CHEMICAL REACTIONS Physical and chemical changes

ESSENTIAL QUESTIONS

How are new substances made? How can you tell a new substance has been formed?

I. Both living and non-living things are made up of matter (atoms).

II.Matter around us undergoes changes. These changes can be classified into two types

- physical changes - chemical changes

“Those who matter don’t mind…”

Chemistry – study of matter.

1. Physical changes in a substance affect its physical properties, such as

Physical changes

• Shape

• Volume

• Density

• State of matter (solid, liquid, gas)

• Colour

• Mass

1. No new substance is formed during a physical change. No bonds between atoms are broken and no new bonds formed.

2. Physical changes are usually reversible. This means that the physical properties of the substance which has undergone a physical change can be changed back.

Physical changes

Water ice water

3. ExamplesPhysical changes

a) Changes of physical state

• Caused by heating

Steam water ice

Ice water steam

• Caused by cooling

• No new substance formed because H2O remains

as H2O

Physical changes b) Dissolving a solid in water

Sugar + water aqueous sugar

• Reverse changes

Aqueous sugar sugar + steam evaporated

dissolved

Aqueous sugar sugar + water

distillation

Describe the behaviour of molecules in physical changes. Move from one station to the next and record your observations.

Exploding can- demo Inflating balloon Deflating balloon Dry ice Classify each according to the type of

physical change

You do -Task

Experiment

Exploding can demo

Inflating balloon

Dry Ice Heating salt solution

Change of shape, state, colour, size?Molecule gain or lose energy

What happens?

1. Changes of matter that produce new substances. Bonds between atoms are broken or new bonds are formed.

2. The new substances have chemical properties which are different from the original substances.

3. Chemical changes are usually difficult to reverse. This means that the product formed cannot be easily changed back into the reactant.

Chemical Changes

4. Example

Chemical Changes

a) Burning of paper

Paper ash + carbon dioxide Burn

but

Ash + carbon dioxide Cannot be

paper

A chemical change has definitely occurred if there is any of these: –

A permanent colour change A gas is given off A change in temperature A precipitate (solid) forms when two liquids

are mixed Light emitted

Signs of Chemical change

Chemical Changesb) Browning of a peeled apple

Peeled apple browning appleExposed to air

Chemical Changesc) Combustion of methane gas

methane + oxygen carbon dioxide + water vapour

Chemical Changesd) Photosynthesis

Carbon dioxide + water glucose + oxygenlight

(in plant)

Examples in daily life Physical changes in daily life

Chemical changes in daily life

Freezing of water to form ice cube

Boiling an egg to get a hard-boiled egg

Boiling of water Burning of fossil fuel in motor vehicles

Dissolving sugar or salt in water

Respiration in living organism

Melting of wax to form different shapes of candles

Photosynthesis in plants

Melting of chocolate Digestion of food in our body

Evaporation of sweat Baking a cake

Comparison between physical and chemical changes

Physical changes

Chemical changes

SIMILARITIESSubstance in both processes undergo changesBoth changes require energy

Comparison between physical and chemical changes.

Chemists often use word equations as a way of quickly telling us what the reactants and products are.

We just write: 1) the names of the reactants - always on

LEFT 2) an arrow (meaning “reacts to give”) 3) the names of the products – always on

RIGHT

Word Equations

Magnesium reacting with copper oxide to form magnesium oxide and copper

Magnesium + Copper Oxide Magnesium Oxide + Copper

Products are

Reactants are

For example:

Steel(Iron)Wool + copper sulphate-> iron sulphate + copper

Products are

Reactants are

“It turned into Copper” “heat released” “blue disappeared – it went clear (the

CuSO4)” “Some of it split apart’ “it turned red” “it absorbed the CuSO4”

Observations:

Magnesium + hydrochloric acid

Magnesium (s) + Hydrochloric acid (aq) -> Magnesium chloride (aq) + Hydrogen(g)

Observations:- bubbles form and gas is released- temperature increase during reaction- removing tarnish has the effect of making the Mg sink

and react faster.

Chemical states:- (s) – solid- (aq) – aqueous solution- (g) – gas

Sodium Sulphate + Barium Chloride Sodium Chloride + Barium Sulphate.

Evidence of a chemical reaction:

A white precipitate is formed

Sodium Sulphate + Barium Chloride

Magnesium burned brightly reacting with the oxygen in the air to form magnesium oxide.

List Products List Reactants

Write word equations for these reactions:

Hydrochloric acid reacted with the calcium hydroxide to give water and calcium chloride

List Products List Reactants

Electrolysis of aluminium oxide produced aluminium metal and oxygen gas

List Products List Reactants

Write word equations for the following chemical reactions.

◦ Octane gas is burnt with oxygen in a car engine to produce carbon dioxide and water.

◦ Sodium metal reacts with chlorine gas to form sodium chloride.

◦ Hydrogen gas and oxygen gas combine to form water.

◦ Zinc metal dissolves in hydrochloric acid to form hydrogen gas and zinc chloride.

Practice Questions:

Factors that affect the rate of reaction: Surface area Temperature Concentration of reactants Rate of reaction – how fast the alka –seltzer

dissolves – gas released is carbon dioxide.

Grasp task rate of reaction – Alka - Seltzer