Physical changes and chemical reactions

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Lesson notes

Physical Changes

In a **physical change**, the chemical composition of a substance does **not** change.
Physical changes are **reversible**.
Example: ice melting to water – both are H₂O.
All **state changes** (melting, freezing, boiling, condensing, subliming) are physical changes.
Another example: dissolving sugar in water – can be reversed by evaporation.
**Mass is conserved** in a physical change (e.g., 100 g ice → 100 g water).

Chemical Reactions

In a **chemical reaction**, the chemical composition of substances changes.
Example: iron rusting – iron reacts with oxygen to form iron oxide (colour change grey/silver → reddish/brown).
Chemical reactions are **not easily reversible**.
**Mass is conserved** in a chemical reaction: mass of reactants = mass of products.
Example: 70 g iron + 30 g oxygen → 100 g iron oxide.

Signs of a Chemical Reaction

**Colour change** (e.g., iron rusting).
**Temperature change** (usually rise).
**Gas given out** – may show as effervescence (bubbles).
**Light emitted**.
**Precipitation** – a solid (precipitate) forms.

Conservation of Mass

In both physical and chemical changes, mass is **conserved**.
The total mass before the change equals the total mass after.
For a chemical reaction: mass of **reactants** = mass of **products**.
Example: 50 g copper + 12 g oxygen → 62 g copper oxide.

Reversing Physical Changes

Physical changes can be reversed by changing conditions.
Melting/freezing: change temperature.
Dissolving: evaporate the solvent to recover the solute.
Example: heat salt solution until water evaporates, leaving salt behind.

Particle arrangement in solid, liquid and gas – all state changes are physical changes.

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Practice questions

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1.Which of the following is a sign that a chemical reaction has taken place?
Easy
- AA colour change
- BA change of state
- CDissolving
- DA change of shape
2.What is the name of the state change from gas to liquid?
Easy
- ACondensation
- BEvaporation
- CMelting
- DFreezing
3.Which of the following is a physical change?
Easy
- AIce melting to form water
- BIron rusting to form iron oxide
- CBurning wood
- DCooking an egg
4.A mass of 50 g of copper reacts with 12 g of oxygen to form copper oxide. What is the mass of copper oxide formed?
Medium
- A62 g
- B50 g
- C12 g
- D38 g
5.When 200 g of water evaporates, what is the mass of the water vapour formed?
Medium
- A200 g
- BLess than 200 g
- CMore than 200 g
- D0 g
6.Which of the following is NOT a sign of a chemical reaction?
Medium
- AA change of state
- BA temperature change
- CGas being given out
- DLight being emitted
7.What is meant by the conservation of mass?
Medium
- AThe mass of the reactants equals the mass of the products
- BMass is always lost during a reaction
- CMass is created during a chemical reaction
- DThe mass of the products is always greater than the reactants
8.Which of the following is a chemical reaction?
Hard
- ADissolving sugar in water
- BMelting ice
- CRusting of iron
- DBoiling water

