DP Chemistry SL S1.1 The Particulate Nature of Matter

DP Chemistry SL S1.1 The Particulate Nature of Matter - Full notes

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DP Chemistry SL | S1.1 The Particulate Nature of Matter

DP CHEMISTRY · STRUCTURE

S1.1

The Particulate Nature of Matter

Full notes for SL · IB-style

Contents

1	Matter is made of particles
2	Elements, compounds and mixtures
3	Separating mixtures
4	The kinetic molecular theory and states of matter
5	Changes of state
6	Temperature and the Kelvin scale
7	Pure substances and purity
8	Choosing a separation method
9	Fractional distillation in detail
10	Paper and thin-layer chromatography
11	The kinetic molecular theory in detail
12	Heating and cooling curves in detail
13	Evaporation and boiling
14	Temperature, kinetic energy and absolute zero
15	State symbols and physical changes
16	Physical and chemical changes
17	Mixtures in everyday life
18	Practical skills: separations in the lab
19	Diffusion experiments
20	Mixed review: worked problems
21	Additional exam-style practice
22	Common misconceptions and links
23	Exam-style question with markscheme
24	Check your understanding
25	Summary and topic map

These are full teaching notes. Read each section in order, try the Think about it question before you read on, and do every Quick check before moving to the next section.

Before you start

Rate yourself on each statement now, and again after you've worked through the notes. Shade R (not yet), A (getting there) or G (confident).

I can…	R	A	G
distinguish elements, compounds and mixtures
distinguish homogeneous and heterogeneous mixtures
explain separation techniques: filtration, crystallization, distillation, chromatography, solvation
use the kinetic molecular theory to describe solids, liquids and gases
explain changes of state in terms of particles and energy
interpret heating and cooling curves
convert between Celsius and kelvin and explain absolute zero

There is no additional HL content in this subtopic. SL and HL students study the same material.

Guiding questions

How can we model the particulate nature of matter? How do particles behave as matter is heated and cooled?

How this topic is examined

S1.1 is the foundation for the whole course. Paper 1 questions test classification of substances, separation techniques and state symbols; data questions use heating curves and temperature conversions.

Use the particle model in every explanation: arrangement, movement and the forces between particles.

Command term	What the examiner wants
State / Identify	a brief answer, no explanation
Outline / Describe	a brief or detailed account, without reasons
Explain	a detailed account with causes, reasons or mechanisms
Calculate / Determine	a numerical answer with working, correct significant figures and units
Deduce / Predict	reach a conclusion from given information
Draw / Sketch	a clear labelled diagram, structure or graph
Evaluate / Discuss	weigh strengths and limitations; reach a judgement

How to use these notes

Each section teaches one part of the syllabus in full. You'll see these boxes throughout; each has one job:

Box	What it's for
Think about it	a question to get you thinking before a section; the answer comes in the section
Key point	the one idea you must not forget
Common mistake	an error examiners see every year, and how to avoid it
In the real world	where the chemistry shows up outside the classroom
Try this	something quick you can do yourself to test the idea
Worked example	full solutions with IB-style marks (M1 method, A1 answer, R1 reasoning)
Quick check	short questions to test yourself at the end of each section

1 Matter is made of particles

Everything is made of tiny particles in constant motion.

Think about it

When you open a bottle of perfume, the smell slowly fills the room. What does this tell you about the particles of perfume?

Matter is anything that has mass and occupies space. All matter is made of extremely small particles: atoms, molecules or ions. These particles are in constant random motion, and there are forces of attraction between them.

Evidence for the particle model comes from diffusion (the spreading of particles from high to low concentration, as with the smell of perfume or a drop of ink in water) and Brownian motion (the random jiggling of pollen grains or smoke particles, caused by collisions with much smaller, invisible water or air molecules).

Chemistry describes matter at three levels: the macroscopic level (what we see and measure), the particulate level (atoms, molecules, ions) and the symbolic level (formulas and equations). Good explanations move between them.

