Physical and chemical changes — Combined Science 0653 - Chemistry Revision Notes & Topical Questions

Section 1 of 27Physical changes

Physical change

• = change that produces no new chemical substance.
• It is often easy to reverse.

Examples

• Examples changing state such as melting, making a mixture and dissolving a solute.
• Mixtures are relatively easy to separate.

Section 2 of 27Chemical changes

Chemical change

• = change in which new substances with very different properties form.
• It is usually difficult to reverse.

Examples

• Examples burning, rusting, cooking and photosynthesis.

Section 3 of 27Comparing the two

Feature	Physical change	Chemical change
New substance	No	Yes
Reversible	Usually easy	Usually difficult
Properties	Substance keeps its properties	Products have different properties
Mass	Conserved	Conserved

Section 4 of 27Signs of a chemical change

Sign	Observation	Example
Gas given off	Fizzing or bubbles	Metal + acid
Colour change	New colour	Iron rusting
Temperature change	Gets hotter or colder	Burning, neutralisation
Precipitate	Solid appears in a liquid	Mixing two solutions
Light or sound	Flash or bang	Fireworks

Boiling

• Boiling also makes bubbles but is physical.

Change of state

• Change of state has no new substance.

Section 5 of 27More from the specification

More from the specification

Monitoring rates of physical dissolution

• Monitoring rates of physical dissolution recording solute mass dissolved per minute provides an accurate means to evaluate dynamic dissolution rates.

Thermal energy

• Total kinetic energy of the particles in a substance.

Collision theory

• Reactions only occur when reacting particles collide with sufficient energy (>= Ea) and correct orientation.

Concentration effect

• Increasing the concentration of reactants increases particle frequency per unit volume, leading to more frequent successful collisions.

Surface area effect

• Dividing a solid into smaller particles increases exposed surface area, increasing collision frequency and overall rate.

Physical vs chemical changes

• Physical changes are easily reversible phase or appearance alterations without forming new chemical substances, unlike chemical reactions.

Gradient

• The change in y divided by the change in x on a graph.

Pressure

• Force of gas particles colliding with a surface.

Theory

• A tested hypothesis that is widely accepted.

Hydrogen

• A gas that gives a squeaky pop with a lighted splint.

Successful collision

• A collision with the correct orientation and enough energy to react.

Activation energy

• The minimum energy colliding particles need to react.

Collision theory

• Particles react only if they collide with at least the activation energy and the correct orientation.

Kinetic energy

• The energy of motion of a particle.

Particle

• A tiny portion of matter, too small to see.

Concentration

• Moles ÷ volume in dm³, or g/dm³.

Reactant

• A substance on the left of the arrow.

Volume

• The space taken up by an object.

Catalyst

• A substance that speeds up a reaction and is not used up.

Section 6 of 27Colour change in displacement

1Copper and silver nitrate

• Copper and silver nitrate orange-brown copper is placed in colourless silver nitrate solution.
• The solid changes from orange-brown to silver as silver is displaced.
• The solution changes from colourless to blue.

2Halide test

• Halide test adding silver nitrate to a halide solution gives a white, cream or yellow precipitate.
• The colour depends on the halide ion.
• A solid also forms, so a change of state is seen.

Section 7 of 27Temperature change

Exothermic

• Exothermic gives out thermal energy and the temperature rises.
• Calcium oxide with water forming calcium hydroxide is highly exothermic.
• Sodium reacting with water is exothermic and fizzes.

Endothermic

• Endothermic takes in thermal energy and the temperature falls.
• Photosynthesis absorbs light energy to make glucose and oxygen.
• Dissolving ammonium chloride in water absorbs heat and is used in cold packs.

Section 8 of 27Effervescence

Effervescence

• = fizzing, a sign that a gas is made.

Alkali metals and water

• Alkali metals and water metal + water → metal hydroxide + hydrogen.

Acids

• Acids many reactions with acids give fizzing.

