Thermal Expansion | Grade 6 | Unit 2 - Concept 1 - Lesson 4 | Science

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In This Lesson:
We can apply what we have learned in the previous lessons on the following graph that shows the different processes that happen when a beaker of ice cubes was heated until the ice (solid) changes to water (liquid), then water (liquid) changes to water vapor (gas).
At area 1, When the ice is heated, the molecules of ice absorb thermal energy and they move faster due to the increase of their kinetic energy.
At area 2, By increasing the temperature, the kinetic energy of ice molecules increases that leads to decrease the force that bonds the molecules of ice together, so they slide over each other and ice (solid) changes to water (liquid), this temperature is called "melting point".
At area 3, By increasing the temperature the force that holds the molecules together becomes weaker and they spread in all directions, so water (liquid) changes to water (gas vapor) and this temperature is called "boiling point".
Thermal Expansion
You have learned that matter behave differently when they are heated or cooled, where:
When we cool a matter, the spaces between its molecules decrease and the molecules come close
together (contract) and this is called "contraction".
When we heat a matter, the spaces between its molecules increase and the molecules spread out (expand) and this is called "expansion".
Thermometer:
Some thermometers contain alcohol (liquid) mixed with color.
When the thermometer is placed in a hot substance, the temperature of alcohol increases and the spaces between its molecules increase, so the molecules of alcohol spread out and expand giving high level of temperature in the thermometer.
When the thermometer is placed in a cold substance, the temperature of alcohol decreases and the spaces between its molecules decrease, so the molecules of alcohol come close together and contract giving low level of temperature in the thermometer.
Jars:
Sometimes it is hard to open the lid of the jar.
When you pour hot water on the lid of the jar, it opens easily, where:
• The lid of the jar is made of metal.
• When hot water is poured on the metal lid, the temperature of the metal lid increases and the spaces between its molecules increase, so the molecules of metal lid spread out and expand, so it can be easily opened.
Bridges:
Bridges are made up of steel (metal) and concrete.
When bridges are exposed to hot weather, the emperature of metal increases and the spaces between its molecules increase, so the molecules of metal spread out and expand.
So, engineers use expansion joints to keep bridges safe from buckling (bending)
when they expand at high temperatures.
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