Introduction To The Particulate Nature Of Matter

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Elements and Compounds

Elements are substances made from one kind of atom and cannot be chemically broken down into simpler substances.
Elements take part in chemical reactions, often with an energy change, to form new substances.
In chemical reactions, atoms combine in fixed ratios to give them full outer shells of electrons, producing compounds.
Compounds are made from two or more elements chemically bonded together.
The properties of a compound can be quite different from the properties of the elements that form it, e.g. sodium chloride is different from sodium and chlorine.

Ball-and-stick models of common elements and compounds

Mixtures

Mixtures contain more than one element or compound in no fixed ratio.
The components of a mixture are interspersed but not chemically combined.
Each component in a mixture retains its own characteristic properties, e.g. nitrogen and oxygen in air keep their properties.
A homogeneous mixture has uniform composition and properties throughout, e.g. air, bronze (an alloy).
A heterogeneous mixture has non-uniform composition, so its properties are not the same throughout, e.g. concrete, orange juice with pulp.
It is often possible to see the separate components in a heterogeneous mixture, but not in a homogeneous mixture.

Particle diagram showing elements, compounds and mixtures

Separating Mixtures: Solubility and Filtration

Separation techniques take advantage of differences in physical properties such as boiling point, solubility, magnetism, or density.
Solvation is the process where a solute dissolves because solvent particles surround and attract the solute particles; in water it is called hydration.
To separate a mixture of salt and sand, add water: only the salt dissolves, then filter.
Filtration separates an insoluble solid from a liquid using filter paper: the liquid (filtrate) passes through, and the solid (residue) remains on the paper.
Filtration works because solid particles are too large to pass through the pores in the filter paper.
Vacuum filtration can be used for very fine solids that clog filter paper under gravity filtration; centrifugation can also separate solid–liquid mixtures.

Filtration apparatus

Separating Mixtures: Crystallisation and Distillation

Crystallisation separates a dissolved solid from a solution when the solid is more soluble in hot solvent than cold, e.g. recovering copper(II) sulfate crystals.
Steps for crystallisation: heat gently to evaporate some solvent, cool until saturated, allow crystals to form, then filter, wash with distilled water, and dry.
Recrystallisation purifies an impure solid by dissolving it in hot solvent and allowing it to crystallise as the solution cools.
Simple distillation separates a solvent from a solute or a pure liquid from a mixture based on differences in boiling point, e.g. separating water from salt solution.
Fractional distillation separates two or more miscible liquids with similar boiling points using a fractionating column, e.g. ethanol and water, or crude oil.
For ethanol (78 °C) and water (100 °C), heat to 78 °C to distil the ethanol first; use an electric heater when flammable liquids are present.

Crystallisation by evaporation and cooling

Separating Mixtures: Chromatography and Other Techniques

Paper chromatography separates dissolved substances based on differences in solubility and adsorption to the paper.
In chromatography, a pencil line is used because pencil does not dissolve in the solvent; the spots must stay above the solvent level.
Substances that are more soluble in the solvent travel further; substances more strongly adsorbed to the paper move more slowly.
The resulting chromatogram shows a pattern of spots representing different components in the mixture.
Magnetic separation uses differences in magnetic properties, e.g. removing iron filings from a mixture.
Summary: filtration (insoluble solid from liquid), crystallisation (dissolved solid), recrystallisation (purify solid), simple distillation (solvent from solute), fractional distillation (miscible liquids), chromatography (dissolved substances).

Paper chromatography

The Three States of Matter

Solids have a fixed shape and volume; particles are closely packed in a regular, ordered pattern and can only vibrate in place.
Solids have a high density and the lowest particle energy compared to liquids and gases.
Liquids have a fixed volume but take the shape of their container; particles are close together but randomly arranged and can slide past one another.
Liquids have a medium density and particles have more energy than in solids but less than in gases.
Gases have no fixed shape or volume and expand to fill the container; particles are widely spaced, randomly arranged, and move quickly in all directions.
Gases can be compressed easily, have very low density, and particle collisions generate pressure; particles have the highest energy.