Section 9 of 27Other signs of a chemical change

Light

• Light is produced in reactions such as burning magnesium.

Smell

• Smell a new smell can appear.

pH

• pH a change in pH can show a reaction.

Precipitate

• Precipitate a solid forms in a solution.

Section 10 of 27Judging the evidence

• New substance
  • only a new substance proves a chemical change.
  • Boiling makes bubbles but no new substance.

Section 11 of 27More from the specification

More from the specification

Reversibility as an analytical means

• Reversibility as an analytical means testing for reversible phase changes versus irreversible chemical synthesis provides a clear means to distinguish physical processes.

Effect of Concentration on Reaction Rates

• Increasing the concentration of reactant particles in solution increases the number of particles per unit volume,
• which increases the frequency of collisions between particles and accelerates the rate of reaction.

Surface Area and Collision Frequency in Solids

• Subdividing a solid reactant into smaller pieces dramatically increases its exposed surface area,
• allowing more reactant particles to collide simultaneously and increasing the frequency of successful collisions.

Interpreting Rate Graphs and Tangents

• On a graph of product volume against time, the initial gradient is steepest because reactant concentration is highest;
• as reactants are consumed, collision frequency drops, gradient decreases,
• and the curve plateaus when the reaction finishes.

Activation Energy and Proportion of Successful Collisions

• For collisions between particles to be successful and cause chemical change, the colliding entities must possess kinetic energy exceeding the activation energy threshold.

Pressure

• Force of gas particles colliding with a surface.

Theory

• A tested hypothesis that is widely accepted.

Successful collision

• A collision with the correct orientation and enough energy to react.

Activation energy

• The minimum energy colliding particles need to react.

Collision theory

• Particles react only if they collide with at least the activation energy and the correct orientation.

Kinetic energy

• The energy of motion of a particle.

Particle

• A tiny portion of matter, too small to see.

Concentration

• Moles ÷ volume in dm³, or g/dm³.

Volume

• The space taken up by an object.

Catalyst

• A substance that speeds up a reaction and is not used up.

Gradient

• The change in y divided by the change in x on a graph.

Section 12 of 27Features of physical changes

Features of physical changes

Same substance

• Same substance no new chemical substance forms.

Examples

• Examples melting, freezing, boiling, condensing, dissolving and making a mixture.

Reversing

• Reversing the change is easy to reverse and mixtures are relatively easy to separate.
• Dissolved salt is recovered by evaporation.

Section 13 of 27Features of chemical changes

Features of chemical changes

New substances

• New substances products have very different properties from the reactants.

Reversing

• Reversing most chemical changes are difficult to reverse.

Signs

• Signs colour change, temperature change and effervescence often show that a new substance has formed.

Section 14 of 27Particle diagrams

Particle diagrams

Physical change

• Physical change the same particles are present before and after, only their arrangement or energy changes.

Chemical change

• Chemical change the particles themselves are different, because atoms are rearranged into new substances.
• Hydrogen and oxygen molecules become water molecules when hydrogen burns.

Atoms

• Atoms the number of each type of atom stays the same, so mass is conserved.

Section 15 of 27Classifying everyday changes

Change	Type	Reason
Ice melting	Physical	Same substance, change of state
Salt dissolving	Physical	Salt recovered by evaporation
Sand mixed with iron filings	Physical	Mixture, easily separated
Iron rusting	Chemical	New substance, hard to reverse
Cooking an egg	Chemical	New substances form
Magnesium burning	Chemical	White powder forms with bright flame

Section 16 of 27Testing whether a change is chemical

1Recover the starting material

• Recover the starting material if evaporation, filtration or cooling restores it, the change was physical.

2Compare properties

• Compare properties a new substance has different properties such as melting point or conductivity.

Section 17 of 27More from the specification

1Dissolution in aqueous solution

• Dissolution in aqueous solution dissolving solutes in aqueous solution provides a physical means to alter chemical reaction rates.

