On burning candle,its particles get energy and start vibration with more force.Particles of candle get free one another and move to some distances.Now they are not in fixed positions,but are still close to each other.
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:
Breakdowns where at a point of instability the constituent particles (molecules) might break down often as a result of heating
Combinations of different types of particles to form new ones or
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
colour change
texture change (irreversible)
formation of solid, liquid or gas bubbles (effervescence)