Particle arrangements in solids, liquids and gases

Changes of State

State changes are physical changes and are reversible; they involve energy transfer but do not alter the chemical identity of the substance.
Names of state changes: melting (solid→liquid), freezing (liquid→solid), vaporisation (liquid→gas), condensation (gas→liquid), sublimation (solid→gas), deposition (gas→solid).
Boiling occurs throughout the liquid at a specific temperature when vapour pressure reaches external atmospheric pressure; evaporation occurs only at the surface below the boiling point.
State symbols: (s) solid, (l) liquid, (g) gas, (aq) aqueous (dissolved in water).
Energy is always required to overcome intermolecular forces, so melting, vaporisation, and sublimation are endothermic.
Energy is released when particles come closer, so freezing, condensation, and deposition are exothermic.

Changes of state

Average Kinetic Energy and Temperature

Temperature is a measure of the average kinetic energy (Ek) of the particles in a substance.
The Kelvin (K) is the SI unit of temperature; temperature in Kelvin is directly proportional to average kinetic energy.
The higher the temperature, the faster the particles move on average.
At the same temperature, all gases have the same average kinetic energy, but lighter gases move faster and heavier gases move slower.
Conversion: Temperature in K = Temperature in °C + 273.15; Temperature in °C = Temperature in K − 273.15.
0 K (absolute zero) is the temperature at which particles have no kinetic energy; a change of 1 K equals a change of 1 °C.

Interpreting State Change Graphs

A heating curve shows the relationship between temperature and energy during changes of state.
While a substance is heating in the solid state, particles vibrate more as they gain kinetic energy, so temperature increases.
During melting (solid→liquid), energy is used to overcome intermolecular forces, not increase kinetic energy, so temperature remains constant.
While heating in the liquid state, particles move more freely and gain kinetic energy, so temperature rises again.
During boiling (liquid→gas), energy goes into overcoming intermolecular forces, so temperature remains constant.
In the gas state, particles move rapidly and have the highest average kinetic energy, so temperature increases.
Observations: melting (solid becomes liquid, shape lost but volume constant), boiling (bubbles form throughout liquid), condensation (gas forms droplets on cooler surfaces), freezing (liquid becomes rigid), sublimation (solid disappears directly into vapour).

A heating curve

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Practice questions

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1.What is the name of the process that occurs when a gas turns into a liquid?
Easy
- ACondensation
- BFreezing
- CDeposition
- DSublimation
2.What is the name of the process that occurs when a solid turns into a gas without first forming a liquid?
Easy
- ASublimation
- BEvaporation
- CCondensation
- DMelting
3.A compound is made from two or more elements chemically bonded together.
Easy
True or false?
4.Which of the following are heterogeneous mixtures? (select all that apply)
Easy
- AConcrete
- BOrange juice with pulp
- CAir
- DBronze
- ESalt solution
5.Match each change of state with its correct description.
Medium
- Melting
- Freezing
- Condensation
- Deposition
- Solid to liquid
- Liquid to solid
- Gas to liquid
- Gas to solid
6.In the kinetic molecular theory, which statement correctly describes the motion of particles in a liquid?
Medium
- AParticles vibrate about fixed positions and cannot move past one another.
- BParticles are closely packed but can slide past one another, allowing the liquid to flow.
- CParticles are widely spaced and move randomly at high speeds.
- DParticles are fixed in a regular lattice and only rotate.
7.A student separates a mixture of propanone (boiling point 56 °C) and water (boiling point 100 °C) by distillation. Which liquid is collected first as the distillate?
Medium
- APropanone, because it has the lower boiling point.
- BWater, because it has the higher boiling point.
- CA mixture of both in equal amounts.
- DNeither; distillation cannot separate them.
8.Temperature is a measure of the average kinetic energy of the particles in a substance.
Easy
True or false?