2Collision frequency and successful collisions

• Chemical reactions occur when reacting particles collide with energy exceeding the activation energy;
• increasing reactant concentration increases particle density and the frequency of successful collisions.

3Effect of surface area on reaction rate

• Increasing the exposed surface area of a solid reactant provides more exposed particles for collisions,
• which directly increases the reaction rate.

4Graphical analysis of gas volume

• On a graph of gas volume produced over time, the initial gradient represents the initial rate of reaction,
• which increases when temperature or reactant concentration is increased.

5Physical vs chemical changes

• Physical changes involve reversible changes in state with no new substances formed,
  • (such as melting and boiling)
• whereas chemical changes involve breaking and forming chemical bonds to produce new substances.

6Pressure

• Force of gas particles colliding with a surface.

7Theory

• A tested hypothesis that is widely accepted.

8Successful collision

• A collision with the correct orientation and enough energy to react.

9Activation energy

• The minimum energy colliding particles need to react.

10Collision theory

• Particles react only if they collide with at least the activation energy and the correct orientation.

11Kinetic energy

• The energy of motion of a particle.

12Thermal energy

• Total kinetic energy of the particles in a substance.

13Concentration

• Moles ÷ volume in dm³, or g/dm³.

14Volume

• The space taken up by an object.

15Catalyst

• A substance that speeds up a reaction and is not used up.

16Gradient

• The change in y divided by the change in x on a graph.

Section 18 of 27Physical changes

Physical changes

Physical change

Physical change a change in which no new substance is formed.

Particles

• Particles they stay the same but their arrangement or energy changes.

Reversibility

• Reversibility the change is usually easy to reverse and the substances easy to separate.

Examples

• Examples melting, freezing, boiling, condensing, dissolving, and mixing sand with iron filings.

Section 19 of 27Chemical changes

1Chemical change (reaction)

Chemical change (reaction) a change in which one or more new substances form with different properties from the reactants.

2Bonds

• Bonds are broken and new bonds are made.

3Reversibility

• Reversibility most chemical changes are difficult to reverse.

4Examples

• Examples burning, rusting, cooking an egg, photosynthesis, digestion, fermentation and fireworks exploding.

Section 20 of 27Physical compared with chemical change

Feature	Physical change	Chemical change
New substance	No	Yes
Reversibility	Usually easy	Usually difficult
Energy change	Small	Often large
Mass	Conserved	Conserved

Section 21 of 27Evidence for a chemical reaction

Gas given off

• Gas given off bubbles or effervescence, often when acids or alkali metals react.
• Examplesodium with water releases hydrogen, metal + water → metal hydroxide + hydrogen.

Colour change

• Colour change for example copper added to colourless silver nitrate.
• The solid changes from orange-brown to silver and the solution turns from colourless to blue.

Precipitate

• Precipitate a solid forms in a solution, such as a silver halide changing the solution from colourless to cream, white or yellow.

Temperature change

• Temperature change exothermic reactions give out heat; endothermic reactions take in heat.
• Exothermic
  • calcium oxide with water and sodium with water;
    • endothermic: photosynthesis and ammonium chloride dissolving, used in cold packs.

Other signs

• Other signs light, a new smell, a flame or a change in pH.

Caution

• Caution some signs also occur in physical changes, so check whether a new substance has really formed.
  • (boiling gives bubbles)

Section 22 of 27Testing whether a change is physical or chemical

Testing whether a change is physical or chemical

Recoverable starting material

• Recoverable starting material if it can be recovered by evaporation, filtration or cooling, the change was physical.

New substance

• New substance if a new substance with different properties has formed, the change was chemical.

Property check

• Property check compare properties such as melting point or conductivity before and after.

Particle models

• Particle models in melting the same particles gain energy and break free from fixed positions;
  • in a chemical change the particles themselves are replaced.

Burning hydrogen

• Burning hydrogen diatomic hydrogen and oxygen molecules are replaced by water molecules, each with two hydrogen atoms bonded to an oxygen atom.

