PHYSICS, CLASS 8
ALL QUESTIONS ARE COMPULSORY.
Q1. WHY DO WE SMELL HOT FOOD FROM A DISTANCE? 2
Q2. WHY IS THERE NO CHANGE IN TEMPERATURE DURING THE MELTING PROCESS? 2
Q3. DIFFERENTIATE BETWEEN BOILING AND EVAPORATION WITH 3POINTS. 3
Q4. STATE AND EXPLAIN ANY THREE POSTULATES OF KINETIC THEORY. 3
Q5. DEFINE TWO TYPES OF INTERMOLECULAR FORCE WITH ONE EXAMPLE OF EACH. 2
Q6 EXPLAIN THE PROCESS OF MELTING ON THE BASIS OF KINETIC THEORY OF MATTER. 2
Q7. EVAPORATION CAUSES COOLING . GIVE REASON WITH ONE EXAMPLE. 2
Q8. WHY DOES A CAMPHOR GET CONVERTED TO FUMES ON HEATING? 2
Q9. WATER ______________HEAT DURING FREEZING AND _________ HEAT DURING BOILING. FILL IN THE BLANK WITH “ GAINS OR LOSES" 2
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Answers:
A1. The molecules of food in gaseous form are free to move in large intermolecular space available between them and the air and these molecules show zigzag motion to different parts of a house. So, we can smell hot food from a distance.
A2. During melting process, the heat given to a solid is utilized to do work against intermolecular force of attraction. The heat energy changes completely into potential energy. Hence temperature does not change.
A3. Boiling I Evaporation
a) Boiling is a fast process. I a) Evaporation is a slow process.
b) Boiling does not cause cooling l b) Evaporation causes cooling.
c) Boiling takes place at a particular I c) Evaporation takes place at any temperature.
Temperature
A4. I) Matter is made of atom or molecule..
Matter is made of small particles called atoms or molecules which cant be seen by naked eyes
ii) The molecules are always inconstant, random motion.
The particles continuously move in a zigzag path .
iii) The molecules possess kintic and potential energy.
Due to their motion the molecules possess kinetic energy. As they interact with other molecules, they change their position and possess potential energy.
A5. I) Force of cohesion : The force of attraction that exists between similar kinds of molecules. Example: The force of attraction between molecules of water.
ii) Force of adhesion: The force of attraction that exists between molecules of different substances. Example: The wetting of glass due to the force of adhesion between the molecules of water and glass.
A6. When a solid is heated, the molecules gain kinetic energy and start overcoming the strong intermolecular force of attraction. At a particular temperature, the molecules completely overcome the force of attraction and the molecules becomes free to flow and the solid changes into liquid . This process is called melting.
A7. Evaporation causes cooling because the liquid absorbs heat from the body and the surrounding to evaporate. Earthen pot keeps water cool by evaporation of water from the surface of the pot.
A8. Camphor molecules have very weak intermolecular force of attraction. By absorbing heat from surrounding these molecules overcome the force easily and molecules becomes free like gaseous molecules. Hence it is directly converted to fumes.
A9. Loses, gains.
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