States of matter: Exploring changes of state
Objectives
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Associate the rate of evaporation with temperature.
Measure the temperature at which materials change state.
Plenary
Ask the pupils to discuss how these changes of state are affected by the weather:
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Will laundry dry faster or slower in the sun?
Will a snowman melt faster or slower in the sun?
Will a puddle evaporate faster or slower in the sun?
Why may these processes be faster on a sunny day? Pupils may suggest that the sun’s heat
speeds up the processes.
Rate of evaporation
EXPERIMENT: In pairs, pupils should put water in two beakers (same amount in both).
Place one beaker in a cool, dark location and the other in a warm, bright location, such as on
a radiator or in direct sunlight. Throughout the lesson, pupils should check their beakers and
record how much water has evaporated. At the end of the lesson, they should compare how
much water has evaporated from the beakers in the two locations and draw a conclusion
about the relationship between temperature and rate of evaporation.
Pupils must note:
Evaporation is faster at higher temperatures.
Change of state in other materials
So far, pupils have only observed changes of state in water. However, many other materials
can be melted, evaporated/boiled, condensed and frozen. We only see metals as strictly solid
because they are solid at room temperature, which is how we are used to seeing them.
We measure temperature in degrees Celsius (°C). 0ºC is the temperature at which water
freezes and 100ºC is the temperature at which water boils. We are used to experiencing room
temperature, which is about 21°C. By measuring things in ºC, we can tell how hot or cold
they are. Things can be much hotter than 100°C or much colder than 0ºC (when we start
using negative numbers).
We do not see many metals melt because metals have very high melting points; the
temperature has to be hundreds or thousands of degrees (°C) before they turn from solids to
liquids. The reason we do not see many gases condense is because they have very low
boiling points so they turn from liquids to gases at temperatures much lower than 0ºC.
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Pupils should use the worksheet to gain a sense of what temperatures are required for various
changes of state.
Boiling point of water: 100°C. Room temperature: 21°C. Melting point of water: 0°C. Body
temperature: 37°C. Melting point of iron: 1540°C. Boiling point of nitrogen: -196°C.
Pupils may note:
We measure temperature in degrees Celsius (°C). Metals have high melting points and gases have
low boiling points.
The melting point of chocolate
When you are cooking, do you need to raise or lower temperature in the following situations?
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To prepare a cheese fondue, need to melt cheese.
To make ice lollies, need to freeze fruit juice.
To steam fish, need to boil water.
To make chocolate dipped strawberries, need to melt chocolate and then let it set.
While the water evaporation experiment continues, another experiment can be carried out to
find the melting point of chocolate. Ask pupils to help break chocolate into small pieces (put
the chocolate in a bag and smack it with heavy objects).
EXPERIMENT: If a stove is available, double boil the chocolate. If a stove is not available,
melt the chocolate in a fondue arrangement using a bowl above a candle. Once the chocolate
becomes soft, begin measuring its temperature with a thermometer every 60 seconds. All
pupils should try using this new measuring tool. A melting temperature of 45ºC should be
found.
Add cornflakes to the chocolate to make chocolate crispy cakes. As they cool, point out that
as the temperature is lowered, the chocolate turns from a liquid to a solid.
Possible extra-curricular questions
Why does water evaporate faster at a high temperature?
A higher temperature means that more heat energy is available. This heat energy is used by
molecules in the liquid to help break out of its arrangement (described in previous lesson
plan). When water molecules escape and whiz around, this is evaporation (the liquid
structure is turning into a gas structure).
What is the melting point of a human (or other material that does not appear prone to
melting)?
While some components of a human can melt or boil (e.g.: blood can boil), most of a human
body would lose its structure and decompose at high temperatures so a human does not have
a clear melting point.
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Why do some objects burn instead of melt when they get hot?
As explained above, some things do not have clear melting points as many different processes
occur when they are exposed to heat. Burning is not a change of state, it is a chemical
change; the chemical properties of a material are changed, not just its state.
How can you have a negative temperature?
You can have a negative temperature because 0°C is not the lowest temperature that exists.
Using °C is a clumsy and arbitrary way of measuring temperature! The lowest temperature
that exists is -273°C, which is called absolute zero. Scientists use a different system for
measuring temperature called Kelvin (K), where the absolute zero is 0K.
© Education Umbrella 2015