More questions

9.A student observes bubbles forming when a solid is added to a liquid. Which conclusion is most likely correct?
Hard
- AA chemical reaction is occurring, producing a gas
- BThe solid is dissolving
- CThe liquid is boiling
- DThe solid is melting
10.Which statement about physical changes is correct?
Hard
- AThey are usually irreversible
- BThe chemical composition of the substance changes
- CMass is conserved
- DThey always involve a colour change
11.A 100 g sample of ice melts to form water. The water is then boiled to steam. What is the mass of the steam?
Hard
- A100 g
- BLess than 100 g
- CMore than 100 g
- D0 g
12.Which of the following is an example of a chemical reaction?
Easy
- AWater freezing to ice
- BSugar dissolving in tea
- CIron reacting with oxygen to form rust
- DCutting a piece of paper
13.A student adds a colourless liquid to a white solid. A gas is produced, and the mixture becomes hot. Which of the following is true?
Medium
- AOnly a physical change has occurred
- BA chemical reaction has occurred, as evidenced by gas production and temperature increase
- CThe gas production indicates a physical change only
- DThe temperature increase shows it is a physical change
14.When 100 g of ice melts, what is the mass of the water formed?
Medium
- A100 g
- BLess than 100 g
- CMore than 100 g
- D0 g
15.A student adds a colourless liquid to a white solid. The mixture becomes hot and a gas is produced. Which of the following is true?
Medium
- AA physical change has occurred because the solid dissolved.
- BA chemical reaction has occurred because there is a temperature change and gas production.
- CA physical change has occurred because the mixture became hot.
- DA chemical reaction has occurred because the solid was white.
16.A student measures the mass of a sealed flask containing 10 g of iron and 5 g of oxygen. After the iron rusts completely, the flask is weighed again. What is the total mass inside the flask?
Hard
- A10 g
- B5 g
- C15 g
- D20 g
17.A student places 50 g of ice in a beaker and leaves it on a bench. After one hour, the beaker contains 50 g of water. Which statement explains this observation?
Hard
- AThe ice gained mass from the air as it melted.
- BMass is conserved in a physical change.
- CThe water evaporated and then condensed back.
- DThe ice lost mass but the water gained mass.
18.A student dissolves 10 g of sugar in 100 g of water. The solution is then heated until all the water evaporates. What mass of sugar remains?
Medium
- A110 g
- B10 g
- C90 g
- D100 g
19.A student places a 100 g sample of ice in a beaker and melts it completely. The water is then boiled until it all becomes steam. What is the mass of the steam?
Hard
- A0 g
- B100 g
- C50 g
- D200 g
20.A student adds hydrochloric acid to a sample of marble chips (calcium carbonate). The mixture fizzes and a gas is produced. Which of the following is the best evidence that a chemical reaction has occurred?
Medium
- AA gas is produced
- BThe marble chips get smaller
- CThe mixture becomes warm
- DThe marble chips are being used up
21.A student heats 50 g of hydrated copper(II) sulfate (blue crystals) in a test tube. The crystals turn white and water vapour condenses on the cooler part of the test tube. After cooling, the white solid is reweighed. Which mass is most likely?
Hard
- A50 g
- BLess than 50 g
- CMore than 50 g
- D0 g
22.A student places 10 g of iron filings in a sealed flask with 5 g of oxygen. The iron rusts completely. Which statement about the mass of the flask and its contents after the reaction is correct?
Medium
- AThe mass is less than 15 g because gas has escaped
- BThe mass is exactly 15 g because mass is conserved
- CThe mass is more than 15 g because iron oxide is heavier
- DThe mass is less than 15 g because some iron is used up
23.Which of the following is a sign of a chemical reaction?
Easy
- AMelting
- BEvaporation
- CA colour change
- DDissolving
24.A teacher demonstrates the reaction between iron and sulfur. A mixture of iron filings and sulfur powder is heated strongly. A black solid forms. The black solid does not stick to a magnet, but the original iron filings did. Which conclusion is correct?
Hard
- AThe black solid is a mixture of iron and sulfur
- BA chemical reaction has occurred because the properties changed
- CThe iron has been removed by the magnet
- DThe sulfur has evaporated
25.A student dissolves 20 g of salt in 100 g of water. The solution is then heated until all the water evaporates. What mass of salt remains?
Medium
- A0 g
- B20 g
- C100 g
- D120 g
26.A student measures the mass of a piece of magnesium ribbon before burning it in air. The ash (magnesium oxide) is collected and weighed. Which mass is expected?
Medium
- ALess than the original magnesium
- BThe same as the original magnesium
- CMore than the original magnesium
- DZero, because it burns away