Quick check

1. Name three types of particles that make up matter.

2. What evidence supports the idea that particles are in constant motion?

Answers: 1. atoms, molecules, ions 2. diffusion; Brownian motion

2 Elements, compounds and mixtures

Pure substances and mixtures are classified differently.

An element is a substance made of only one type of atom (all with the same atomic number), such as iron, oxygen (O2) or carbon. It cannot be broken down into simpler substances by chemical means. About 118 elements are known.

A compound is a substance made of two or more different elements chemically combined in a fixed ratio, such as water (H2O) or sodium chloride (NaCl). Compounds have properties very different from their elements: sodium is a reactive metal and chlorine a toxic gas, but sodium chloride is table salt. Compounds can only be separated into their elements by chemical reactions.

A mixture contains two or more substances (elements or compounds) that are not chemically combined. The components keep their own properties, can be present in any proportion, and can be separated by physical methods.

A homogeneous mixture has a uniform composition throughout, with only one phase visible, such as salt solution or air. A heterogeneous mixture has a non-uniform composition, with visibly different parts or phases, such as sand in water or oil and water.

	Element	Compound	Mixture
Composition	one type of atom	elements combined in fixed ratio	substances not combined; variable ratio
Separation	cannot be broken down chemically	only by chemical reactions	by physical methods
Properties	its own	different from its elements	those of its components
Example	copper, O2	CO2, NaCl	air, seawater

Common mistake

Calling air a compound. Air is a mixture of nitrogen, oxygen, argon, carbon dioxide and other gases, in variable proportions, not chemically combined.

Quick check

1. Define compound.

2. Classify: (a) bronze (b) carbon dioxide (c) helium.

3. Is salt water homogeneous or heterogeneous?

Answers: 1. two or more elements chemically combined in a fixed ratio 2. (a) mixture (alloy) (b) compound (c) element 3. homogeneous

3 Separating mixtures

Physical differences allow components to be separated.

Because the components of a mixture are not chemically bonded, they can be separated by exploiting differences in their physical properties.

Filtration separates an insoluble solid from a liquid: the liquid (filtrate) passes through filter paper, and the solid (residue) is trapped. Example: sand from water.

Evaporation and crystallization separate a dissolved solid from a solution: heating evaporates the solvent; for crystallization, the solution is concentrated and allowed to cool slowly so that pure crystals form. Example: obtaining copper(II) sulfate crystals.

Solvation and dissolving: a soluble component can be separated from an insoluble one by dissolving it in a suitable solvent and filtering; for example, separating salt from sand.

Distillation separates a liquid from a solution, or liquids with different boiling points. In simple distillation, the solution is boiled, the vapour is condensed in a condenser and collected as the pure liquid (distillate). Fractional distillation uses a fractionating column to separate miscible liquids with close boiling points, such as ethanol and water, or the fractions of crude oil.

Paper chromatography separates dissolved substances with different solubilities in a solvent (mobile phase) and different attraction to the paper (stationary phase). Each substance is identified by its Rf value = distance moved by substance ÷ distance moved by solvent.

Method	Separates	Property used	Example
Filtration	insoluble solid from liquid	particle size / solubility	sand from water
Crystallization	dissolved solid from solution	solubility changes with temperature	copper(II) sulfate
Simple distillation	solvent from solution	boiling point	pure water from seawater
Fractional distillation	miscible liquids	different boiling points	ethanol from water; crude oil
Paper chromatography	dissolved substances	solubility and attraction to paper	dyes in ink

WORKED EXAMPLE 1 Rf value

In a chromatogram, the solvent front moved 9.0 cm and a dye moved 6.3 cm. Calculate the Rf value.

Solution

Rf = 6.3 ÷ 9.0 = 0.70A1

Rf has no units and is always less than 1R1

Quick check

1. Which method separates ethanol from water?

2. Why is the Rf value always less than 1?

Answers: 1. fractional distillation 2. substances cannot travel further than the solvent

4 The kinetic molecular theory and states of matter

Arrangement, motion and forces explain solids, liquids and gases.

