الأسبوع 12 · المحور الثاني «المادة والطاقة»، الوحدة الثانية «حركة الجسيمات»، المفهوم 2.3 «مقارنة تغيرات المادة» (ب): تغير الحالة والمخاليط · 3 دروس
1
Chocolate in the sun
Discovery Education Science P5: Term 1, Unit 2, pp. 178–185
GoalExplain that adding or taking away heat changes the state of matter, using melting chocolate and particles that move faster or slower.
New words
thermal energy · heat: the energy that makes things warm and moves from warmer things to cooler onesfreeze · to change from a liquid to a solid when cooledreversible · able to change back again
Explanation
Heat is a form of energy, also called thermal energy. It is not a material you can hold. It is the energy that can make a cup of tea hot. All matter is made of tiny particles that are always moving. When matter takes in thermal energy, its particles move and vibrate faster, and it feels warmer. When it loses thermal energy, its particles slow down. A bar of chocolate shows this. Heat flows into it from the warm air, its particles move faster, and the solid melts into a liquid. Put the liquid somewhere cool and heat flows out, the particles slow down, and the chocolate freezes into a solid again. This is a physical change: it is still chocolate, and only its state and shape changed. Water is a liquid between 0 °C and 100 °C, and it freezes to ice below 0 °C.
In a hot cup of tea the water particles move fast. In a cup of cooled tea they move more slowly. The hot tea has more thermal energy, so it feels hotter.
Time
What Tarek saw
after 5 min in the sun
the pieces are soft
after 15 min in the sun
all liquid
after 15 min in a cool place
solid again, new shape
Tarek puts chocolate pieces in a sealed bag in the sun and writes what he sees. The chocolate softens, melts, and sets again when the bag is put in a cool place. The same chocolate changes state twice.
Temperature
State of water
below 0 °C
solid (ice)
0 °C to 100 °C
liquid
100 °C
boils into a gas
The state of water depends on its temperature. Below 0 °C it is ice. Between 0 °C and 100 °C it is liquid. At 100 °C it boils and becomes a gas.
Class activity
Chocolate bag (a grown-up helps, hats on, nobody looks at the Sun). Seal a few small chocolate pieces in a resealable bag. Lay the bag on a tray in a warm, sunny place and look every 5 minutes until the chocolate melts. Then put the bag in a cool place or on a bowl of cold water and look again. Record what you see in a table. Do not eat the chocolate afterwards, and wash your hands.
Practice
1
What makes solid chocolate melt?
Heat flows into it. Its particles gain energy and move faster, so it changes from a solid to a liquid.
2
Is melting chocolate a physical change? Give a reason.
Yes. It is still chocolate, only its state changed, and cooling can change it back.
3
A bowl of water is left outside on a very cold night in the Sinai mountains, and the thermometer shows its temperature. Is the water a solid, a liquid or a gas?
It is a solid. The temperature is below 0 °C, so the water has frozen into ice.
4
What happens to the particles of a liquid when heat is taken away?
They slow down. If enough heat is taken away, the liquid freezes into a solid.
