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Heat, melting and mixtures

الأسبوع 12 · المحور الثاني «المادة والطاقة»، الوحدة الثانية «حركة الجسيمات»، المفهوم 2.3 «مقارنة تغيرات المادة» (ب): تغير الحالة والمخاليط · 3 دروس
2

From water to steam and back

Discovery Education Science P5: Term 1, Unit 2, pp. 186–188
GoalDescribe how water changes between ice, liquid water and water vapour, and what happens to its particles.
New words
boil · to bubble and change quickly into a gas when very hotcondense · to change from a gas into a liquid when cooledwater vapour · water as an invisible gas in the air
Explanation

Water can be a solid, a liquid or a gas. When ice warms up, it melts into liquid water. When liquid water gets more heat, its particles move faster. A few at the surface escape slowly into the air, which is evaporation. At 100 °C the water boils, and vapour forms all through it. Water vapour is a gas, and we cannot see it. The white cloud above a boiling kettle is steam, which is not vapour but tiny drops of liquid water. Those drops form when vapour meets cooler air. Taking heat away reverses each step. Water vapour that cools slows down and condenses into drops. Liquid water that cools below 0 °C freezes into ice. In every change it is still water, so these are physical changes. A grown-up always handles a kettle or a stove, and children watch from a safe distance.

Worked examples
cold glassdrops on the outside
On a hot day Mariam pours cold water into a glass. Soon drops cover the outside. They did not leak through the glass. Water vapour from the warm air touched the cold glass, cooled and condensed.
The changes can run backwards when heat is taken away. Cooling turns vapour into liquid, and cooling more turns the liquid into ice.
kettle: a grown-up’s jobwhite mist of tiny drops
A grown-up boils a kettle while Omar watches from the other side of the kitchen. Right at the spout the vapour is invisible. A little further away the vapour cools, and we see a white mist of tiny drops.
Class activity

Cold glass test (no heat needed; a grown-up helps). Fill one glass with cold water and ice cubes, and another with water from the tap, and put both on a table in a warm room. Predict which one will get drops on the outside. Look after 10 minutes and write what you see. Explain it with the words vapour and condense.

Practice
1
Name each change: (a) liquid water to water vapour, (b) water vapour to liquid water.
2
Why do drops appear on the outside of a cold glass?
3
True or false: the white cloud above a kettle is water vapour.
4
Which box shows water vapour? Say the letter.
ABC