Cambridge IGCSE Chemistry Topic 1: The particulate nature of matter


State the distinguishing properties of solids, liquids and gases

Solids

  • Strong forces of attraction between particles, particles are packed very closely together in a fixed and regular pattern.
  • Atoms vibrate in position but can’t change position or move.
  • Solids have a fixed volume, shape and high density.

Liquids

  • Weaker attractive forces in liquids than in solids, particles are close together in an irregular, unfixed pattern.
  • Particles can move and slide past each other which is why liquids adopt the shape of the container they’re in and also why they are able to flow.
  • Liquids have a fixed volume but not a fixed shape and have a moderate to high density.

Gases

  • No intermolecular forces and, since particles are in random movement, there is no defined pattern.
  • Particles are far apart and move quickly (around 500 m/s) in all directions, they collide with each other and with the sides of the container (this is how pressure is created inside a can of gas).
  • No fixed volume, since there is a lot of space between the particles, gases can be compressed into a much smaller volume. Gases have low density.

The arrangement of particles in solids, liquids and gases


● The three states of matter are solid, liquid and gas
● Melting and freezing take place at the melting point
● Boiling and condensing take place at the boiling point
● They can be represented by a simple model , particles are represented
by small solid spheres
● solids- particles have a regular arrangement and are close together
● liquids- particles have a random arrangement and are close together
● gases- particles have a random arrangement and are spread apart
Describe the structure of solids, liquids and gases in terms of particle
separation arrangement and types of motion
● Gas: particles have the most energy – shown by the diagram, as the particles are
the most spread apart, motion is more random and frequent
● Liquid: particles have more energy than those in a solid, but less than those in a
gas
● Solid has least energy – particles are not moving/are just vibrating
Describe changes of state in terms of melting, boiling, evaporation, freezing,
condensation and sublimation
● Physical changes – therefore involves the forces between the particles of the
substances, instead of these changes of state being chemical changes
o Evaporation = happens at the surface, molecules have enough energy to
evaporate – i.e. go from liquid to gas
o Freezing = liquid to solid
o Melting = solid to liquid
o Boiling = happens throughout the liquid, liquid to gas
o Condensation = gas to liquid
o Sublimation = solid to gas

(Extended only) Explain changes of state in terms of the kinetic theory
● Kinetic theory can help to explain melting, boiling, freezing and condensing…
o The amount of energy needed to change state from solid to liquid and
from liquid to gas depends on the strength of the forces between the
particles of the substance.
o The nature of the particles involved depends on the type of bonding and
the structure of the substance.
o The stronger the forces between the particles the higher the melting
point and boiling point of the substance.
o The more kinetic energy (from increased temperature) particles have, the
more movement, which causes a change of state from (s) to (l) to (g)
Describe qualitatively the pressure and temperature of a gas in terms of the
motion of its particles
● The higher the pressure = the more motion of a gas’ particles
● The higher the temperature = the more motion of a gas’ particles
Show an understanding of the random motion of particles in a suspension
(sometimes known as Brownian motion) as evidence for the kinetic particle
(atoms, molecules or ions) model of matter
● Particles in liquids and gases (known as fluids) move randomly (this is called
Brownian motion)
● This happens because they collide with other moving particles in the fluid
● This is evidence for the kinetic particle model of matter- it shows that there are
individual particles which make up solids/liquids/gases
(Extended only) Describe and explain Brownian motion in terms of random
molecular bombardment
● Particles in liquids and gases move randomly because they are bombarded by
the other moving particles in the fluid. Larger particles can be moved by light,
fast-moving molecules
(Extended only) State evidence for Brownian motion
● Robert Brown observed the random movement of pollen grains within water,
which showed that there were separate particles within the water that were
moving randomly and caused the grain to move (kinetic theory)

Describe and explain diffusion
● Movement of particles from an area of high concentration to an area of low
concentration
● For this to work, particles must be able to move
o Therefore, diffusion does not occur in solids, since the particles cannot
move from place to place (only vibrate)
o A smell does not travel very fast, because the particles collide with
particles of air, changing direction randomly when they collide, taking
much longer to travel from place to place
(Extended only) Describe and explain dependence of rate of diffusion on
molecular mass
● The smaller the molecular mass, the greater the average speed of the molecules
(but all gases have the same average kinetic energy at the same temperature)
o Therefore, the smaller the molecular mass, the faster the gas diffuses

Apparatus used in Chemistry

Name appropriate apparatus for the measurement of time, temperature,
mass and volume, including burettes, pipettes and measuring cylinders
● Appropriate apparatus for measuring…
o Time: stopwatch/clock
o Temperature: thermometer
o Mass: balance
o Volume: measuring cylinder, burette, pipette

