Physical and Chemical Changes Virtual Lab: Evidence and Mass

This physical and chemical changes virtual lab combines the classic "is it a new substance?" practical with the sealed-flask demonstration of conservation of mass. Students run seven changes on a balance, from melting ice to burning steel wool and baking soda with vinegar. They weigh before and after, watch the thermometer and a particle view, tick the evidence they saw and classify each change. Then they repeat the changes that gained or lost mass in a sealed flask. Every value below was read from the simulation.

Curriculum links

• NGSS MS-PS1-2: analyze data on the properties of substances before and after they interact to decide whether a chemical reaction occurred. MS-PS1-5: atoms are rearranged, so mass is conserved.
• AQA GCSE Chemistry (8462), 4.3.1.1 and 4.3.1.3: conservation of mass, and mass changes when a reactant or product is a gas.
• Key Stage 3 science (England) and Cambridge IGCSE Chemistry (0620), 6.1: the difference between physical and chemical changes.

Simulic is not affiliated with or endorsed by AQA or Cambridge.

Before the lab (5 min)

Ask students to commit to a prediction, on paper or as question 1 of the class link:

"Baking soda is tipped into vinegar in an open beaker standing on a balance. What happens to the balance reading by the end?"

Expect plenty of "it stays the same, because mass is conserved".

Method in the simulation

1. The page opens on Baking soda + vinegar and Open to the air. Press Weigh before, then Start, and watch the balance, thermometer and particle view. Press Weigh after.
2. Tick the evidence you saw, press Check evidence, choose Physical change or Chemical change, and press Record result.
3. Press Test properties and Try to reverse for more evidence.
4. Click Sealed (closed system) and repeat.
5. Run the other six changes in the open container, then repeat Burning steel wool and Boiling water sealed.

Change	Container	m before (g)	m after (g)	Δm (g)	Evidence	Physical or chemical?
Baking soda + vinegar	open
Baking soda + vinegar	sealed

Expected results

The simulation has no random error, so every student gets these values. In the sealed flask, Δm was 0.00 g for all seven changes.

Change (open)	Before → after (g)	Δm (g)	T (°C)	Type
Melting ice	81.84 → 81.84	0.00	−5.0 → 8.0	physical
Dissolving salt	118.36 → 118.36	0.00	20.0 → 18.6	physical
Boiling water	111.84 → 103.84	−8.00	20.0 → 100.0	physical
Burning steel wool	63.84 → 64.26	+0.42	about 750 at peak	chemical
Rusting iron (7 days)	68.84 → 68.93	+0.09	20.0 → 20.0	chemical
Baking soda + vinegar	115.36 → 114.31	−1.05	20.0 → 16.6	chemical
Making a precipitate	141.99 → 141.99	0.00	20.0 → 23.3	chemical

• Open container: mass fell when a gas left and rose when oxygen joined a solid. The precipitate is chemical, yet its mass did not change.
• Sealed flask: baking soda and vinegar read 200.88 g before and after; steel wool read 149.36 g both times.
• Try to reverse gave back ice, salt and water for the physical changes, and nothing for the chemical ones.
• Check evidence accepts gas, energy change, new substance and mass change for baking soda and vinegar, but only energy change and "easily reversed" for dissolving salt.

Questions for students

1. (Prediction, asked again after the lab) What happens to the balance reading when baking soda reacts with vinegar in an open beaker?
2. What is the independent variable when you compare the open beaker with the sealed flask?
3. What is Δm for burning steel wool in the open?
4. Which conclusion do the open-container results support?
5. Why is boiling water a physical change but baking soda with vinegar a chemical change?

Answers for teachers: (1) It falls, from 115.36 g to 114.31 g, because carbon dioxide escapes. (2) Whether the container is open or sealed. (3) +0.42 g (accept 0.41–0.43). (4) An open-container mass change shows that a gas left or joined, not that the change is chemical. (5) Steam is still water and condenses back; carbon dioxide and sodium ethanoate are new substances that cannot be turned back.

Common misconceptions

• "Burning makes things lighter." Steel wool gained 0.42 g of oxygen from the air.
• "Bubbles mean a chemical change." Boiling water bubbles too, but the bubbles are steam.
• "A temperature change proves a reaction." Dissolving salt cooled the water to 18.6 °C, and it is a physical change.

Extension

• Run baking soda and vinegar in the sealed flask and count the atoms of each element in the particle view before and after (MS-PS1-5). In the open beaker the carbon dioxide drifts out of view, just as it leaves the balance.
• Compare burning steel wool (+0.42 g in one minute) with rusting (+0.09 g in seven days): both are iron combining with oxygen, at very different rates.

FAQ

Do I need to change the class link's starting values?

No. The defaults, Change Baking soda (NaHCO₃) with vinegar and Container Open to the air (gases can enter and leave), match question 1. Leave Show the answers as soon as the change ends (teacher demo) unticked so students tick the evidence themselves.

Why does the sealed flask read so much more than the open beaker?

The balance weighs the container too, and the stoppered flask is heavier. Compare Δm.

Will every student get the same numbers?

Yes. There is no random error, and the balance reads to 0.01 g.

Related simulations and guides

Reactants, products and leftovers – sandwiches and molecules States of matter – particle model on heating and cooling

For changes of state, see the states of matter lesson plan. For counting atoms in equations, see the balancing chemical equations lesson plan.