27.A student adds a colourless solution of hydrogen peroxide to a black powder (manganese dioxide). Bubbles of gas are produced and the test tube feels warm. Which statement is correct?
Hard
- AThe black powder is used up in the reaction
- BThe gas produced is oxygen
- CThe temperature decrease shows an endothermic reaction
- DThe bubbles are due to boiling
28.A student carries out an experiment to show conservation of mass. They place 25 g of sodium hydrogencarbonate in a flask and add 50 g of dilute ethanoic acid. The flask is connected to a gas syringe. After the reaction, the flask contains a colourless solution and the gas syringe contains 12 g of carbon dioxide. What mass of solution is in the flask?
Hard
- A63 g
- B75 g
- C87 g
- D25 g
29.A student wants to separate a mixture of sand and salt. Which sequence of steps is correct?
Medium
- AAdd water, filter, evaporate
- BAdd water, evaporate, filter
- CEvaporate, add water, filter
- DFilter, add water, evaporate
30.A student dissolves 15 g of salt in 85 g of water. The solution is then heated until all the water evaporates. What mass of salt remains?
Medium
- A15 g
- B85 g
- C100 g
- D70 g
31.A student measures the mass of a sealed flask containing 10 g of iron wool and 5 g of oxygen. The iron rusts completely. Which statement about the mass of the flask and its contents after the reaction is correct?
Hard
- AThe mass is 15 g.
- BThe mass is less than 15 g.
- CThe mass is more than 15 g.
- DThe mass cannot be determined without knowing the mass of iron oxide formed.
32.A 50 g sample of ice is left in a beaker at room temperature. After melting, the water is boiled until it all becomes steam. What is the mass of the steam?
Medium
- A50 g
- BLess than 50 g because some water evaporates during boiling
- CMore than 50 g because steam is less dense
- D0 g because the water disappears
33.A student adds 25 cm³ of hydrochloric acid to 25 cm³ of sodium hydroxide solution. The mixture is stirred and the temperature rises from 20°C to 32°C. A colourless solution remains. Which statement is correct?
Hard
- AA chemical reaction has occurred because there is a temperature change.
- BA chemical reaction has occurred because the volume is conserved.
- CA physical change has occurred because the solution is still colourless.
- DA physical change has occurred because no gas is produced.
34.A 100 g sample of copper(II) sulfate crystals (blue) is heated strongly. The crystals turn white and 36 g of water vapour is given off. What is the mass of the white solid remaining?
Medium
- A64 g
- B100 g
- C136 g
- D36 g
35.A student dissolves 20 g of sugar in 100 g of water. The solution is then heated until all the water evaporates. What mass of sugar remains?
Medium
- A20 g
- B100 g
- C120 g
- D80 g
36.A student places 10 g of iron filings and 5 g of sulfur powder in a test tube and heats the mixture. A black solid forms. The test tube is weighed before and after the reaction. Which mass is expected?
Hard
- A15 g
- BMore than 15 g because the reaction absorbs air
- CLess than 15 g because gas escapes
- DCannot be predicted
37.A student heats a blue solution of copper(II) sulfate until all the water evaporates, leaving white copper(II) sulfate powder. This change is reversed by adding water to the white powder. Which statement is correct?
Medium
- AThe change is physical because it is reversible and no new substances are formed.
- BThe change is chemical because a colour change occurred.
- CThe change is physical because the mass decreased.
- DThe change is chemical because heat was applied.
38.A student adds a colourless solution of hydrogen peroxide to a black powder (manganese dioxide). Bubbles of gas are produced and the test tube feels warm. Which of the following is the most likely conclusion?
Medium
- AA chemical reaction has taken place because a gas is produced and the temperature changes.
- BA physical change has taken place because the black powder does not dissolve.
- CA physical change has taken place because the mixture is still colourless.
- DA chemical reaction has taken place because the black powder acts as a catalyst.
39.A student heats a blue solution of copper(II) sulfate until all the water evaporates, leaving white copper(II) sulfate powder. This change is reversed by adding water to the white powder. Which statement about this process is correct?
Medium
- AThe evaporation is a physical change and the reversal is a chemical reaction.
- BBoth the evaporation and the reversal are physical changes.
- CBoth the evaporation and the reversal are chemical reactions.
- DThe evaporation is a chemical reaction and the reversal is a physical change.
40.A student places 10 g of iron filings and 5 g of sulfur powder in a test tube and heats the mixture. A black solid forms. The test tube is weighed before and after heating. Which mass is expected after heating?
Hard
- A5 g
- B10 g
- C15 g
- DMore than 15 g

