Week 2 · Lesson 2 Changes in the States of Matter (A): heating curve of water, energy and particle motion, melting and boiling points, effect of pressure and impurities · 3 lessons
1
Heating ice: temperature against time
Science Prep 2 Term 1, Unit 1: pp. 10–14
GoalRead and draw a heating curve of water and explain why the temperature rises while a substance warms.
New words
thermal energy · the energy a substance gains when it is warmedheating curve · a graph of temperature against time while a substance is heated steadilymelting point · the temperature at which a solid changes to a liquid
Explanation
When a substance gains thermal energy, its particles move faster: their kinetic energy rises, and so does the temperature. When it loses energy, both fall. A heating curve shows this for ice warmed steadily. Part 1: the ice warms from a cold start up to 0 °C. Part 2: at 0 °C the curve goes flat, because the ice is melting. Part 3: the liquid water warms. Part 4: at 100 °C the curve is flat again, because the water is boiling. Part 5: the vapour warms. The numbers 0 °C and 100 °C hold at normal atmospheric pressure, the pressure at sea level. The curve here is an idealised sketch: the flat parts are not drawn to scale. To plot a graph, put time on the horizontal axis and temperature on the vertical axis, then join the points.
Worked examples
Read the curve above. In which part is the water a liquid and warming? Part 3, between 0 °C and 100 °C. In which parts does the temperature stay constant? Parts 2 and 4.
Karim measures a cup of ice and a little water every two minutes: 0 °C, 0 °C, 0 °C, 1 °C, 3 °C. The first three readings stay at 0 °C because ice is still there; the temperature rises only when the ice has gone.
To plot Karim’s numbers, put time (0, 2, 4, 6, 8 minutes) along the bottom and temperature up the side. Mark the points and join them: the line is flat at first and then slopes up.
Class activity
Melting watch, with a grown-up. Put 4 ice cubes and a spoon of water in a cup at room temperature. With a digital kitchen thermometer, read the temperature every two minutes for about 20 minutes and write the numbers in a table. Draw the graph. Describe what you see while ice is still in the cup and after the ice has gone. Do not heat anything on a stove.
Practice
1
What happens to the particles of a substance when its temperature rises?
They move faster: their kinetic energy increases.
2
At normal pressure, at what temperatures does the heating curve of water have flat parts?
0 °C and 100 °C.
3
A student says the water in Part 3 is a solid. Use the curve to answer.
No: it is between the melting point and the boiling point, so it is a liquid.
4
Which axis shows time and which shows temperature in a heating curve?
Time on the horizontal axis, temperature on the vertical axis.
