Melting point
| Y10 (08/2009) | |||
| a) | i) | Yes | moderate |
| ii) |
Candles made from the same substance will have the same
melting point. Related partially correct ideas (the candles are assumed to be made from the same substance), e.g.,
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very difficult very difficult |
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Why do students need to know about the melting points of substances?
Different substances have different properties and this means they are useful for different things. Melting point is one property which is used to decide if a substance will be useful for a particular purpose. For example electrical insulators and pots and pans need to be made from a substance with a high melting point, and candles and cooking fats require a substance with a low melting point.
One of the purposes of science is to develop better explanations. Therefore shaping an explanation about the science ideas is an important Nature of science idea related to understanding about science.
| Common response | Likely misconception |
| More than half of the students answered that different sized candles will have different melting points. |
Most students confused the rate of melting with the melting point (temperature at which a solid changes to its liquid form) of a substance, e.g., A bigger candle will take longer to melt than a smaller candle. |
Examples of good explanations
Students who gave the following explanations recognised that the candles needed to be of the same substance to have the same melting point, e.g.,
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Because most candles are made out of the same substance they melt at the same temperature.
If the candles are made out of the same material they will have the same melting point no matter how much of it there is. |
and that the other variables affect the rate of melting, e.g.,
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As long as the wax is the same throughout it should melt at the same temperature provided all other factors are the same, i.e. flame, heat. |
Examples of explanations that were not sufficient
Some students recognised that temperature was a factor but they did not connect this to differences in melting points of different substances as the reason for temperature related differences:
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It doesn’t matter the size of the candles but it does matter the temperature of the heat being used. |
Other students did understand melting point but they assumed all candles are made of the same substance, e.g.,
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Candles all have the same melting point just it will take longer to burn if the candle is longer. |
Rate of melting and melting point
Students need to distinguish between the rate at which a substance melts and its melting point. Most students focused on the time a substance took to completely melt rather than the temperature at which the substance begins to melt i.e. changing from a solid state into a liquid state. To help students understand melting point they could:
| 1. |
Investigate the melting points of various substances, e.g., different types of wax.
Note: Palm wax, gels, synthetic waxes, and synthesized waxes are also now used in candles, as are a variety of wax blends and customized wax formulations. Therefore many candles are not a pure substance but a mixture of different compounds so their melting points vary. |
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| 2. | Discuss, by looking at the particle theory, why some substances have higher or lower melting points than other substances (each substance has unique particles that are different to the particles of other substances and also behave differently). | |||||||||||||||||||||
| 3. | Research the melting points of various substances (i.e., plastics, metals, etc) and determine how their structure (formulae) affects their melting point. | |||||||||||||||||||||