The kinetic molecular theory states that all matter consists of particles in constant random motion, and that the average kinetic energy of the particles is proportional to the absolute temperature.

In a solid, particles are packed closely in a regular arrangement, held in fixed positions by strong forces of attraction. They vibrate about fixed positions. Solids have a fixed shape and volume and cannot be compressed.

In a liquid, particles are close together but arranged randomly. Forces between particles are weaker than in solids, so particles can move past each other. Liquids have a fixed volume but take the shape of their container, and are almost incompressible.

In a gas, particles are far apart and arranged randomly, moving rapidly in straight lines in all directions, colliding with each other and the container walls. Forces between particles are negligible. Gases have no fixed shape or volume, fill their container and are easily compressed.

	Solid	Liquid	Gas
Arrangement	regular, close	random, close	random, far apart
Movement	vibrate in place	move past each other	move rapidly and freely
Forces between particles	strong	moderate	very weak
Shape / volume	fixed / fixed	not fixed / fixed	not fixed / not fixed
State symbol	(s)	(l)	(g)

State symbols are written after formulas in equations: (s) solid, (l) liquid, (g) gas and (aq) aqueous (dissolved in water). For example: NaCl(s) → Na+(aq) + Cl−(aq).

Quick check

1. Describe the movement of particles in a liquid.

2. What does (aq) mean?

Answers: 1. move past each other randomly while staying close 2. dissolved in water

5 Changes of state

Energy changes the forces between particles, not the particles themselves.

Changes of state are physical changes: no new substances are formed. They involve energy transfer that changes the arrangement and motion of particles.

Melting (solid to liquid) and boiling/vaporization (liquid to gas) require energy to overcome forces between particles: they are endothermic. Freezing (liquid to solid) and condensation (gas to liquid) release energy: they are exothermic. Sublimation is the direct change from solid to gas (dry ice, CO2(s), or iodine); deposition is the reverse.

Evaporation occurs at the surface of a liquid at any temperature, when the most energetic particles escape; boiling occurs throughout the liquid at the boiling point.

On a heating curve, the temperature rises while one state is heated, but stays constant during a change of state, because the energy supplied is used to overcome intermolecular forces rather than increase kinetic energy (temperature).

Figure 1. Heating curve of water: flat sections at 0 °C (melting) and 100 °C (boiling).

WORKED EXAMPLE 2 Interpreting a heating curve

Explain why the temperature remains at 100 °C while water boils, even though heat is still supplied.

Solution

Energy is used to overcome the intermolecular forces (hydrogen bonds) between water moleculesR1

not to increase the average kinetic energy of the particlesR1

so temperature stays constant until all liquid has changed to gasR1

Common mistake

Saying covalent bonds break when water boils. Boiling overcomes forces between molecules; the O–H bonds within molecules are unchanged.

Quick check

1. Which changes of state are exothermic?

2. Why is the line flat during melting?

Answers: 1. freezing, condensation, deposition 2. energy overcomes forces between particles; kinetic energy unchanged

6 Temperature and the Kelvin scale

Absolute zero is the lowest possible temperature.

Temperature is a measure of the average kinetic energy of the particles in a substance. The Kelvin scale is the absolute temperature scale: its zero, absolute zero (0 K = −273.15 °C), is the temperature at which particles have minimum kinetic energy and cannot lose any more. Kinetic energy is directly proportional to temperature in kelvin, so the Kelvin scale is used in gas law calculations (S1.5).

A change of 1 K is the same as a change of 1 °C. To convert: T(K) = T(°C) + 273.15 (often rounded to 273).

WORKED EXAMPLE 3 Converting temperatures

Convert (a) 25 °C to kelvin and (b) 77 K to °C.

