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Particle Theory

Topics covered

Physical Changes

Chemical Changes

Physical Changes

Changes of state such as melting or boiling are physical changes and are generally easy to reverse though the end product may not always look exactly the same as the starting material. In physical changes no new materials are formed and the particles do not change apart from gaining or losing energy. Melted wax solidifies when cooled but unless it is shaped or moulded it will not be the same shape as at the start. On a microscopic level although the same particles are present they may be in different places within the solid.
Ice is made up of particles of water. When it melts the water which is formed is made up the same water particles and when it boils the steam is also made up of the same water particles. Particles stay the same unless there is a chemical change whether the matter is solid, liquid or gas. Only their arrangement, energy and movement changes.
When substances change state there is no change in mass so if 100 g of ice is melted 100g of water are formed this will boil to form 100g of steam (this is called "conservation of mass"). If this steam could be collected, cooled and condensed it would form 100g of water which could be frozen to give 100g of ice. Children often think that solids get lighter when they melt as liquids are lighter than solids.
Dissolving is a reversible process represented by considering particles mixing closely together and reversed by separating the particles. Some further interpretation must be given in to try to explain why some materials are soluble and some are not. When a solid is mixed with a solvent the solid will dissolve if its particles and the solvent particles are attracted together. Particles in sugar are able to form weak links to water particles. Consequently sugar is soluble in water. Sand on the other hand is insoluble as sand particles are not able to form weak links with the water particles.

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Chemical Changes

For some materials chemical changes rather than physical changes are initiated by heat. In chemical changes new substances are formed and the process is often difficult to reverse. During chemical changes particles do change with atoms or ions regrouping . Bonds (links) between atoms break and new ones form and energy is either given out or taken in. Some chemical changes are initiated by mixing.
The overall changes can be:

Re-arrangement of particles either by rearrangement within complex particles or as a result of breakdown and combination
The particle model can be adapted to represent these changes but only if different particles are represented in different ways and if complex particles are represented as such.
The chemical changes most likely to be encountered by primary children are those involving cooking and those involving combustion (burning). Changes due to cooking are often complex as the substances contained within foods are complex but they do provide vivid examples of changes with which children may be familiar. Common examples include eggs and cake mixtures going hard when they are cooked or potatoes and other vegetables going soft. Generally those changes are not reversible. Children may be familiar with combustion reactions such as burning candles, wood or other fuel.
In chemical changes there are often signs of change such as

But it is not always easy to see exactly what has happened during a chemical change.
Sometimes the reactants (starting materials) and products (end materials) may be difficult to see especially if they are gases For example when petrol (a liquid hydrocarbon made up of carbon and hydrogen) burns it reacts with oxygen gas (from the air) to produce water (vapour/ liquid) and carbon dioxide(gas). So within this reaction there are two reactants and two product and two of these are colourless gases (oxygen and carbon dioxide) and the water formed as a result of the burning may be formed as steam and be difficult to see. Consequently all that would be observed apart from the flame is that the petrol is disappearing.
The rate of chemical changes varies enormously from those which are very slow (such as rusting) to those which happen instantaneously (such as fireworks exploding).
In chemical changes the amount of particular substances may change but the total amount of materials does not. In chemical changes as in physical changes matter cannot disappear nor appear from nowhere. It can only react to form something else. So if 16g of methane gas burns in oxygen (a total of 80g the resulting products will weigh 80g i.e.44g of carbon dioxide and 36g of water).

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Website maintainer: R. Jones Updated: November 13, 2000