Physical and chemical changes

Apprends en jouant

Réponds à ces questions pour gagner de l'énergie, puis pêche et explore. Sans compte.

Pour les enseignants : diapos de leçon, notes de révision prêts à l'emploi pour Physical and chemical changes (MYP Chemistry, Year 1) — utilise-les en cours, ou lance le thème en activité de classe interactive que tes apprenants jouent en direct.

Notes de leçon

Big idea: how matter changes

Big idea (key concept): Change. Everything around us is always changing: ice melts, bread toasts, iron goes brown. Scientists sort these changes into two kinds so they can explain and predict them.
Related concept: Transformation. In some changes one substance is transformed into a completely different one. In others the substance stays the same and only its form changes.
Global context: Scientific and technical innovation. Chemists use reactions to make fuels, metals, medicines and materials. To do that they need to know exactly when a new substance has been made.
A physical change changes the shape, size or state of a substance, but no new substance is made. Folding, tearing, melting and dissolving are physical changes.
A chemical change (also called a chemical reaction) makes one or more new substances with different properties. Burning, rusting and cooking an egg are chemical changes.
Look at the paper: folding, tearing and crumpling only change its shape, but burning turns it into ash, smoke and gases. Those are new substances, so only burning is a chemical change.

Paper folded, torn, crumpled and burned

Physical changes

Common physical changes are melting, freezing, boiling, evaporating, condensing, dissolving, cutting, bending and mixing.
In a physical change the particles stay the same. Ice, liquid water and steam are all made of the same H₂O particles. Only the way the particles are arranged and how they move has changed.
Many physical changes can be reversed. Freeze water and you get ice again. Evaporate salty water and the salt is left behind.
Dissolving is a physical change. The sugar seems to disappear in your tea, but it is still there: the tea tastes sweet, and if you evaporate the water the sugar comes back.
Energy is still involved. Melting takes in energy from the surroundings. Freezing gives energy out. This does not make a new substance, so it is still physical.
The mass of the substance does not change in a physical change. Ice that melts weighs exactly the same as the water it makes.

Changes of state between solid, liquid and gas

Chemical changes and how to spot them

In a chemical reaction the reactants (the substances you start with) are changed into products (the new substances). The atoms are joined up in a different way, so the products have different properties from the reactants.
Chemical changes are usually hard to reverse. You cannot turn ash and smoke back into paper.
Clues that a reaction may have happened: gas bubbles form, there is a colour change, a solid appears when two liquids are mixed (a precipitate), the temperature changes without anyone heating or cooling it, or there is light or a flame.
A clue is not proof. Bubbles also appear when water boils, but that is just water changing into steam, which is a physical change. The strongest evidence is a new substance with different properties.
Test for carbon dioxide: a teacher bubbles the gas through limewater, which is clear. If the gas is carbon dioxide the limewater turns milky. No other common gas does this.
Stay safe: school reactions are teacher-run or follow school rules. Wear eye protection, never taste or sniff chemicals unless told to, and never mix cleaning products at home.

Carbon dioxide turns limewater milky

Word equations and conservation of mass

A word equation shows a reaction in words: reactants → products. The arrow means 'react to make'. For example: carbon + oxygen → carbon dioxide.
More examples: magnesium + oxygen → magnesium oxide. Methane + oxygen → carbon dioxide + water.
In a reaction atoms are rearranged. They are not made and they are not destroyed. So the total mass of the products always equals the total mass of the reactants. This is conservation of mass.
Example: 2.4 g of magnesium joins with 1.6 g of oxygen to make 4.0 g of magnesium oxide. Check: 2.4 + 1.6 = 4.0.
Sometimes mass seems to change. If a gas escapes from an open container, the container gets lighter. If a gas from the air joins in, for example when iron wool burns, the solid gets heavier.
In a sealed container nothing can get in or out, so the mass stays the same. That is how scientists show that the mass really is conserved.

A carbon atom and an oxygen molecule make carbon dioxide

Burning, rusting and thermal decomposition

Combustion (burning) is a fuel reacting with oxygen and giving out energy as heat and light. Methane + oxygen → carbon dioxide + water. A fire needs three things: fuel, oxygen and heat. Take one away and it goes out.
Rusting is iron reacting slowly with oxygen and water: iron + oxygen + water → rust. It needs both. Salt in water makes it faster.
To stop rusting, keep oxygen and water away from the iron with paint, oil, plastic or a coating of another metal such as zinc.
Thermal decomposition is when heat breaks one compound into simpler substances. Calcium carbonate → calcium oxide + carbon dioxide. Green copper carbonate → black copper oxide + carbon dioxide. Teachers run these tests with safety equipment.
An exothermic reaction gives energy out to the surroundings, so the surroundings get warmer. Burning and hand warmers are examples.
An endothermic reaction takes energy in from the surroundings, so they get colder. Thermal decomposition needs a constant supply of heat, so it is endothermic.