More questions

9.Which of the following changes of state are exothermic? (select all that apply)
Medium
- AMelting
- BFreezing
- CCondensation
- DDeposition
- ESublimation
10.Which of the following statements about mixtures are correct? (select all that apply)
Medium
- AComponents of a mixture are chemically combined.
- BComponents of a mixture retain their individual properties.
- CMixtures have a fixed ratio of components.
- DA homogeneous mixture has uniform composition throughout.
- EConcrete is a heterogeneous mixture.
11.Match each separation technique with the physical property difference it relies on.
Easy
- Filtration
- Simple distillation
- Chromatography
- Magnetic separation
- Boiling point
- Solubility
- Magnetic properties
- Adsorption and solubility
12.Place the following steps for separating a mixture of salt and sand using filtration in the correct order.
Medium
- Add water to the mixture and stir.
- Pour the mixture into the filter funnel.
- The salt solution passes through as the filtrate.
- The sand remains on the filter paper as the residue.
13.Which of the following is a heterogeneous mixture?
Medium
- AAir
- BBronze
- CConcrete
- DSalt solution
14.Which technique is most suitable for separating two miscible liquids with similar boiling points, such as ethanol (78 °C) and water (100 °C)?
Medium
- ASimple distillation
- BFractional distillation
- CFiltration
- DCrystallisation
15.In a gas, particles are widely spaced and move randomly at high speeds.
Easy
True or false?
16.During boiling, the temperature of a pure liquid increases steadily as energy is added.
Medium
True or false?
17.A sealed container holds nitrogen gas at 25 °C. It is heated to 125 °C. Which statement correctly describes the change in the average kinetic energy and average speed of the nitrogen molecules?
Medium
- ABoth the average kinetic energy and the average speed increase.
- BThe average kinetic energy increases but the average speed stays the same.
- CThe average speed increases but the average kinetic energy stays the same.
- DBoth the average kinetic energy and the average speed decrease.
18.A sample of gas is cooled until it condenses to a liquid. Which statements about this change are correct? (select all that apply)
Medium
- AThe particles lose kinetic energy.
- BThe particles become more closely spaced.
- CThe process is exothermic.
- DThe chemical identity of the substance changes.
- EThe particles stop moving completely.
19.During boiling, the temperature of a pure liquid stays constant even though energy is being supplied continuously.
Easy
True or false?
20.Place the steps for the recrystallisation of an impure solid in the correct order.
Medium
- Allow the solution to cool slowly.
- Collect the crystals by filtration.
- Add a minimum amount of hot solvent to the impure solid.
- Wash the crystals with cold solvent and dry them.
21.A mixture contains water, sugar, dyes and flavouring concentrates, and has become contaminated with an unknown solid. Which methods would be suitable to isolate the solid particles and then recover the dissolved solids? (select all that apply)
Hard
- AFiltration to remove the insoluble solid.
- BEvaporation or crystallisation to recover dissolved solids.
- CSimple distillation to separate the dyes from the flavourings.
- DChromatography to separate the dyes and flavourings.
- EUsing a magnet to remove the solid.
22.A sample of pure solid sulfur dioxide is heated from −100 °C to 0 °C. Its melting point is −72 °C and its boiling point is −10 °C. Which row correctly shows the state of SO₂ at −50 °C and at 0 °C?
Medium
- AAt −50 °C: solid; at 0 °C: gas
- BAt −50 °C: liquid; at 0 °C: gas
- CAt −50 °C: solid; at 0 °C: liquid
- DAt −50 °C: gas; at 0 °C: gas
23.A mixture of sand and salt is separated using the method shown: water is added and stirred, the mixture is filtered, and the filtrate is then heated to remove the water. Which statements about this procedure are correct? (select all that apply)
Medium
- AWater is a suitable solvent because it dissolves the salt but not the sand
- BThe sand is collected as the residue on the filter paper
- CThe salt solution passes through the filter paper as the filtrate
- DThe salt is recovered by evaporating the water from the filtrate
- EThe salt and sand are separated because they have different magnetic properties
