Kerala Syllabus Class 7 Basic Science: Chapter 06 Heat in Everyday Life - Questions and Answers


Questions and Answers for Class 7 Basic Science (English Medium) താപം നിത്യജീവിതത്തിൽ | Text Books Solution Basic Science (English Medium) Chapter 06 Heat in Everyday Life - Teaching Manual | Teachers Handbook

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ഈ ബ്ലോഗ് അഡ്‌മിൻറ രേഖാമൂലമുള്ള അനുമതിയില്ലാതെ ഈ ബ്ലോഗിൽ നൽകിയിരിക്കുന്ന ചോദ്യോത്തരങ്ങൾ, ഇതേരീതിയിലോ പി.ഡി.എഫ് രൂപത്തിലോ, മറ്റേതെങ്കിലും ഡിജിറ്റലോ, പ്രിന്റഡ് ഉൾപ്പെടെയുള്ള ഏതെങ്കിലും രൂപങ്ങളിലേക്കോ മാറ്റി മറ്റൊരു വെബ്സൈറ്റിലോ, ബ്ലോഗിലോ, യുട്യൂബ്, സോഷ്യൽ മീഡിയാ ഗ്രൂപ്പുകളിലോ ഉൾപ്പെടെ ഒരിടത്തും പ്രചരിപ്പിക്കാൻ പാടില്ലാത്തതാകുന്നു.

Std 7: Basic Science Chapter 06: Heat in Everyday Life - Questions and Answers
♦ Observe the picture. Which energy forms are utilized here? (Textbook Page No: 111)
• Light energy
• Solar energy
• Heat energy
• Wind energy
• Wave energy
• Electrical energy
• Mechanical energy

♦ Which are the various situations where heat energy is used? List them.
• For cooking
• For ironing cloths
• For drying cloths
• For welding purposes
• Heaters
• Driers

♦ Ice is the solid form of water. What happens when ice is exposed to air?
When ice is exposed to air it melts by absorbing the heat. 

♦ What happens when water is boiled? 
• When water is boiled it becomes steam
• Steam is the gaseous form of water.

♦ Take water at normal temperature in one glass tumbler and hot water in another one. Put some ice cubes in both glasses. Ice cubes in which glass melts faster? Why? 
• Ice cubes in hot water melt faster
• Ice requires heat for melting. heat is absorbed more easily from hot water.

♦ Which form of energy caused the change of state of ice?
Heat is a form of energy that can change the state of matter.

♦ Have you noticed coconut oil solidifying during winter? What is the reason?
During winter the temperature decreases and coconut oil solidifies 

♦ How did the rice get the heat energy radiated from the flame of the gas stove? 
Rice gets cooked when the heat radiating from the flame of the stove is transferred through the pot and water and reaches the rice.

♦ Complete the flowchart by writing the different ways by which heat was transferred. 
♦ What is Transmission of Heat?
Transmission of heat refers to the flow of heat from one place to another.

♦ What happens to the pot after turning off the gas stove for a while? 
Heat is transferred to the surroundings, not only from the pot but also from the substances inside it. This results in heat loss to the vessel and its contents.
♦ What is conduction?
When heat is received at one end of a metal rod, the molecules at that end receive the heat and transfer it to the nearest molecules. This method of transmission of heat is called conduction. In solids, heat is transmitted through conduction.

♦ Complete the table by classifying substances that conduct heat well and those that don’t conduct heat well. 
Substances that conduct heat well Substances that don't conduct heat well
• Aluminium• Wood
• Copper• Glass rod
• Iron 
♦ What are good conductors and poor conductors?
Substances which allow heat to pass through them well by conduction are called good conductors and those substances which do not allow heat to pass through them well are called poor conductors.

♦ Find out more examples of good conductors and poor conductors
● Good Conductors:
• Copper
• Silver
• Gold
• Aluminum
• Steel

● Poor Conductors:
• Rubber: 
• Wood
• Plastic
• Paper


♦ In the following situations, do we use good conductors or poor conductors?
• To remove a hot vessel from the stove - poor conductor (cloth, paper etc.)
• To make handles of cooking utensils - poor conductor (plastic, rubber, wood etc.)
• To make cooking utensils - good conductor (Aluminium, iron, copper, steel etc.)

♦ Check the pictures below.
Both good conductors and poor conductors are used in the same vessel. Explain the reason for this. Find more examples of such vessels.

● Good Conductor: Utensils like pressure cookers and frypans have bases made of materials like aluminium and steel to ensure even heat distribution.

● Poor Conductor: The handle is made of materials like plastic or wood to prevent heat transfer and keep it cool to touch.

● more examples of such vessels: kadai, saucepan, wok, grill pan

♦ By which method does the transmission of heat occur in solids?
Conduction

♦ Write an experiment showing the transmission of heat in liquids.
Materials required: A transparent plastic jar with a lid, a glass tumbler with the same diameter as that of the jar, mason pipe, Potassium permanganate, hot water, cold water.

Procedure: Make two holes on the lid of the plastic jar. Fix a 10 cm long mason pipe in each hole in an airtight manner. Take hot water in the glass tumbler having the same diameter as that of the plastic jar. Add some Potassium permanganate granules to it. Take some cold water in the plastic jar and close it with the lid fitted with the mason pipe. Place this plastic jar upside down over the glass tumbler filled with hot water containing Potassium permanganate.

