Heating & Cooling Changes

Activities to try

1. 1

   Heat-and-cool bench: watch the change track the temperature

   Why it works: For water, melting, freezing and boiling do not happen vaguely 'when it gets warm': they happen at specific, readable temperatures (0°C and 100°C at normal atmospheric pressure) that are the same every time. The reading pauses at that temperature while the change is happening, and melting/freezing are the same boundary crossed in opposite directions. (Evaporation is different: it can happen slowly at temperatures below boiling.)

   1. Drag a heat dial under a beaker of ice and watch a thermometer climb in °C.
   2. Notice the ice stays solid as the temperature rises, and observe that it begins to melt at 0°C, with a marker snapping onto the scale at that reading; the reading holds at 0°C while there is still ice melting.
   3. Keep heating: once all the ice has melted the water warms to 100°C, where the reading holds while the water boils, with a marker at 100°C.
   4. Reverse the dial to cool: watch steam recondense and the water refreeze at 0°C, showing the change runs both ways.
   5. Use a side panel to run melting ice next to dissolving sugar and describe how the two processes differ.
2. 2

   Real thermometer in melting ice: predict the sticking point

   Why it works: While a melting ice-and-water mixture still has ice in it the temperature holds at about 0°C (the energy first goes into changing state), then climbs once all the ice has melted; the melt-water is the same material as the ice.

   1. Put a thermometer into a beaker of crushed melting ice (a genuine ice-and-water mixture), stir gently and let it warm slowly (for example in a warm room or with gentle warming), reading the temperature aloud.
   2. Before starting, predict what the thermometer does WHILE the ice is still melting, and say why.
   3. Observe whether the reading climbs steadily or sticks, and record the temperature where it holds.
   4. Afterwards, refreeze the melt-water and check it is the same water (e.g. mass stays the same if the container is covered so no water evaporates).
3. 3

   Melting is not dissolving — and freezing is heat leaving

   Why it works: Melting is a heat-driven change of state of ONE substance; dissolving spreads a solid into a second liquid and does not need heating (though warming can speed it up). Freezing is heat energy leaving, not 'cold' moving in; a melted material is still the same material.

   1. Compare ice melting in a warm room with sugar dissolving in water, and say what is different about each.
   2. State which process needs heat and which needs a second substance.
   3. Explain what happens when water freezes: does cold go IN, or does heat come OUT?
   4. Decide whether melted chocolate is a new material or the same chocolate.