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| Properties of Matter |
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| A solid has certain characteristics: |
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| According to modern theory, the molecules of a solid are fixed in relation to each other. They vibrate in place, but do not wander around. This accounts for the fact that a solid keeps its own shape. Solids are usually crystalline. A solid is a crystalline substance in which the molecules, atoms and ions are believed to be arranged in definite geometrical patters. This geometrical pattern determines the shape of the crystal. |
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| A few solids are not distinctly crystalline. The black soot that forms on the bottom of a saucer when you
hold it above a candle flame is an almost non-crystalline form of carbon. It is called amorphous. An amorphous substance is a solid in which the atoms or molecules are not clearly arranged in a definite geometrical pattern. Crystals form when a substance solidifies from a liquid state. For example, freezing water forms ice crystals. Crystals may also form in a solution that is highly concentrated. |
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| Molecular forces determine the properties of solids like hardness, tensile strength and elasticity. |
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| Liquids and gases are almost always homogeneous, which means that every portion of a particular sample is exactly like every other portion. Solids may be either homogeneous or heterogeneous. In a heterogeneous substance same portions of a particular sample may be different from others. A bar of gold, for example is a homogeneous solid, while a piece of wood is heterogeneous one. Homogeneous substances may be further classified into elements, compounds and solutions. |
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| Why does a gas exert pressure on the walls of container? Consider only those molecules which are moving horizontally. Each molecule hits the container wall and bounces back with the same speed, because the collision is elastic. The molecule then travels in the opposite direction. During one second the molecule crosses the container many times because of its greater speed, and makes a very large number of impacts. The change in momentum in each second gives the force exerted on the wall by the molecule. There are very large numbers of molecules involved, so a large force is exerted on the wall. This force acting on a unit area of the wall gives the pressure exerted on the wall. |
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| When common salt is added to crushed ice
to make a freezing mixture the temperature is lowered below 0oC.
This is because ice absorbs heat from salt which is at a higher temperature
and some ice melts dissolving some salt in it. This process continues till
all the salt is dissolved or all the ice is melted. Because the heat is
absorbed from the mixture there will be a considerable drop in the
temperature. Calcium chloride and ice in the right proportion can produce a
temperature of nearly - 50oC. |
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| The experiment illustrated in figure shows the effect of pressure on the melting point of ice. A thin copper wire passes round a large block of ice and two weights are attached to the free ends of the wire. The pressure of the wire lowers the melting point of ice in contact with the wire. The ice absorbs the heat from the wire for melting and lowers itself in the water. As soon as the water passes above the wire it is no longer under pressure and therefore freezes. In this manner the wire gradually sinks through the ice block and comes out of the block. |
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| The melting under pressure and freezing again after the pressure is released is called regelation. |
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| When a snow ball is made, the binding together of the snow is an example of 'regelation'. When the snow is pressed between the hands the increase in pressure lowers the melting point. The snow starts to melt and when the pressure is released the water freezes again and holds the particles of snow together. |
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| Skating on ice is another practical example of pressure affecting the melting point. The area of the blades of the skates in contact with the ice is small and the pressure on the ice is large. This pressure lowers the melting point of ice. Melting of ice provides a thin film of water which acts as a lubricant for the skates to glide over. |
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| Substances which contract in volume on solidifying have their melting points raised by pressure. |
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| The boiling point of liquid is raised by increasing the pressure and reduced by lowering the pressure. It can be demonstrated by the apparatus shown in figure. |
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| Water boils at a lower temperature when the pressure is reduced. |
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| The water in the round bottom flask is heated till it starts boiling. By pressing the clip on the rubber tube, allow the steam to escape. Now you will notice that the thermometer reads 100oC. Trap the steam inside the flask and the temperature of boiling water starts increasing. This is because by increasing the pressure, boiling point also increases. The flask is inverted and cold water is poured on the top of the inverted flask to condense the steam. Now you will notice that the water starts boiling at a low temperature below 100o C under a low pressure. |
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| As one goes higher the atmospheric pressure decreases. At about 300 m above sea level water boils at 90oC. Certain pulses cannot be properly cooked in the open in very high regions as the boiling point of water in these places is too low. |
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| These pulses can be easily cooked with the help of a pressure cooker. The water in the pressure cooker boils at a very high temperature nearly 120oC as the pressure is increased to two atmospheres. Thus the food cooked in water boiling at a high temperature takes less time than that at 100oC. |
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| When salt is added to boiling water the temperature of boiling water is raised above 100oC. This is because the salt absorbs heat energy from the water for dissolving in it and its average kinetic energy decreases. When gases dissolve in water it lowers the boiling point. While when solids dissolve in water it raises the boiling point. |
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