the total mass of the products. It holds in open and closed systems.

A precipitation reaction runs in a closed flask on a balance. What does the reading do, and why?

It stays the same, because nothing has been lost and nothing has been gained.

Why does the reading fall when a gas product escapes from an open system?

The gas leaves, and the balance only measures what is still on it. All gases have mass, so mass is still conserved.

Calcium carbonate reacts with hydrochloric acid in an open flask. Does the number of products explain the fall in mass?

No. The number of products compared with the number of reactants is irrelevant to the total mass. The reading falls because carbon dioxide escapes.

Why does the reading rise when magnesium is heated with the lid lifted?

A gas can enter an open system. Oxygen from the surroundings gets in, so the measured mass increases while mass is still conserved.

Magnesium reacts with acid in a flask with a balloon fitted over it. What happens to the mass?

The hydrogen can't escape, so the start mass equals the end mass. That is evidence that mass is conserved.

How do you use conservation of mass to find an unknown mass?

Write the equation, put the masses below the substances, change the plus signs and arrow into mass of reactants = mass of products, then solve.

What is true about relative formula masses in a reaction?

The sum of the relative formula masses of the reactants equals the sum for the products, taking account of the balancing coefficients.

What is a coefficient, and what does a balanced symbol equation show?

A coefficient is the number in front of a formula; it multiplies all the atoms in that formula. A balanced equation has equal numbers of atoms of each element on both sides.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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What does the balance reading do?

Why did the reading fall?

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