Nails in air and water, water without air, and air without water

Think like a scientist: what makes iron rust faster?

Question: does salt in the water make iron nails rust faster? A teacher sets up nails in tap water and in salty water and leaves them for a few days.
The independent variable is the one you change on purpose: the type of water (tap water or salty water).
The dependent variable is the one you measure: how much rust has formed, for example the percentage of the nail covered in rust after 5 days.
For a fair test keep everything else the same: the size and type of nail, the volume of water, the temperature, the container and the number of days.
Repeat with several nails in each liquid and take an average. One nail might be different from the rest.
Inquiry task: plan an investigation to find out which of three coatings (paint, oil, nothing) protects nails best. State what you change, what you measure, three variables you will control, and how you will judge whether your results are reliable.

Diapos

Step through every slide for this topic — plus flashcards and revision notes — with a free account.

S'INSCRIRE — GRATUIT CONNEXION

Questions d'entraînement

Aperçu gratuit — 8 sur 51 questions. Inscris-toi pour toutes les voir.

1.Which of these is a physical change?
Easy
- AWood burning in a fire
- BIce melting into water
- CIron going rusty
- DMilk turning sour
2.What always happens in a chemical change?
Easy
- AOne or more new substances are made
- BThe substance changes shape
- CThe substance gets hotter
- DThe substance becomes a liquid
3.When ice melts, what happens to the particles of water?
Easy
- AThey turn into different particles
- BThey are destroyed
- CThey turn into oxygen and hydrogen gas
- DThey stay the same but move differently
4.Melting ice is a chemical change.
Easy
True or false?
5.Burning a candle makes new substances.
Easy
True or false?
6.Which of these is a clue that a chemical reaction may be happening?
Easy
- AA solid is cut into smaller pieces
- BA liquid is poured into a different beaker
- CGas bubbles form when two liquids are mixed
- DSugar is stirred into water until it disappears
7.Which gas turns limewater milky?
Easy
- AOxygen
- BCarbon dioxide
- CHydrogen
- DNitrogen
8.In the word equation magnesium + oxygen → magnesium oxide, which substance is a product?
Easy
- AMagnesium oxide
- BMagnesium
- COxygen
- DBoth magnesium and oxygen