Section 23 of 27Practical examples

Practical examples

Hydrated copper(II) sulfate

• Hydrated copper(II) sulfate heating removes the water of crystallisation, turning blue crystals into white powder.
• Adding water restores the blue colour and releases heat, so it is a reversible chemical change.

Copper carbonate

• Copper carbonate heating turns the green solid into black copper oxide and releases carbon dioxide, which turns limewater cloudy.

Magnesium ribbon

• Magnesium ribbon burning gives a bright white flame and a white powder, a chemical change.

Distillation and evaporation

• Distillation and evaporation physical separations.

Change of state

• Change of state physical, because the same particles are present in a different arrangement and only their kinetic energy changes.

Section 24 of 27Conservation of mass

Conservation of mass

Law

• Law atoms cannot be created or destroyed in a chemical reaction, so the total mass of reactants equals the total mass of products.

Apparent changes

• Apparent changes when mass seems to change, a gas has usually been lost or gained.

Magnesium burning in air

• Magnesium burning in air gains mass because it combines with oxygen from the air.

Calcium carbonate heated in an open crucible

• Calcium carbonate heated in an open crucible loses mass because carbon dioxide escapes.

Sealed container

• Sealed container the total mass stays the same.

Section 25 of 27Writing equations

1

Write the word equation: reactants → products.

2

Replace names with correct formulae.

3

Balance by placing numbers in front of formulae, never by changing the subscripts, until each element has equal atoms on both sides.

4

Add state symbols: (s), (l), (g), (aq).

Section 26 of 27Key terms

Term	Meaning
Physical change	Change with no new substance.
Chemical change	Change that forms new substances.
Reactant	Starting substance.
Product	Substance formed.
Effervescence	Fizzing as gas is given off.
Reversible	Able to change back.

Term	Meaning
Displacement	A more reactive metal replacing a less reactive one in a compound.
Effervescence	Fizzing due to gas being made.
Exothermic	Gives out thermal energy.
Endothermic	Takes in thermal energy.
Precipitate	Insoluble solid formed in solution.
Halide	Compound containing a Group VII ion.

Term	Meaning
Physical change	Change in which no new substance forms.
Chemical change	Change that forms new substances.
Mixture	Substances together but not chemically joined.
Particle diagram	Picture of particles before and after a change.
Rearrangement	Atoms joining differently in new substances.
Reversible change	Change that can be undone.

Term	Meaning
Physical change	A change in which no new substance forms.
Chemical change	A change in which new substances form.
Precipitate	An insoluble solid formed in a solution during a reaction.
Conservation of mass	Total mass of reactants equals total mass of products.
Reactant	A substance that takes part in a reaction.
Product	A substance formed in a reaction.
Exothermic	A reaction that gives out heat to the surroundings.
Endothermic	A reaction that takes in heat from the surroundings.

Section 27 of 27Exam technique

1

Saying dissolving is a chemical change — it is physical.

2

Saying boiling is a chemical change because it bubbles.

3

Saying mass changes in a closed chemical reaction — it is conserved.

4

Saying all chemical changes are reversible — most are not.

1

Saying a colour change always proves a chemical reaction — dissolving a coloured solid is physical.

2

Saying all temperature changes show a chemical reaction — a change of state also changes temperature.

3

Confusing bubbles of boiling with a gas produced by reaction.

4

Forgetting that endothermic reactions also happen.

1

Saying a change of state is chemical — the same particles are present.

2

Thinking particles change in a physical change — only their arrangement and energy change.

3

Saying atoms are lost in a chemical change — they are rearranged.

4

Assuming every irreversible change is chemical without checking for a new substance.

1

Saying dissolving is a chemical change — the substance can be recovered by evaporation, so it is physical.

2

Changing subscripts to balance an equation — only numbers in front of formulae may be changed.

3

Saying mass is lost in a reaction — it is conserved, but gases may escape.

4

Thinking all colour changes prove a chemical reaction.

5

Forgetting state symbols.