C1.1 The particle model
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C1.1 The particle model
C1.1a Describe the main features of the particle model in terms of states of matter
Learners should have some familiarity with the use of the particle model to explain the different states of matter. They will therefore know that:
i) solids comprise particles in fixed arrays with movement restricted to vibrations around fixed points
ii) liquid particles have a greater freedom of movement and are able to move in any direction around each other allowing liquids to flow into and take the shape of their container
iii) gas particles have a far greater freedom of movement and can be far apart from each other. Gas particles move to completely fill the container they are held in and will exert a pressure when held in a closed container by means of impact on the walls of the container.
Neither solids nor liquids can be compressed as the particles are very close together while the spaces between particles in a gas allow it to be compressed.
C1.1b Explain in terms of the particle model the distinction between physical changes and chemical changes:
Learners must be clear about the difference between a physical and a chemical change. Physical changes are brought about by differences in energy within a single substance. For example, as the energy increases, particles gain energy to the point that the number of attractions between particles become less. Therefore, a solid can change to a liquid or a gas, a liquid can change to a solid or a gas and a gas can become a solid or a liquid depending on whether energy is being gained or lost. These changes are all reversible with no change in the mass of the substance.
Chemical change results from collisions between particles at an appropriate energy level. The result of such collisions is the production of one or more new substances and a change in the nature of the particles present. While some reactions may be reversible, many are not. They also bring about changes in the mass and the properties of the new particles present.
C1.1c explain the limitations of the particle model in relation to changes of state when particles are represented by inelastic spheres (e.g. like bowling balls)
that it does not take into account the forces of attraction between particles, the size of particles and the space between them
Learners are required to understand that the particle model is simplistic in its treatment of changes in state and although it gives a clear idea of what might happen, in its most basic form it takes no account of the nature of the particles involved.
Particles can vary in many ways, such as size, mass and their power of attraction between themselves and other different particles. Larger particles with greater mass tend to have a greater momentum though they usually need more energy to acquire that momentum.
The forces of attraction that hold a material together can vary from the various types of bonding, from the very strong to the much weaker interatomic forces. Metals, with a few exceptions, have high melting and boiling points, as do those substances that exist in large lattice arrangements such as sodium chloride and diamond. Meanwhile, molecular and atomic substances such as sulfur and argon rely solely on Van der Waal’s forces to hold them together and have low melting and boiling points as a result.
Approaches to teaching the content
Particle theory is fundamental to understanding a wide range of chemical kinetic ideas and properties hence it is essential that learners get a clear sense of what particles are and how they interact. The basic idea of particles behaving with perfect elasticity needs to be extended to cover real-life situations where this is not the case. All matter differs in their properties depending on the interactions between the particles making up that matter.
Common misconceptions or difficulties learners may have
Learners carry several misconceptions into their dealings with particles. These include the idea that matter is continuous. The particles involved are so small that any spaces between them cannot be observed therefore learners find it difficult to accept that there can be a huge amount of empty space between gas particles. Learners also think that particles expand as they are heated and that they change mass as they change between states.
Activities address these misconceptions.
Conceptual links to other areas of the specification – useful ways to approach this topic to set learners up for topics later in the course
Ideas of energy versus particle movement build into ideas regarding reaction kinetics and bond breaking and making during chemical reactions. A good grounding in particle theory allows learners the ability to cope with visualising collisions during reactions and the differences that might be seen between particles of different character. Building a strong mental image is a means of ensuring a higher level of understanding.
- Take three identical 500 cm3 glass jars and lids and a number of 19 mm polythene spheres. Label the jars 1, 2 and 3.
- Put 10 spheres into jar 1, fill jar 2 to one-third full and fill jar 3 to the top.
- Seal the lids using adhesive tape (insulation is best).
Jar 1 should be shaken vigorously and observations made of the behaviour of the spheres in terms of contact between the spheres themselves, the spheres and the walls of the container and the amount of space between them.
Jar 2 should be swirled around to show that the level of the ‘liquid’ remains constant while the learner observes the freedom of movement of the spheres and the continuous contact between them.
Jar 3 should be shaken gently to allow vibration to happen. Learners should see that the spheres stack into an arrangement showing layers stacked onto layers and when shaken the spheres should not be free to move beyond vibrating.
This activity is to demonstrate that particles do not expand but simply have a greater degree of freedom of movement due to having greater energy. It also demonstrates the difference in energy between the three states.
Approaches to teaching the content
'Compression' is designed to address our inability to see ‘real’ particles.
'Compression' looks at the physical properties that we can see resulting from the arrangement of particles in the three states of matter and it should be clear that gas particles have space between them as they are compressible.
'Particles in motion' uses a model that the learners can build for themselves to observe the differences. When they draw diagrams they should be discouraged from putting spaces between liquid
'What do chemical words mean?' is a fully documented AfL activity from the RSC that looks at the language of particles and should encourage a deeper understanding of some of the differences between particles.
'Chemical change' uses small segments of Dr Peter Wothers’ RI Christmas lecture to highlight some fun aspects of particles (with a serious edge!).
'Particles in motion' and 'Experiments with particles' both develop the ideas on particles further by looking into gas diffusion and mixing respectively and are designed to reinforce the idea of space between particles.
'States of matter' is a learner–produced YouTube video that gives a sound overview of this whole area which leads learners to other videos covering other areas of the specification.
Learners are provided with plaster of Paris, distilled water, air (or helium from a balloon cylinder), three 25 cm3 syringes labelled 1, 2, 3, and Blue tac to seal the ends of the syringes.
- Mix the plaster of Paris as required and suck into syringe 1, wipe the inlet to the syringe and seal with blue tac. Put aside to set.
- Fill syringe 2 with 25 cm3 of distilled water ensuring there is no air between the plunger and the liquid. Seal with blue tac.
- Fill syringe 3 either by connecting this to the helium cylinder or by sucking in 25 cm3 of air. Seal with blue tac.
- When the plaster of Paris has set, compress each syringe in turn using the plunger and record the movement seen in cm3 for each syringe.
Learners should be encouraged to discuss their hypothesis as to what will happen before carrying this out and subsequently discuss their findings using the particle model.
If using air, be sure to discuss the gases that might be present in the syringe to discourage the thought that it might be empty.
The reaction of plaster of Paris and water is exothermic; avoid any contact with eyes or skin while still wet.
If using helium, this must not be allowed to be inhaled as it may be harmful.
For helium: Normal laboratory suppliers
A teaching resource linked to Peter Wother’s RI Christmas Lectures of 2012.
Contains some dramatic moments.
Classic Chemistry Experiments No 36.
When materials are added together, they may acquire new properties. When a solid and a liquid are mixed, the solid may or may not dissolve. When two liquids are mixed they may become one liquid or stay separate.
These experiments provide an opportunity to predict and then observe what happens.
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