Plus de questions

9.What do we call the substances you start with in a chemical reaction?
Easy
- AProducts
- BPrecipitates
- CSolvents
- DReactants
10.Complete the sentence.
Easy
The new substances made in a chemical reaction are called ____.
11.What is the scientific word for burning a fuel in oxygen?
Easy
- ACondensation
- BCorrosion
- CCombustion
- DEvaporation
12.Which two things must be present for iron to rust?
Easy
- ACarbon dioxide and heat
- BOxygen and water
- CNitrogen and light
- DHydrogen and salt
13.Rust is a new substance that is made when iron reacts with oxygen and water.
Easy
True or false?
14.What is thermal decomposition?
Easy
- AHeat breaking a substance into simpler substances
- BA substance dissolving in hot water
- CA gas cooling into a liquid
- DTwo substances joining to make one
15.What do we call a reaction that gives out energy to its surroundings?
Easy
- AEndothermic
- BReversible
- CNeutral
- DExothermic
16.Match each word to its meaning.
Easy
- Reactant
- Product
- Combustion
- Thermal decomposition
- Burning a fuel in oxygen
- Heat breaking a compound down
- A substance you start with
- A substance that is made
17.Put these in the order they form when ice is heated and then keeps being heated.
Easy
- Liquid water
- Ice
- Steam
18.In a chemical reaction in a sealed container, how does the total mass of the products compare with the mass of the reactants?
Easy
- AIt is always bigger
- BIt is always smaller
- CIt is equal
- DIt is zero
19.Dissolving sugar in water makes a new substance.
Easy
True or false?
20.Why is dissolving salt in water a physical change?
Medium
- AThe salt turns into water
- BThe salt can be got back by evaporating the water
- CThe water turns into salt
- DThe salt is destroyed by the water
21.Two clear liquids are mixed and a white solid appears. What is the solid called?
Medium
- AA precipitate
- BA solvent
- CA reactant
- DA filtrate
22.Which of these could be signs of a chemical reaction? (select all that apply)
Medium
- AIce cubes get smaller on a warm day
- BA gas is given off
- CA piece of paper is cut in half
- DA permanent colour change
- EThe temperature rises with no heating
23.Bubbles appear when water is heated to 100 °C. Why is this not a sign of a chemical reaction?
Medium
- AThe bubbles are oxygen from the water
- BHeating never causes reactions
- CWater cannot make bubbles
- DThe bubbles are water turning into steam
24.Every change that gives out energy is a chemical reaction.
Medium
True or false?
25.What is the missing product? carbon + oxygen → ?
Medium
- ACarbon monoxide gas only
- BCarbonic water
- CCarbon dioxide
- DOxygen carbide
26.Methane burns in plenty of oxygen. Which pair of substances are the products?
Medium
- ACarbon and hydrogen
- BCarbon dioxide and water
- CMethane and oxygen
- DOxygen and water
27.A fire needs fuel, oxygen and one more thing. What is it?
Medium
- AHeat
- BWater
- CWind
- DCarbon dioxide
28.A teacher sets up four nails. Which nail would rust fastest?
Medium
- AA nail in a sealed tube with a drying agent
- BA nail in boiled water under a layer of oil
- CA nail covered in a thick layer of grease
- DA nail in tap water in a tube open to the air
29.Why does painting an iron gate stop it rusting?
Medium
- AThe paint removes the iron from the gate
- BThe paint makes the iron colder
- CThe paint stops oxygen and water reaching the iron
- DThe paint turns rust back into iron
30.Green copper carbonate is heated by a teacher and turns black. What is the best explanation?
Medium
- AIt melted and then froze again
- BIt broke down into a new black substance and a gas
- CIt dissolved in the air
- DIt only changed colour because it got hot
31.Heating calcium carbonate strongly makes calcium oxide and carbon dioxide.
Medium
True or false?
32.A hand warmer gets hot after it is opened. What type of reaction is happening?
Medium
- AExothermic
- BEndothermic
- CA physical change
- DThermal decomposition
33.Two solids are mixed in a cup and the thermometer falls from 21 °C to 15 °C. What does this show?
Medium
- AThe reaction is exothermic
- BEnergy was destroyed
- CNo reaction happened
- DThe reaction is endothermic and takes in energy
34.Match each everyday change to its name.
Medium
- A fuel burning in oxygen
- Iron reacting with oxygen and water
- Heating calcium carbonate
- Ice turning to water
- Thermal decomposition
- Melting (a physical change)
- Combustion
- Rusting
35.Put the steps of a sealed-flask mass investigation in order.
Medium
- Compare the two masses
- Weigh the sealed flask again
- Let the reactants mix inside the sealed flask
- Weigh the sealed flask with the reactants
36.A sealed flask holds a reacting mixture. It has a mass of 212.4 g before and 212.4 g after. Why?
Medium
- AThe reaction did not happen
- BMass is destroyed then remade
- CNo atoms can escape, and atoms are not destroyed
- DGas has no mass
37.An open flask has a mass of 80.0 g before a reaction and 78.8 g after. How much mass escaped as gas?
Medium
38.Which of these are physical changes? (select all that apply)
Medium
- AA match burning
- BWater boiling
- CAn egg frying
- DChocolate melting
- ESugar dissolving in water
39.2.4 g of magnesium burns completely with 1.6 g of oxygen. What mass of magnesium oxide is made?
Hard
40.10.0 g of calcium carbonate is heated strongly and decomposes completely into 5.6 g of calcium oxide and carbon dioxide gas. What mass of carbon dioxide escapes?
Hard
41.5.6 g of iron wool is burned in air by a teacher and the solid left weighs 8.0 g. Why is it heavier?
Hard
- AMass was created by the heat
- BOxygen from the air joined with the iron
- CIron turns into a heavier atom when heated
- DThe balance was wrong
42.
A teacher sets up three nails. Results after a week:
Tube Conditions Rust
A Air and water Lots
B Boiled water under oil, no air None
C Air and a drying agent, no water None
Which conclusion is best supported by the results?
Hard
- ARusting needs both air and water
- BRusting needs water but not air
- CRusting needs air but not water
- DRusting does not need either
43.A student claims: 'Bubbles prove that a chemical reaction has happened.' Which statements show the claim is too strong? (select all that apply)
Hard
- AA new gas is never a sign of a reaction
- BBoiling water makes bubbles of steam
- CChemical reactions never produce bubbles
- DDissolved air can come out of tap water as bubbles when it warms
- EA test such as limewater is needed to identify the gas
44.A student burns a candle on a balance and sees the mass fall. She says 'mass is not conserved'. What is the best response?
Hard
- AShe is right, the mass was destroyed
- BCandles do not react
- CThe balance must have been broken
- DThe wax became gases that escaped into the air
45.When a candle burns, the products weigh less than the wax and oxygen that reacted.
Hard
True or false?
46.Explain why a sealed flask and an open flask can give different mass readings for the same reaction.
Hard
47.Explain why burning paper is a chemical change but tearing paper is a physical change.
Hard
48.Plan a fair test to find out whether salt makes iron nails rust faster. Say what you change, what you measure and two things you keep the same.
Hard
49.Complete the sentence.
Medium
In a chemical reaction atoms are not made or destroyed, they are ____.
50.Complete the sentence.
Medium
Carbon dioxide turns clear limewater ____.
51.Complete the sentence.
Medium
A reaction that takes in energy and makes its surroundings colder is called ____.