Solution

(a) 25 + 273 = 298 KA1

(b) 77 − 273 = −196 °C (the boiling point of nitrogen)A1

Quick check

1. Define absolute zero.

2. Convert 100 °C to kelvin.

Answers: 1. 0 K, the temperature at which particles have minimum kinetic energy 2. 373 K

7 Pure substances and purity

How do we know a substance is pure?

A pure substance contains only one element or one compound. Pure substances have sharp, fixed melting and boiling points. Impurities usually lower the melting point and widen the melting range, and raise the boiling point.

Chemists check purity by measuring the melting point (compared with a data-book value), by chromatography (a pure substance gives a single spot), and by spectroscopic methods.

Sample	Melting behaviour	Conclusion
A	melts sharply at 122 °C	pure benzoic acid (data value 122 °C)
B	melts over 115–119 °C	impure: lower and broader range
C	one spot on chromatogram	probably pure

WORKED EXAMPLE 4 Purity from data (standard)

A sample of aspirin melts between 128 °C and 133 °C. The data-book value is 136 °C. Comment on its purity.

Solution

Melting point lower than the pure valueR1

Melts over a range rather than sharply: impureR1

In the real world

Pharmaceutical companies test every batch of a drug for purity using melting points and chromatography, because impurities can be toxic or reduce the effectiveness of the medicine.

Quick check

1. Why does a mixture not have a single fixed melting point?

2. Suggest how to check whether a sample of water is pure.

Answers: 1. its composition can vary; components melt at different temperatures 2. measure its boiling point (100 °C at 1 atm) and freezing point (0 °C)

8 Choosing a separation method

Match the method to the mixture.

To choose a separation method, ask: Is the mixture solid-liquid, liquid-liquid or solid-solid? Is the solid soluble? Are the liquids miscible? Do the components differ in boiling point, solubility, magnetism or particle size?

Many real separations use several steps in sequence. For example, rock salt (salt and sand) is separated by dissolving in water, filtering off the sand, then evaporating or crystallizing the salt.

WORKED EXAMPLE 5 Multi-step separation (exam level)

Describe how to obtain pure, dry samples of both components from a mixture of copper(II) oxide (insoluble) and copper(II) sulfate (soluble).

Solution

Add water and stir to dissolve copper(II) sulfateR1

Filter: copper(II) oxide is the residue; wash and dry itR1

Heat the filtrate to concentrate it, then cool to crystallize copper(II) sulfate; filter and dry the crystalsR1

Quick check

1. How would you separate iron filings from sulfur powder?

2. How would you separate oil from water?

3. How would you obtain pure water from seawater?

Answers: 1. use a magnet (iron is magnetic) 2. a separating funnel (immiscible liquids) 3. simple distillation

9 Fractional distillation in detail

Separating miscible liquids.

In fractional distillation, the vapour rises through a fractionating column packed with glass beads. Repeated condensation and evaporation on the beads enriches the vapour in the more volatile component (lower boiling point), which reaches the top first and is condensed and collected.

The thermometer bulb is placed at the side arm so that it measures the temperature of the vapour entering the condenser. When the temperature stays constant, a pure fraction is being collected; when it rises, the next component is starting to distil.

WORKED EXAMPLE 6 Ethanol and water (standard)

Ethanol boils at 78 °C and water at 100 °C. Explain how fractional distillation separates them.

Solution

Ethanol has the lower boiling point, so it is more volatileR1

Its vapour reaches the top of the column first and is condensedR1

The temperature stays near 78 °C while ethanol distils, then risesR1

In the real world

Crude oil is separated into fractions (refinery gases, petrol, kerosene, diesel, fuel oil, bitumen) in huge fractionating towers. Each fraction contains hydrocarbons with similar boiling points.

Quick check

1. Why is the thermometer placed at the side arm, not in the liquid?

2. Why is water passed into the condenser at the bottom?

Answers: 1. to measure the temperature of the vapour being collected 2. so the condenser stays full of cold water and cools efficiently

10 Paper and thin-layer chromatography

Separating and identifying substances.

