Changes of state
In this topic you will:
- practise measuring the volume and the temperature of a liquid
- learn what happens when matter changes state
- investigate the temperature increase when you heat water.
Key words
- boil
- boiling point
- change of state
- condensation
- condense
- evaporation
- freeze
- measuring cylinder
- melt
- melting point
- meniscus
- steam
- thermometer
- water vapour
Changing state
If you leave ice in a warm place it melts and becomes liquid water. The temperature at which a solid melts is called the melting point.
Water on the ground will gradually disappear as it changes to water vapour, an invisible gas. This is called evaporation. The warmer the water, the more quickly it evaporates.
If you heat water until its temperature reaches 100 °C, it will boil. All of the water rapidly changes to steam. Steam is water heated to the point that it turns into a gas. 100 °C is the boiling point of water.
If the water vapour or steam touches something cold, it condenses and changes back to liquid water. This is called condensation.
If you put liquid water in the freezer, it freezes and becomes ice.
These changes are known as changes of state.
Measuring
Measuring volume
When you measure the volume of a liquid you use a measuring cylinder. The liquid forms a curve at the top. This is called the meniscus.
You measure the volume from the bottom of the meniscus. To do this, you must make sure that your eye is level with the meniscus.
Measuring temperature
When you measure the temperature you use a thermometer. The liquid inside the thermometer expands as it gets hotter, so it rises up inside the thermometer. You read the temperature from the scale. Make sure that your eye is level with the top of the liquid in the thermometer.
Questions
1 Look at the diagram. What is the volume of water in each measuring cylinder?
Show Answer
A: 20 cm³
B: 50 cm³
C: 80 cm³
2 What are the temperatures shown on the thermometers?
Show Answer
A: 24 °C
B: 15 °C
C: 29 °C
Think Like a Scientist
Investigation: Measuring the temperature when you heat water
In this task, you will investigate how the temperature of water changes as it is heated over time.
You will need: beaker, Bunsen burner, clamp stand, gauze, measuring cylinder, thermometer, tripod, stopwatch, heat-proof mat, safety spectacles
Safety: Wear safety spectacles. Take care when handling hot water. Ensure your workspace is clear and follow your teacher’s safety instructions.
Steps:
1. Use the measuring cylinder to pour a known volume of water into a beaker.
2. Place the thermometer into the water. Use the clamp stand to hold it in position so that the bulb is submerged but not touching the bottom of the beaker.
3. Set up the Bunsen burner beneath the tripod and gauze. Place the beaker on top.
4. Light the Bunsen burner and begin heating the water. Start your stopwatch.
5. Record the temperature every minute using the thermometer.
6. Continue taking readings until the water is boiling vigorously.
7. Record your results in a table like this:
| Time (minutes) | Temperature (°C) |
|---|---|
| 0 | |
| 1 | |
| 2 | |
| 3 | |
| 4 |
Questions:
Show Answer
A graph should show temperature increasing steadily over time until it levels off near boiling point.
• When we heated the water, the temperature ________
• The longer we heated the water, the ________ the temperature ________
• The increase in temperature was ________
Show Answer
• ...the temperature increased.
• ...the higher the temperature rose.
• ...steady at first, then slower as boiling approached.
Show Answer
The temperature stopped rising and remained constant at around 100°C.
Show Answer
At boiling point, the added energy is used to change water into steam, not to raise temperature.
Show Answer
To avoid measuring the temperature of the glass, which may be hotter than the water itself.