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Density is an abstract concept which is very difficult for students in elementary class to grasp. Through these experiements, they'll find an answer to why some objects sink and some others float.
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Every object has some physical properties like mass, volume and density. Density is the ratio of the mass to volume of that object. Hence two different objects occupying the same volume have different density as mass is an intrinsic property which varies from one object to another.
An object will sink or float in liquid according to its density. If it is denser than the liquid then it will sink and if it is lighter than the liquid then it will float freely. On mixing liquids with different density the denser will settle beneath the lighter one.
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Activities to Try
- Vegetable Oil
- Wine ( grape wine will do as it is colorful)
- Larger glass bowls
- Paper plates ( plastic coated)
Begin your class by asking your students the following questions:
Have you ever seen oil from a car form a layer over a puddle?
How do people locate the crash site of an airplane looking at the water surface in oceans?
- Instruct them to find answers to these questions after closely watching the following experiments. Take equal volumes of water and oil and measure its density using the hydrometer. Let them note down the density.
- Pour oil and water into one glass bowl and stir. Set aside for some time, after a few minutes oil will settle on the top of water.
- Make paper boats using paper plates. Take two bowls and pour water in one and oil in the other. Place the paper boat in water. As soon as it settles in water add sand slowly till it sinks. Now place another paper boat of the same size and material in oil and add sand till it sinks in oil. Drain the liquids using a filter so that sand will be collected and instruct the students to note the amount of sand used each time.
- Divide your students into three groups. Provide each with different liquids, like water, vegetable oil and wine. Let them float a paper boat in the liquid given to them and allow it to sink by adding sand into it. Let them note down their findings.
- Now take a large glass bowl and add each of the liquids one after another. Use a sufficient amount of each liquid so that it forms a clear and distinct layer with enough thickness so that children can distinctly make out each layer. Now place a paper boat and watch how it sinks in each liquid column on adding sand. Once it is inside one liquid you can take the help of a straw to put more sand into it without disturbing the liquid columns.
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Analysis and Conclusion
From the recorded data children will understand that the paper boat sinks only when it becomes heavier. Since the amount of sand in each case is different they will understand that the more dense liquid can balance more weight and the less dense liquid balance less weight. Now go back to the questions asked at the beginning of class and let your students find the answers.