In chromatography, a mixture is carried by a mobile phase (a solvent) over a stationary phase (paper or a thin layer of silica on a plate). Components that are more soluble in the mobile phase, or less attracted to the stationary phase, travel further.

A substance can be identified by comparing its Rf value with that of a known reference run under the same conditions (same solvent and temperature). Colourless spots are made visible with ultraviolet light or a locating agent such as ninhydrin (for amino acids).

WORKED EXAMPLE 7 Identifying a dye (exam level)

A food colouring gives two spots with Rf values 0.32 and 0.75. Reference dyes have Rf values: E102 0.75, E110 0.55, E133 0.32. Identify the dyes present.

Solution

0.32 matches E133; 0.75 matches E102A1 A1

The food colouring contains E102 and E133A1

Common mistake

Drawing the start line in ink. Ink would itself separate in the solvent. The start line must be drawn in pencil, and must be above the solvent level.

Quick check

1. A spot moved 3.4 cm when the solvent moved 8.5 cm. Calculate Rf.

2. Why must the start line be above the level of the solvent?

Answers: 1. 0.40 2. otherwise the sample would dissolve into the solvent reservoir instead of moving up the paper

11 The kinetic molecular theory in detail

Assumptions and evidence.

The kinetic molecular theory states that: matter is made of particles in constant random motion; the average kinetic energy of the particles is proportional to the absolute temperature; particles collide elastically; and attractive forces exist between particles, which are strongest in solids and weakest in gases.

These assumptions explain the properties of the three states: solids have a fixed shape because particles vibrate about fixed positions; liquids flow because particles can slide past each other; gases fill their container and are compressible because the particles are far apart and move freely.

Property	Solid	Liquid	Gas
Arrangement	regular, close-packed	irregular, close	random, far apart
Movement	vibrate about fixed positions	slide past each other	fast, random, in all directions
Forces between particles	strong	moderate	negligible
Compressibility	very low	very low	high
Shape and volume	fixed shape and volume	fixed volume, takes shape of container	fills container

WORKED EXAMPLE 8 Explaining a property (standard)

Use the kinetic molecular theory to explain why gases exert pressure.

Solution

Gas particles move randomly at high speedR1

They collide with the walls of the container, exerting a forceR1

Pressure is the force per unit area from these collisionsR1

Quick check

1. Explain why liquids cannot easily be compressed.

2. Explain why a gas spreads to fill a room.

Answers: 1. particles are already close together 2. particles move randomly and the forces between them are negligible

12 Heating and cooling curves in detail

Reading the plateaus.

On a heating curve, sloping sections show a single state being heated: the energy supplied increases the average kinetic energy of the particles, so the temperature rises. Flat sections (plateaus) show a change of state: the energy is used to overcome the forces between particles (increasing potential energy), so the temperature stays constant.

The boiling plateau is usually longer than the melting plateau because much more energy is needed to separate the particles completely (liquid to gas) than to loosen them (solid to liquid). On a cooling curve, the plateaus occur at the same temperatures, and energy is released to the surroundings as forces form.

Figure 2. Heating curve of water: ice warms, melts, water warms, boils, steam warms.

WORKED EXAMPLE 9 Interpreting a cooling curve (exam level)

A liquid cools from 90 °C. Its temperature stays at 54 °C for 6 minutes, then falls again. Explain this.

Solution

At 54 °C the liquid is freezing (54 °C is its melting point)A1

Forces between particles form, releasing energyR1

The energy released balances the energy lost to the surroundings, so the temperature stays constantR1

Quick check

1. Why is the boiling plateau longer than the melting plateau for water?

2. During melting, what happens to the kinetic and potential energy of the particles?

Answers: 1. more energy is needed to separate particles completely than to loosen them 2. kinetic energy constant; potential energy increases

13 Evaporation and boiling

Two ways a liquid becomes a gas.

Evaporation happens at any temperature, only at the surface of a liquid: the particles with the highest kinetic energy escape, so the average kinetic energy of those left decreases and the liquid cools. Boiling happens at a fixed temperature (the boiling point) throughout the liquid, when bubbles of vapour form inside it.

