Chapter 1 Matter Class 8th physics chemistry ICSE скачать в хорошем качестве

Chapter 1 Matter Class 8th physics chemistry ICSE

Class 8th Physics Chapter 1 Matter What is Matter Composition of matter Kinetic theory of matter What is matter? Anything that occupies space and has mass is called matter. Composition of matter If the matter consists of only one kind of an elements, then the smallest unit of that element is called an atom Molecules: If the matter Consists of two or more elements combined together then the smallest unit of that matter is called the molecule. A molecule may consist of two or more atoms. Kinetic theory of matter: Kinetic theory of Matter was developed about 200 years ago by the works of scientists Dalton, joule David, Maxwell, Boltzmann Main points of kinetic theory are as follow : Molecular arrangement in matter- three States of matter Solids: 1.The molecules in solids are closely packed and the intermolecular spaces are extremely small. 2.The intermolecular forces are very strong and hence position of molecules are fixed as a result solid has a definite shape and volume. 3. In solid , molecules can only have vibrational motion. 4. A solid cannot be compressed on account of close packing of molecules. Thus, solids are hard and rigid. 5. A solid cannot flow and it can be stored independently without a vessel. Liquids: 1.The molecules in liquids are less tightly packed or the intermolecular space is somewhat more than that in solids. 2.The intermolecular forces are weak and hence a position of molecules is not fixed. Thus, a liquid has no shape of its own and can flow. 3. The molecular forces do not allow the molecules to leave the liquid easily. So, a liquid has a definite volume, though it does not have a definite shape. 4. Liquids can be compressed only a little. 5. Liquid can be stored in a container. A liquid has a definite and only one free surface. Gases: 1.The molecules in gases are far apart or the intermolecular space is much larger than liquids. 2. The intermolecular forces are extremely weak or almost zero. 3. In a gas, each molecule move independently and therefore a gas has neither a definite shape not a definite volume. It can be stored in a closed vessel only. 4. A gas can be compressed easily. 5. A gas can flow very easily. Moreover, a gas has no free surface of its own. Energy of molecules in three States of matter: Solids: In solid state, the intermolecular spaces are extremely small and the molecules are closely packed. Therefore, molecules cannot have translatory or rotational motion. The molecules can only vibrate to and fro about their respective mean position and so energy of movement of molecules in solid state is minimum. At low temperature, the amplitude of vibration of molecules is small. As a solid is heated, the vibrational amplitude increases and kinetic energy of molecules increases. At a certain temperature, called the melting point, the kinetic energy becomes a sufficient to overcome the intermolecular force and molecules leave their fixed position,i.e, the solid is transformed into liquid. Liquids: In liquid state, the intermolecular spaces are more as compared to solids and intermolecular forces are quite weak. As a result, liquid molecules can move freely within the liquid but cannot leave the free surface of the liquid. Thus, kinetic energy of movement of molecules in the liquid is greater than that of solid. As a liquid is heated the kinetic energy of the molecules goes on increasing. At a certain temperature, called the boiling point, the energy of molecules is sufficient so as to overcome the forces of attraction of each other. As a result the liquid is transformed into gaseous state. Gases: In gaseous state , the intermolecular spaces are very large and the intermolecular forces are almost zero. Thus, gas molecules can have independent translatory motion in all possible directions throughout the space. So, kinetic energy of movement of molecules in a gas is maximum as compared to solids and liquids. Change of state: Freezing or solidification Vaporization or boiling Condensation or liquefaction Sublimation Deposition or de- sublimation Evaporation Factors affecting evaporation Evaporation causes cooling Applications and examples of evaporation 1. During summer water contain in an earthen pot get cooled. 2. We feel more comfortable under a fan when we perspire 3. Doctor advises to put the strip of vatic cotton cloth on the forehead of a patient suffering from high fever 4. To cool a hot tea faster, pour it from a cup into a saucer 5. During summer, one should wear cotton clothes Boiling Evaporation 1. Boiling takes place at a definite temperature only called the boiling point of the liquid. Evaporation takes place at any temperature. 2. Boiling takes place throughout the mass of the liquid. Evaporation takes place only at the liquid surface. 3. During boiling, the temperature of a liquid remains constant. Evaporation leads to fall in liquid temperature. 4. Boling is a rapid and violent process. Evaporation is a slow and silent process.