Properties of Fluids

Background Both air and water are examples of fluids. Any substance that flows and takes the shape of its container is considered a fluid. When heat is applied, the molecules in a fluid begin to move more rapidly, stretching the bonds that hold the molecules together. This is why fluids expand when they are heated. This expansion is also another way of understanding how a fluid can be less dense (more spread out) than another fluid.

Warm air and warm water are less dense than cold air and cold water and will, therefore, rise upward to be replaced by the cooler, more dense water or air underneath. This property of fluids is what allows hot air or helium balloons to float. As the amount of warmer, less dense gas is increased in the balloon, the amount of “lift” increases.

The fluid in a thermometer is an obvious example of how temperature affects fluid- as the temperature warms, the liquid inside the thermometer expands (and, therefore, rises): as the temperature cools, the liquid inside the thermometer contracts (and, therefore, falls).

Activity One Air and Water How are air and water alike? How are they different? What is a fluid? Is water a fluid? Is air a fluid? Both air and water are fluids- this means that they have the ability to flow. Any substance that can flow is a fluid.

What are some examples that prove that water flows? Water flows from a tap, through a garden hose, in a river. What are some examples that prove that air flows? Air flows through a flute, through vents that heat and cool houses, through the hose on a bicycle pump.

Fluids Water Milk Molasses Air Helium Oxygen Fluids allow other objects to move through them – planes through air, so if we understand fluids then flight is easier!

Activity Two Each group needs a container of water, a cup, and a paper towel. Crumple up the paper towel and stick it in the bottom of the glass. What do you think will happen when you turn the cup upside down and push it into the container of water? What will happen to the paper towel?

Do you think it will get wet? Why? Why not? Test your predictions. Hold the cup immersed in water and observe what is happening. Is the towel getting wet? Why not?

Remove the cup and take out the towel and observe. Is it dry? The water does not come in contact with the paper towel because air is taking up space in the cup and is pushing against the water. Both the air and the water are exerting pressure.

Activity Three You are now going to see what happens when fluids are heated and cooled. You will need a container of hot water and one of cold water, a 2 litre pop bottle, a balloon and activity sheet Do the investigation.

Questions What have you learned about what happens to air when it is heated? Why do you think this happened? What happens to air as it is cooled? Why do you think the air contracts?

As air is heated, molecules move around more and, therefore, take up more space (or expand). When air is cooled, the molecules move around less and, therefore, remain closer together. As a result, cooled air takes up less space (or contracts).

Test you hot water container with a thermometer. What happens to the liquid in the thermometer? Based on what you learned about air expansion and contraction, why do you think the liquid in the thermometer rises when the thermometer is placed in hot water?

What do you think will happen to the thermometer when it is placed in cold water? Test your predictions. Place the thermometer in cold water and observe. What have you learned about what happens to fluids when they are heated and when they are cooled? Expansion and contraction!

Activity Four Motion of fluids at different temperatures. Activity sheet Fill one of the pop bottles with cold tap water until there is not air left in the bottle. 2. Fill another pop bottle with hot tap water until there is no air left in the bottle. Add a few drops of food coloring to the hot water in the bottle.

3. Place the index card over the opening of the bottle of hot water, turn the bottle over, and place it very carefully onto the lower bottle of cold water. 4. Carefully slide the index card out from between the two bottles and have the students observe what happens. 5.Repeat steps 1-4 with the bottle of hot water on the bottom.

Record observations. What happens to the hot coloured water as it comes in contact with the cold water? What does this tell you about hot and cold water? Warm or hot fluids rise and cold fluids sink. Concept map