Evaporation is faster at higher temperature, with a larger surface area, with wind (removing vapour) and for liquids with weaker intermolecular forces.

	Evaporation	Boiling
Temperature	any temperature	fixed (boiling point)
Where	at the surface only	throughout the liquid
Bubbles	no	yes
Rate	slow	fast

WORKED EXAMPLE 10 Cooling by evaporation (standard)

Explain why sweating cools the body.

Solution

The most energetic water particles escape from the skin surfaceR1

The average kinetic energy of the remaining particles falls, so the temperature fallsR1

In the real world

Clay water pots and evaporative coolers keep water and rooms cool in hot, dry climates by using the cooling effect of evaporation.

Quick check

1. Why does a puddle dry faster on a windy day?

2. Why does ethanol feel colder on the skin than water?

Answers: 1. wind removes vapour and increases evaporation 2. ethanol evaporates faster (weaker intermolecular forces)

14 Temperature, kinetic energy and absolute zero

Why kelvin is needed.

The kelvin scale starts at absolute zero (0 K = −273.15 °C), where particles have the minimum possible kinetic energy. The average kinetic energy of particles is directly proportional to the absolute temperature, so doubling the kelvin temperature doubles the average kinetic energy. A change of 1 K equals a change of 1 °C.

WORKED EXAMPLE 11 Conversions (basic)

Convert 25 °C and −196 °C (boiling point of nitrogen) to kelvin.

Solution

25 + 273 = 298 KA1

−196 + 273 = 77 KA1

WORKED EXAMPLE 12 Doubling kinetic energy (standard)

A gas is at 27 °C. At what Celsius temperature is the average kinetic energy of its particles doubled?

Solution

27 °C = 300 K; double: 600 KM1

600 − 273 = 327 °CA1

Common mistake

Thinking that doubling the Celsius temperature doubles the kinetic energy. Only the kelvin temperature is proportional to the average kinetic energy.

Quick check

1. Convert 0 K to °C.

2. Convert 373 K to °C.

3. Explain why negative kelvin temperatures are impossible.

Answers: 1. −273 °C 2. 100 °C 3. kinetic energy cannot be less than its minimum value at absolute zero

15 State symbols and physical changes

Writing equations correctly.

State symbols show the physical state of each species: (s) solid, (l) liquid, (g) gas, (aq) aqueous (dissolved in water). They are required in thermochemical equations and are good practice in all equations.

Changes of state are physical changes and can be written as equations, such as H2O(s) → H2O(l) for melting, and I2(s) → I2(g) for sublimation.

WORKED EXAMPLE 13 Writing state symbols (standard)

Write an equation with state symbols for magnesium reacting with hydrochloric acid to give magnesium chloride solution and hydrogen.

Solution

Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g)A2

Quick check

1. Write an equation with state symbols for the deposition of iodine.

2. What is the difference between (l) and (aq)?

Answers: 1. I2(g) → I2(s) 2. (l) is a pure liquid; (aq) is dissolved in water

16 Physical and chemical changes

Has a new substance formed?

In a physical change, no new substance is formed; the change is usually easily reversed (melting, dissolving, boiling). In a chemical change, new substances with different properties are formed; there is often a colour change, gas production, a precipitate or a temperature change, and it is usually difficult to reverse.

WORKED EXAMPLE 14 Classifying changes (standard)

Classify: (a) dissolving sugar; (b) burning magnesium; (c) rusting iron; (d) condensing steam.

Solution

(a) and (d): physicalA1

(b) and (c): chemical (new substances: MgO, hydrated iron(III) oxide)A1

Quick check

1. Is heating copper(II) sulfate crystals until they turn white a physical or chemical change? Explain.

Answers: 1. chemical: water of crystallization is removed and a new substance (anhydrous CuSO4) forms

17 Mixtures in everyday life

Alloys, solutions, colloids.