24.A mixture of iron filings and sulfur powder is to be separated. Which property difference makes this separation possible?
Medium
- AIron is magnetic but sulfur is not
- BIron has a higher boiling point than sulfur
- CIron is soluble in water but sulfur is not
- DIron has a higher density than sulfur
25.In paper chromatography, the solvent is allowed to travel up the paper. A component that is more soluble in the solvent and less strongly adsorbed to the paper will:
Medium
- Atravel further up the paper
- Btravel less far up the paper
- Cremain at the pencil line without moving
- Dmove down the paper against the solvent
26.A student separates a mixture of ethanol (boiling point 78 °C) and water (boiling point 100 °C) using fractional distillation. Why is a fractionating column used instead of simple distillation?
Medium
- AIt gives better separation of liquids with similar boiling points
- BIt prevents the ethanol from boiling at all
- CIt raises the boiling point of the water to above 100 °C
- DIt removes dissolved solids from the mixture
27.An impure solid is purified by recrystallisation. Why is only the minimum amount of hot solvent used to dissolve the solid?
Hard
- ATo avoid losing product, since a larger volume of solvent would keep more of the product dissolved on cooling
- BTo make the solid dissolve more slowly
- CTo increase the boiling point of the solvent
- DTo ensure the impurities also crystallise out
28.In the crystallisation of copper(II) sulfate from solution, why should the solution not be boiled to dryness?
Medium
- ABoiling to dryness may prevent proper crystal formation and can decompose the solid
- BBoiling to dryness would dissolve the impurities again
- CBoiling to dryness would change the copper(II) sulfate into a different compound
- DBoiling to dryness would make the crystals too large to filter
29.Which of the following are correct applications of separation techniques? (select all that apply)
Medium
- AFiltration separates an insoluble solid from a liquid
- BCrystallisation recovers a dissolved solid from solution
- CFractional distillation separates the gases in air
- DChromatography separates dissolved substances by differences in solubility and adsorption
- EFiltration separates two miscible liquids with similar boiling points
30.A chromatogram of a sample solution X is compared with reference flavourings A, B and C. A spot in X at the same height as the spot for flavouring A indicates that:
Hard
- Aflavouring A is present in sample X
- Bflavouring A is absent from sample X
- Csample X contains only flavouring A
- Dsample X contains no flavourings at all
31.In a chromatography experiment, sample U shows spots that do not match the spots of either solution Y or solution Z, while sample S shows spots matching both Y and Z. Which statement is correct?
Hard
- AU contains ions different from those in Y and Z, while S contains the ions present in both Y and Z
- BU contains the same ions as Y only, while S contains the same ions as Z only
- CU contains the same ions as both Y and Z, while S contains no matching ions
- DU and S both contain exactly the same ions as each other
32.In paper chromatography, the pencil line is drawn near the bottom of the paper because pencil does not dissolve in the solvent.
Easy
True or false?
33.In paper chromatography, the spots of the sample must be placed below the level of the solvent in the container.
Medium
True or false?
34.Solvation is the process in which solvent particles surround and attract solute particles, and in water this process is called hydration.
Medium
True or false?
35.Match each separation technique with the physical property difference that it relies on.
Medium
- Filtration
- Crystallisation
- Chromatography
- Magnetic separation
- Solubility decreasing with temperature
- Difference in magnetic properties
- Solubility and adsorption
- Insolubility of a solid in a liquid
36.Place the steps for purifying an impure solid by recrystallisation in the correct order.
Medium
- Add a minimum amount of hot solvent to the impure solid until it dissolves
- Perform hot filtration to remove any insoluble solid impurities
- Allow the solution to cool slowly to room temperature so crystals form
- Collect the crystals by filtration and wash with fresh cold solvent
- Dry the crystals on filter paper
37.Place the steps for separating the pigments in a food colouring by paper chromatography in the correct order.
