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Chemistry IGCSE Revision Notes - Chapter 1 –The Particulate Nature of Matter

                Matter

·         Anything that has mass and occupies space (has volume)

·         Matter is composed of particles (molecules, ions, atoms)

o        Spaced apart and seen with scanning electron microscope

o        Are in constant motion attracting one another with inter-particle forces (or cohesive)

o        Strength of interparticle force and space between particles determines state

·         Universe à matter or energy

Kinetic Particle Theory

·         Matter is made up of particles that are in constant motion

·         The higher the temperature, the faster they move (more energy)

·         Temp increase weakens interparticle forces, particles spread apart and increase in volume/size (ie. Expansion)

·         Gases have greatest average energy

·         Solids have smallest average energy

·         Average energy is measure of temperature

Solids

·         Have greater resistance to penetration b other materials (particles packed tight together in fixed pattern) and are hard to touch

·         Definite shape and volume

·         Only vibrate

·         When heated, solid particles vibrate more vigorously, bonds weaken and will expand

Liquids

·         Move freely because their particles slide over each other as they move (interparticle forces are weak but keep particles in close range) – soft to touch

·         No definite shape

·         Definite volume cannot be squashed

·         Can flow because interparticle forces between liquid particles are weak and so the particles can slide over/past each other.

Gases

·         Offer least resistance

·         Occupies greater volume than mass of solids/liquids

·         Particles are widely spaced apart (weak interparticle forces) and move with great speed

·         No fixed volume, no fixed shape

·         Only restricted by shape of room/container

·         Particles are far apart and can be pushed together (can be easily compressed)

·         Move around easily, quickly and randomly colliding with each other and bounce off, spacing out.

Low Temp                                                         High Temp

Low Energy                                                        High Energy

Order                                                                Disorder

         SOLID –- melting à  LIQUID  -- boiling à   GAS

         SOLID ß freezing -- LIQUID  ß condensing -- GAS

 Diffusion

·         The movement by which substances disperse themselves spontaneously

·         Two gases eventually become mixed when they are mixed

·         The movement of particles of fluid from a region of higher concentration to a region of lower concentration in order to maintain a constant concentration in the given space.

Demonstrating Diffusion in Gas:

·         A jar of dense green gas, chlorine, is put underneath a jar of air

·         Because gases consist of fast-moving particles, when the jars are exposed to each other, molecules of air and chlorine spread themselves between the two jars

·         With time, both gas jars consist of green gas of uniform intensity

·                      Also, an aerosol spray can be released at the front of a room, with time the occupants at the back of the room can smell the spray because the gas moved gradually to the back of the room.

Demonstrating Diffusion in Liquids:

·                      When a potassium permanganate crystal is suspended at the bottom of a beaker, ¾ filled with water, the permanganate crystals will tend to disperse themselves evenly throughout the beaker. After 2 hours, a purple solution of the same shade was observed.

·                      Precautions (a) Do not stir or shake the mixture in anyway that may aid it to mix

                        (b) Provide a short distance for diffusion, in other words do not over fill the beaker, prolonging the time taken for distribution.

The Rate of Diffusion

·                      Gases with larger molecules have higher densities than gases which have small molecules.

·                      E.g. Chlorine molecules are 35.5 larger than hydrogen molecules and chlorine is denser than hydrogen.

·                      Large heavy  molecules move more slowly than small, light molecules

·                      Therefore, dense gases diffuse more slowly than gases of low density

·                      The rate of diffusion depends on the molecular mass/density of gas

·                      Rate of diffusion is inversely proportional to mass

Demonstrating the Rate of Diffusion

If particles of ammonia gas half the mass of particles of hydrogen chloride gas are soaked in cotton wool and placed at two ends of a tube (corked on both sides) then;

When these gases come in contact, white smoke of Ammonium Chloride forms. Because the smoke forms closer to the HCl, it shows that ammonia gas molecules traveled faster in the same length of time.