Leonie Boshoff-Mostert Edited by Anne Starace

Transcription

1 GASES, LIQUIDS AND SOLIDS Density Leonie Boshoff-Mostert Edited by Anne Starace Abstract Matter is sorted into three groups: solids, liquids and gases. Solids, liquids and gases each have characteristic properties. This activity illustrates some of these properties. Keywords: molecule, density "Science is simply a way of looking at the world with an open, inquiring mind. It usually starts with an observation" Dr. David Darling Funded by the National Science Foundation and the University of Nebraska

2 Content Standards K History & Process Standards K Skills Used/Developed: 2

3 TABLE OF CONTENSE I. OBJECTIVES...4 II. SAFETY...4 III.LEVEL, TIME REQUIRED AND NUMBER OF PARTICIPANTS... 4 IV. LIST OF MATIRALS...4 V. INTRODUCTION... 4 IV. PROCEDURE... 6 VI. FREQUENTLY ASKED QUESTIONS... 8 VII. REFERENCES

4 I. OBJECTIVES Students will: -learn about the properties of solids, liquids, and gasses. -understand the concept of density. II. SAFETY Participants must not drink the oil/water mixture. III. LEVEL, TIME REQUIRED AND NUMBER OF PARTICIPANTS LEVEL This activity is suitable for grades k-6. TIME REQURED minutes NUMBER OF PARTICIPANTS Up to about 15. IV. LIST OF MATERIALS 3 empty pop bottles of the same size marked air, water and cotton balls cotton balls water 3 cylinders of the same size made of different metals 2 plastic cups 600 ml Pyrex beaker mineral oil blue and red food coloring In addition for hands-on version: clear straws plasticine clay droppers another cup V. INTRODUCTION Molecules Atoms are the building blocks for all matter. In nature most atoms do not exist alone. They combine with other atoms to form molecules. Molecules are very, very small and you can't see them with your naked eye - you need a very powerful microscope to see them. Solids, liquids and gases are the three different forms of matter. If something changes from one state of matter to the other, say from a liquid to a gas, it is still made of 4

5 the same molecules. Changing states of matter is a physical change, not a chemical one. Everything and anything in the world is matter in one stage or another 1. As different as they are, the three forms of matter are the same in a few ways: all matter occupies space all matter has mass all matter consists of tiny units called atoms Under normal conditions, most substances occur in just one of these states. For example, we think of iron as being a solid. But if it is heated enough, iron will turn in to a liquid, and eventually into a gas 2. What do we know about gases, liquids and solids? Then gases, liquids and solids will be explained intuitively, making use of posters and encouraging a lot of audience participation - i.e. asking them what they already know about gases, liquids and solids. Can we touch it? Feel it? What does it do? Does it flow? SOLIDS Solids have a definite size and shape. If you look at a solid through a powerful microscope you see that the molecules are packed tightly together. Though the individual molecules are shaking and vibrating, they are all fixed in one spot. The molecules of most (not all - think of an exception! wood) solids are carefully arranged in set patterns. Any material with its atoms and molecules in a definite pattern that is repeated throughout the material is called a crystal. layer C layer B layer A Figure 1 Crystalline Solid LIQUIDS Did you know? three quarters of the earth's surface is covered with liquid water over half of your body weight is water only two liquids, water and oil, are found in nature (But often these liquids have tiny particles in them that make them look different from pure water or oil. For example, blood is water with blood cells in it. ) 5

6 If you could see the molecules in a liquid you would notice that they are packed together, but not as tightly as in a solid. And they are not in any particular pattern or order. They are randomly scattered around. One property of all liquids is that they flow. Although liquids take up a definite amount of space, they have no shape - liquids take the shape of the container they are in. Liquids always seek their own level. Think of water in a bowl, no matter how you lean the bowl, the water does not tilt. Liquids always stay level because of gravity and because the molecules in liquids attract each other. Figure 2 Liquids always stay level GASES Gases are a form of matter in which the molecules are much farther apart than in either solids of liquids. The size of the gas molecules is tiny compared to the distances between them. Gas molecules move around constantly and therefore they spread and fill up any container. Like liquids, gases have no shape, but take on the shape of the container they are in. When a gas spreads out we say it diffuses. One way to test this is to go into a room and shut the door and windows. Then stand in one corner with a closed bottle of a smelly liquid, say vanilla essence. Have a friend stand in the opposite corner. Open the bottle and slowly wave it back and forth in the air to help the smell get out. See how long it takes for diffusion to carry the smell - which is, of course, vanilla essence gas molecules - across the room. Gases change size. If you have a balloon filled with Helium gas and you let the gas escape into a room it will diffuse through the room. The Helium gas changed from the size of the balloon to the size of the room - even though it is spread very thinly. The gas we know the best is air! VI. PROCEDURE Density SETUP Fill the pop bottle marked "water" with water if it is not full. Fill the bottle marked cotton with cotton. Make sure to fill the bottles all the way to the top. Set out the 3 bottles. EXECUTION The object of the demonstration is to let the participants discover the principle of density through their intuitive knowledge of the world around them. 6

7 Ask the following questions: What is the same? The kind of bottle, the space the stuff in the bottles takes up (volume), size, shape, etc. What is different? The stuff inside the bottles, the weight, color, etc. We conclude that different materials have different masses even when they fill the same volume. We call that the density of a material: the mass of a material that fills a certain volume is different for each material. Also tell the participants that, in general, the solid form of a material is more dense than the liquid form, and the liquid form is more dense than the gaseous form. We also see that: Solids have their own shape - the cotton balls are round. Gases and liquids don't have their own shape, but take on the shape of the containers they are in. Cylinders of different density SETUP Set out the 3 cylinders. EXECUTION Pass the cylinders around. They look exactly the same - what is different? Some are heavier than others. They have different densities. Liquid Layers SETUP Have the materials ready. EXECUTION Pour water into a cup and color blue. Pour oil into a cup and color red. Pour the water in the beaker and then the oil. What do we see? The water and the oil don't mix The oil forms a layer on top of the water We see that because the 2 liquids have different densities the oil floats on the water. Materials that are more dense sink to the bottom and materials that are less dense float on top. So, we conclude that water is more dense than oil. 7

8 Hands on version: 1. Make up 1 cup of water - color red Make up 1 cup of oil - color blue 2. Take small ball of clay and push straw into clay so that it stands vertically on a table 3. Let the participant drop the water into the straw with dropper and then the oil. Ask them if they can get the liquids to form 2 layers. CLEANUP The hands on version can become messy. After the participants have made their liquid layers let them take the straw by the top and pinch it closed. Let them pull it out of the clay and go around to each participant and let them run the contents into one of the cups. VII. FREQUENTLY ASKED QUESTIONS VIII. REFERENCES 1. M Berger, "Solids, Liquids and Gases", G. P. Putnam's Sons, New York, D Darling, "From Glasses to Gases", Dillon Press, New York,