Experimental Techniques and Criteria of purity

Demonstrate knowledge and understanding of paper chromatography


● Paper chromatography:
o Used to separate mixtures and give information to help identify
substances
o Involves a stationary phase and a mobile phase
o Separation depends on the distribution of substances between the
phases
o To carry it out: place substances on line near bottom of paper, place in
solvent and observe how far the substances travel up the paper
Interpret simple chromatograms
● if two substances are the same, they will show up the same on the paper after
carrying out the chromatogram
● if the substance is a mixture, it will separate on the paper to show all the
different components as separate spots
● an impure substance will show up with more than one spot, a pure substance
should only show up with one spot
Paper Chromatography is an Analytical technique separating compounds by their
relative speeds in a solvent as it spreads through paper.
The more soluble a substance is, the further up the paper
it travels.
Separates different pigments in a coloured substance.
Pigment Solid, coloured substance
Including the use of Rf values
● Rf value = distance moved by substance ÷ distance moved by solvent
● Different compounds have different Rf values in different solvents, which can be
used to help identify the compounds
o Compounds in a mixture may separate into different spots depending on
the solvent but a pure compound will produce a single spot in all solvents

(Extended only) Outline how chromatography techniques can be applied to
colourless substances by exposing chromatograms to substances called
locating agents; knowledge of specific locating agents is not required
● You can analyse colourless mixtures of chemicals if the ‘spots’ can be coloured
by a chemical or light treatment
o Examples include…
▪ Ninhydrin used with proteins – breaks them down into amino
acids and colours them purple
▪ UV light – fluoresce many colourless organic molecules
▪ (you don’t need to remember these specific locating agents)
o These are all known as locating agents, allowing Rf values to be taken and
(previously colourless) molecules to be identified
Identify substances and assess their purity from melting point and boiling
point information
● Pure substances melt and boil at specific temperatures
o This melting and boiling points data can be used to distinguish pure
substances from mixtures (which melt over a range of temperatures due
to them consisting of 2 or more elements or compounds)
Understand the importance of purity in substances in everyday life, e.g.
foodstuffs and drugs
● A mixture:
o Consists of 2 or more elements or compounds not chemically combined
together
o Chemical properties of each substance in the mixture are unchanged
● A pure substance = a single element or compound, not mixed with any other
substance
● In everyday language, a pure substance = substance that has had nothing added
to it, so it is unadulterated and in its natural state, e.g. pure milk
● to have a pure substance for food or drugs is very important as impurities could
be dangerous even in small amounts

SEPARATION METHODS

Describe and explain methods of purification by the use of a suitable
solvent, filtration, crystallisation and distillation (including use of
fractionating column). ; (Refer to the fractional distillation of petroleum and
products of fermentation)


● Solvent = liquid in which a solute dissolves
o Need a suitable solvent to ensure the liquid dissolves – preventing any
impurities from dissolving with the pure liquid
● Filtration


o If you have produced e.g. a precipitate (which is an insoluble salt), you
would want to separate the salt/precipitate from the salt solution.
▪ You would do this by filtering the solution, leaving behind the
precipitate


● Crystallisation
o If you were to have produced a soluble salt and you wanted to separate
this salt from the solution that it was dissolved in
▪ You would first warm the solution in an open container, allowing
the solvent to evaporate, leaving a saturated solution
▪ Allow this solution to cool
▪ The solid will come out of the solution and crystals will start to
grow, these can then be collected and allowed to dry
● Simple distillation


o Used to separate a pure liquid from a mixture of liquids
▪ Works when the liquids have different boiling points
▪ Commonly used to separate ethanol from water
▪ (Taking the example of ethanol…) ethanol has a lower bp than
water so it evaporates first. The ethanol vapour is then cooled and
condensed inside the condenser to form a pure liquid.
▪ Sequence of events in distillation is as follows: heating ->
evaporating -> cooling -> condensing


● Fractional distillation
o The oil is heated in the fractionating column and the oil evaporates and
condenses at a number of different temperatures.
o The many hydrocarbons in crude oil can be separated into fractions each
of which contains molecules with a similar number of carbon atoms
o The fractionating column works continuously, heated crude oil is piped in
at the bottom. The vaporised oil rises up the column and the various
fractions are constantly tapped off at the different levels where they
condense.
o The fractions can be processed to produce fuels and feedstock for the
petrochemical industry.


Suggest suitable purification techniques, given information about the
substances involved
● use information given above in application to a problem/question with given
information about the substances involved

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