Revision notes

## Physical Changes vs Chemical Reactions ### Physical Changes - **No change in chemical composition** – the substance stays the same (e.g., ice and water are both H₂O). - **Reversible** – e.g., melting/freezing, dissolving/evaporating. - **Mass is conserved** – 100 g of ice melts to 100 g of water. - **Examples**: state changes (melting, freezing, boiling, condensing, sublimation), dissolving sugar in water. ### Chemical Reactions - **Chemical composition changes** – new substances are formed (e.g., iron + oxygen → iron oxide). - **Not easily reversible**. - **Signs of a chemical reaction**: - Colour change - Temperature change - Gas given off (effervescence) - Light emitted - Precipitation (solid formed) - **Mass is conserved**: mass of reactants = mass of products (e.g., 70 g iron + 30 g oxygen → 100 g iron oxide). ### Key Points - Physical changes: same substance, reversible, mass conserved. - Chemical reactions: new substances, not easily reversible, mass conserved. - Conservation of mass: total mass before = total mass after.

Physical change: A change where the chemical composition of the substance does not change; it is reversible.
Chemical reaction: A process where the chemical composition of substances changes, forming new products; not easily reversible.
Conservation of mass: The principle that the mass of the reactants equals the mass of the products in a chemical reaction.
Effervescence: Bubbles in a liquid caused by gas being given out during a chemical reaction.
Precipitate: A solid formed in a chemical reaction.

Flashcards

What is a physical change?

A physical change does not change the chemical composition of a substance and is reversible.

Is ice melting to water a physical change? Why?

Yes, because both ice and water have the same chemical formula H2O, and the change is reversible.

What are state changes?

State changes are physical changes between solid, liquid, and gas (e.g., melting, freezing, boiling, condensing, sublimation).

Give an example of a physical change besides state changes.

Dissolving sugar in water is a physical change; it can be reversed by evaporating the water.

Is mass conserved in a physical change?

Yes. For example, if 100 g of ice melts, it forms 100 g of water.

What is a chemical reaction?

A chemical reaction changes the chemical composition of substances and is not easily reversible.

Give an example of a chemical reaction.

Rusting of iron: iron reacts with oxygen to form iron oxide.

What are five signs of a chemical reaction?

1. Colour change. 2. Temperature change. 3. Gas given out (effervescence). 4. Light emitted. 5. Precipitation (solid formed).

What colour change occurs when iron rusts?

Iron changes from grey/silver to reddish/brown.

Is a chemical reaction easily reversible?

No, because the chemical composition changes, it cannot be easily reversed.

What is the conservation of mass?

In a chemical reaction, the total mass of reactants equals the total mass of products.

If 70 g of iron reacts with 30 g of oxygen, what mass of iron oxide is formed?

100 g of iron oxide (conservation of mass).

True or false: A chemical reaction is easily reversible.

False.

True or false: When 200 g of water evaporates, the mass of water vapour is less than 200 g.

False. Mass is conserved; the water vapour mass is still 200 g.

True or false: A change in colour is a sign of a physical change.

False. A colour change is a sign of a chemical reaction.

True or false: Gas being given out is a sign of a chemical reaction.

True.

Describe what is meant by the conservation of mass.

In any chemical reaction, the total mass of reactants equals the total mass of products.

50 g of copper reacts with 12 g of oxygen to form copper oxide. What is the mass of copper oxide formed?

62 g (50 g + 12 g).

In the copper-oxygen reaction, the colour changes from reddish/brown to black. Why is this a chemical reaction?

Because there is a colour change and the composition changes (copper and oxygen form copper oxide).

List three other signs of a chemical reaction besides colour change.

Temperature change, gas given out (effervescence), light emitted, or precipitation.

What is the name of the state change from gas to liquid?

Condensation.

Is condensation a physical change or a chemical reaction?

Physical change.

Explain why condensation is a physical change.

The chemical composition does not change; it is reversible (the liquid can be evaporated back to gas).

How can you reverse dissolving table salt in water to get salt back?

Heat the solution until all the water evaporates, leaving the salt behind.

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