Many useful materials are mixtures: alloys (such as brass, steel and solder) are homogeneous mixtures of metals; solutions (such as seawater and vinegar) are homogeneous mixtures of a solute in a solvent; colloids and suspensions (such as milk, fog and muddy water) are heterogeneous on a small scale.

Because the composition of a mixture can be varied, its properties can be tuned: adding more carbon to steel makes it harder but more brittle.

In the real world

Air is a mixture of about 78% nitrogen, 21% oxygen, 0.9% argon and 0.04% carbon dioxide. These gases are separated industrially by fractional distillation of liquefied air.

Quick check

1. Explain why brass is classified as a mixture, not a compound.

2. Name the method used to separate the gases in air.

Answers: 1. copper and zinc are not chemically combined and the proportions can vary 2. fractional distillation of liquid air

18 Practical skills: separations in the lab

Doing it safely and well.

Good practice in separation experiments includes: using fluted filter paper for faster filtration; washing the residue with a small amount of cold solvent to remove dissolved impurities; heating solutions gently (water bath) to avoid spitting; using anti-bumping granules when distilling; and never heating a flammable liquid with a naked flame.

WORKED EXAMPLE 15 Evaluating a method (exam level)

A student obtains salt from rock salt but the yield is low. Suggest two reasons.

Solution

Not all salt dissolved (too little water or stirring)R1

Salt lost in the residue or by spitting during evaporationR1

Try this

Draw a simple chromatogram with spots from several felt-tip pens on a strip of filter paper. Stand it in a beaker with a little water and watch the inks separate. Which colours are mixtures?

19 Diffusion experiments

Evidence that particles move.

In the classic ammonia and hydrogen chloride experiment, cotton wool soaked in concentrated ammonia solution and concentrated hydrochloric acid are placed at opposite ends of a long glass tube. A white ring of ammonium chloride forms where the gases meet: NH3(g) + HCl(g) → NH4Cl(s).

The ring forms closer to the hydrochloric acid end, because ammonia molecules (M = 17 g mol−1) are lighter than hydrogen chloride molecules (M = 36.5 g mol−1) and, at the same temperature, move faster on average. Diffusion is also faster at higher temperature, because the particles have more kinetic energy.

WORKED EXAMPLE 16 Predicting the ring (standard)

In a 100 cm tube, explain where the white ring forms and why.

Solution

Closer to the HCl end (about 60 cm from the ammonia end)A1

NH3 has a lower molar mass, so its molecules move faster at the same temperatureR1

Common mistake

Saying the gases 'flow' along the tube. They diffuse: random motion and collisions with air molecules cause a slow net spreading from high to low concentration.

Quick check

1. Why does diffusion take several minutes even though gas molecules move at hundreds of metres per second?

2. Why does a crystal of potassium manganate(VII) colour water faster in hot water?

Answers: 1. molecules collide with air molecules and change direction constantly 2. particles move faster at higher temperature

20 Mixed review: worked problems

Bringing the ideas together.

WORKED EXAMPLE 17 Mixed problem (exam level)

A substance X melts at −95 °C and boils at 56 °C. (a) State its physical state at 25 °C and at −100 °C. (b) Convert its boiling point to kelvin. (c) Sketch and label a heating curve from −120 °C to 80 °C.

Solution

(a) liquid at 25 °C; solid at −100 °CA1 A1

(b) 329 KA1

Quick check

1. Bromine melts at −7 °C and boils at 59 °C. What is its state at 0 °C and at 70 °C?

2. Which separation method would you use to obtain ethanol (boiling point 78 °C) from wine?

Answers: 1. liquid; gas 2. fractional distillation

21 Additional exam-style practice

Paper 1 and Paper 2 questions.