Notes de révision

## The big idea - Matter changes in two ways: **physical** (no new substance) and **chemical** (new substances) (**Change**, **Transformation**). ## Physical changes - Melting, freezing, boiling, condensing, dissolving, cutting. The particles stay the same. - Often reversible. Mass does not change. ## Chemical changes - **Reactants** → **products**. Clues: gas, colour change, precipitate, temperature change, light. - Carbon dioxide turns **limewater** milky. - Atoms are rearranged, so mass is **conserved** (2.4 g + 1.6 g → 4.0 g). ## Examples - **Combustion**: fuel + oxygen. **Rusting**: iron + oxygen + water. **Thermal decomposition**: heat breaks a compound down. - **Exothermic** gives energy out. **Endothermic** takes energy in. ## Investigating - Change one variable, measure one, control the rest, and repeat.

Physical change: A change of shape, size or state in which no new substance is made.
Chemical change: A change in which one or more new substances are made.
Chemical reaction: Another name for a chemical change.
Reactant: A substance you start with in a chemical reaction.
Product: A substance that is made in a chemical reaction.
Precipitate: A solid that forms when two solutions are mixed.
Limewater: A clear solution that turns milky when carbon dioxide is bubbled through it.
Word equation: A reaction written in words, with reactants on the left and products on the right.
Conservation of mass: The total mass of the reactants equals the total mass of the products.
Combustion: Burning a fuel in oxygen, giving out heat and light.
Rusting: Iron slowly reacting with oxygen and water to make rust.
Thermal decomposition: Heat breaking a compound down into simpler substances.
Exothermic: A change or reaction that gives out energy to the surroundings.
Endothermic: A change or reaction that takes in energy from the surroundings.

Cartes mémo

What is a physical change?

A change of shape, size or state in which no new substance is made.

What is a chemical change?

A change in which one or more new substances are made.

Give three physical changes.

Melting, freezing, boiling, dissolving, cutting (any three).

Give three clues that a chemical reaction may have happened.

Gas bubbles, a colour change, a precipitate, a temperature change, light (any three).

Why are bubbles in boiling water not a sign of a reaction?

They are steam, which is still water. No new substance is made.

What is the test for carbon dioxide?

It turns clear limewater milky.

What are reactants and products?

Reactants are the substances you start with. Products are the substances that are made.

Write the word equation for burning carbon.

Carbon + oxygen → carbon dioxide.

What does conservation of mass mean?

The total mass of the products equals the total mass of the reactants.

Why can an open flask get lighter during a reaction?

A gas product escapes into the air.

Why does burning iron wool make a heavier solid?

Oxygen from the air joins with the iron.

What is combustion?

Burning a fuel in oxygen, giving out heat and light.

What three things does a fire need?

Fuel, oxygen and heat.

What does iron need to rust?

Both oxygen and water.

Name two ways to stop iron rusting.

Paint, oil, plastic or a zinc coating (any two).

What is thermal decomposition? Give an example.

Heat breaking a compound into simpler substances, for example calcium carbonate into calcium oxide and carbon dioxide.

What is the difference between exothermic and endothermic?

Exothermic gives energy out and the surroundings get warmer. Endothermic takes energy in and the surroundings get colder.

Unlock all 51 questions, flashcards & more

Crée un compte gratuit pour voir toutes les questions, les diapos, les cartes mémo et les notes de révision de ce thème.

S'INSCRIRE — GRATUIT CONNEXION

Annales

Les annales d'entraînement pour ce thème arrivent bientôt.

Bientôt disponible

Tube	Conditions	Rust
A	Air and water	Lots
B	Boiled water under oil, no air	None
C	Air and a drying agent, no water	None