Medium
- Draw a pencil line near the bottom of the chromatography paper
- Place small spots of the sample mixture on the pencil line
- Suspend the paper in a container with a shallow layer of solvent, keeping the spots above the solvent level
- Allow the solvent to move up the paper by capillary action
- Allow the paper to dry and observe the separated spots
38.Which technique is most suitable for isolating the insoluble solid particles from the mixture?
Medium
- AFiltration
- BSimple distillation
- CFractional distillation
- DCrystallisation
39.Which method could be used to isolate the dissolved solids, such as the dyes and flavourings, from the mixture?
Medium
- ACrystallisation
- BFiltration
- CMagnetic separation
- DPaper chromatography
40.Once the dyes and flavourings have been isolated, which method could be used to separate them from one another?
Medium
- APaper chromatography
- BSimple distillation
- CFiltration
- DMagnetic separation
41.Complete the sentence about states of matter.
Easy
In a solid, particles are closely packed in a ____ pattern and can only ____ in place.
42.What is the name of the separation technique that uses a difference in boiling point to separate a solvent from a solution?
Medium
43.Convert a temperature of 300 K to degrees Celsius.
Medium
44.What is the name of the process where a gas turns directly into a solid?
Easy
45.What is the temperature in Kelvin equivalent to 25 °C?
Medium
46.Complete the table for the three states of matter by filling in the missing type of motion.
Easy
Solid: particles ____ only; Liquid: particles ____ past one another; Gas: particles move ____ in all directions.
47.A gas has a temperature of 25 °C. Convert this to kelvin using the conversion T/K = T/°C + 273.15. Give your answer to two decimal places.
Medium
48.Complete the sentence about the separation of a mixture of sand and salt.
Medium
The mixture is added to water so that the salt dissolves. The mixture is then filtered, and the salt solution that passes through the filter paper is called the ____.
49.Name the process in which a gas changes directly into a solid without passing through the liquid state.
Easy
50.In a heating curve for a pure substance, the flat section between 2 and 3 represents melting. If the substance is heated at a constant rate of 50 J per minute and melting takes 4.0 minutes, calculate the energy absorbed during melting.
Medium
51.Convert a temperature of 5 °C to kelvin using T/K = T/°C + 273.15. Give your answer to two decimal places.
Easy
52.Convert a temperature of 400 K to degrees Celsius using T/°C = T/K − 273.15. Give your answer to two decimal places.
Easy
53.Explain why the temperature remains constant during melting and boiling, even though energy is being added.
Hard
54.Explain why the temperature remains constant during melting, even though energy is being supplied.
Hard
55.A mixture of ethanol (boiling point 78 °C) and water (boiling point 100 °C) is separated by fractional distillation. Explain how the two liquids are separated, referring to boiling points and intermolecular forces.
Medium
56.Explain why the temperature remains constant during boiling, even though energy is being supplied continuously.
Hard
57.Describe how an increase in temperature affects the movement of particles in a liquid.
Hard
58.Convert a temperature of 300 K to degrees Celsius.
Medium
- A32.23 °C
- B26.85 °C
- C29.54 °C
- D24.16 °C
59.What is the temperature in Kelvin equivalent to 25 °C?
Medium
- A327.96 K
- B357.77 K
- C268.34 K
- D298.15 K
60.A gas has a temperature of 25 °C. Convert this to kelvin using the conversion T/K = T/°C + 273.15. Give your answer to two decimal places.
Medium
- A357.77 K
- B327.96 K
- C298.15 K
- D268.34 K
61.In a heating curve for a pure substance, the flat section between 2 and 3 represents melting. If the substance is heated at a constant rate of 50 J per minute and melting takes 4.0 minutes, calculate the energy absorbed during melting.
Medium
- A240 J
- B220 J
- C200 J
- D180 J
62.Convert a temperature of 5 °C to kelvin using T/K = T/°C + 273.15. Give your answer to two decimal places.
Easy
- A305.96 K
- B333.77 K
- C278.15 K
- D250.34 K
63.Convert a temperature of 400 K to degrees Celsius using T/°C = T/K − 273.15. Give your answer to two decimal places.
Easy
- A126.85 °C
- B152.23 °C
- C139.54 °C
- D114.16 °C

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