WORKED EXAMPLE 18 Paper 1 question (standard)

Which of the following is a heterogeneous mixture? A. brass B. air C. sea water D. sand and water

Solution

D: the sand and water are visibly separate phasesA1

WORKED EXAMPLE 19 Paper 2 question (exam level)

A student separates a mixture of two inks by paper chromatography. Ink P gives spots at 2.1 cm and 5.6 cm; the solvent front is at 7.0 cm. (a) Calculate both Rf values. (b) State what the result shows about ink P. (c) Suggest why the experiment should be repeated with a different solvent.

Solution

(a) 0.30 and 0.80A1 A1

(b) P is a mixture of at least two dyesR1

Quick check

1. Which change is exothermic: melting, boiling, sublimation or condensation?

Answers: 1. condensation

22 Common misconceptions and links

Students often think…	Actually…
air is a compound	air is a mixture
temperature stays constant during state changes because no energy is supplied	energy is used to overcome forces between particles
boiling breaks covalent bonds	it overcomes intermolecular forces
0 °C is absolute zero	absolute zero is 0 K = −273 °C

How S1.1 connects to the rest of the course

Topic	Connection
S1.5 Ideal gases	kinetic theory and kelvin
S2.2 Covalent model	intermolecular forces
R1.1 Enthalpy	endothermic and exothermic changes
S3.2 Functional groups	chromatography and distillation

23 Exam-style question with markscheme

Try this under exam conditions (about 15 minutes) before reading the markscheme.

Figure 3. Heating curve of a substance.

(a) State the melting and boiling points of the substance. [2]

(b) Explain the shape of the curve between 4 and 8 minutes in terms of particles. [2]

(d) Describe how to obtain pure water from salt solution. [3]

Markscheme

Part	Answer	Marks
(a)	melting point 0 °C	A1
	boiling point 100 °C	A1
(b)	liquid heated	R1
	average kinetic energy of particles increases	R1
(c)	boiling	R1
	energy overcomes intermolecular forces	R1
	kinetic energy (temperature) unchanged	R1
(d)	simple distillation: heat solution	R1
	water vapour condensed in condenser	R1
	pure water collected; salt remains	R1

Examiner's comments. In (c), answers that said 'no energy is being absorbed' lost marks.

24 Check your understanding

Section A: multiple choice

1. Which is a compound?

A. water B. air C. brass D. oxygen

2. A homogeneous mixture is

A. sand and water B. oil and water C. salt solution D. granite

3. Fractional distillation separates

A. insoluble solids B. miscible liquids with different boiling points C. dyes D. salt from water only

4. Particles in a gas

A. vibrate in fixed positions B. are close together C. do not move D. move rapidly with negligible forces between them

5. Sublimation is

A. liquid to gas B. solid to gas C. gas to liquid D. solid to liquid

6. −73 °C in kelvin is

A. 200 K B. 346 K C. −200 K D. 73 K

Section B: structured

7. Distinguish elements, compounds and mixtures. [3]

8. Explain how paper chromatography separates dyes. [3]

9. Use the kinetic molecular theory to explain why gases can be compressed. [2]

Answers

1. A 2. C 3. B 4. D 5. B 6. A

7. Element: one type of atom; compound: elements chemically combined in fixed ratio; mixture: not combined, variable ratio.

8. Mobile solvent; stationary paper; different solubilities and attractions; different distances; Rf values.

9. Particles far apart with large spaces; can be pushed closer.

25 Summary and topic map

Idea	Remember
Element / compound / mixture	one atom type / fixed ratio combined / not combined
Homogeneous / heterogeneous	uniform / non-uniform
Separation	filtration, crystallization, distillation, chromatography
States	solid, liquid, gas: arrangement, movement, forces
Changes of state	melting, boiling endothermic; freezing, condensation exothermic
Kelvin	T(K) = T(°C) + 273; absolute zero

Before the exam, make sure you…

•classify substances precisely
•match separation method to property
•explain flat sections of heating curves
•use kelvin for temperature calculations

Particulate Nature of Matter on one page

Cover this page with a sheet of paper and try to rebuild it from memory. Whatever you can't recall is what to revise next.

Figure 4. Topic map for Particulate Nature of Matter.

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