Observation: When hot water flows up through one mason pipe, cold water flows down through the other pipe. This cold water becomes hot and rises up again. Thus heat spreads throughout the liquid. 

Inference: Heat was transmitted from hot water to cold water by the displacement of molecules. Here the heat energy is transferred when molecules move across. Heat transmission occurs in all liquids by the displacement of molecules.

♦ Write an experiment showing the transmission of heat in gas.
Materials required: One PVC pipe of 5 cm diameter and 30 cm length, an incense stick, a matchbox, a candle.

Procedure: Make a pencil-sized hole at a height of 8 cm on one end of the PVC pipe. Place a lighted candle on the table. Arrange the pipe as shown in the figure so that the candle comes inside the PVC pipe. While arranging the pipe, the portion with the hole should come at the bottom. Light the incense stick and watch the smoke rising up. Bring the lighted incense stick near the hole on the pipe. Observe the direction of flow of smoke. 

Observation: As the air inside the PVC pipe warms up and rises, cool air flows into that space through the hole. Along with this flow of air, the smoke from the incense stick also enters. The air, thus entered, also gets heated and rises up.

Inference: Heat is transmitted from one part to another by the displacement of molecules as in liquids. In this way, the heat spreads in the air inside the pipe.
♦ What is convection?
Convection is the method of transmission of heat in gases and liquids by the displacement of molecules.

♦ By which method does the transmission of heat occur in liquids and gases?
Convection

♦ What is the role of molecules in the transmission of heat by conduction and convection?
Here the molecules serve as the medium.

♦ What is Radiation?
Radiation is the method by which heat is transmitted without the help of a medium. Heat is transmitted to all directions through radiation. Heat from the Sun reaches the Earth by radiation. White or smooth surfaces reflect radiant heat more than black or rough surfaces. 

♦ Write down some situations where heat is transmitted by radiation.
• Heat reaches down from a glowing filament bulb.
• Sitting near a campfire or fireplace, we feel warmth
• Hot vessel spreads heat to the surroundings

♦ Don’t you adopt various methods at home to keep cooked food hot for a long time? Which are the commonly used devices to reduce heat loss? List them. 
• Casserole
• Thermos flask
• Rice cooker
• Hotbox
• Icebox

♦ What is hot box used for?
A hot box is a system used to reduce the use of cooking fuel and to keep cooked food items from losing heat. 

♦ How is heat loss avoided in a hot box? 
Thermocol is used in the hot box. As it is a poor conductor, the heat loss through conduction is reduced. Heat loss through convection is reduced as the hot box is kept closed.

♦ What is Ice box used for?
Ice box is used to keep ice from melting too quickly. 

♦ How can we make an ice box?
Materials Required: A small box, thermocol, glue, white enamel paint.
Cut the thermocol and glue it inside the small box. The thermocol should be glued on the sides, bottom and the lid. Apply white enamel paint inside and outside the box. 
♦ Write the experiment showing the Thermal Expansion of Solids.
Materials required: A battery, a bulb, a connecting wire, two Aluminium plates, a candle, a matchbox.
Procedure: As shown in the figure, arrange an electric circuit on a board using battery, bulb, connecting wire and the two Aluminium plates.
The Aluminium plates should be placed on both sides of the wire very closely, without touching each other. Heat the Aluminium plates using the candle.

Observation: the bulb glows.

Inference: When heated, the Aluminium plates get expanded, the circuit is completed and thus the bulb glows. 

♦ Why did the circuit become open on cooling?
When heat is lost, the plates contract and the circuit is disconnected and the bulb goes off.

♦ Thermal Expansion of Solids
Solids expand when heated and contract on cooling. 

♦ Which are the situations you have noticed in daily life related to thermal expansion of solids?
• Electrical lines sagged in summer because the metal wires expand as the temperature increases.
• On very hot days, metal doors or window frames may expand and become difficult to open or close.
• Gaps between sections of railway tracks. These gaps allow for the expansion of the metal rails in hot weather.

♦ Why do the wires on the electric pole for distributing electricity get sagged in summer?  
Electrical lines sagged in summer because the metal wires expand as the temperature increases.

♦ Find out the reason behind this by analysing the following situations.
● A very tight pen cap is removed by heating gently.
When heated the tight pen cap expands and becomes loosened. And it can easily be removed.

● The tight lid of a steel tiffin box is opened by gently heating.
By heating the tight lid of the tiffin box expands and loosens.  And it can easily open.

♦ Do Liquids Expand When Heated?
Experiment.
Materials Required:  An injection bottle, a cork that fits the injection bottle, an empty refill tube of a pen, a bowl, potassium permanganate, hot water, water at normal temperature.

Procedure: Fill the injection bottle with water and add some Potassium permanganate granules. Make a small hole in the rubber cork. Fix the refill tube in the hole. Close the injection bottle with the cork. Tie a thread to mark the water level in the refill. Take hot water in the bowl and place the injection bottle in it.

Observation: The Water level in the refill rises

Inference: The water in the bottle expands and rushes into the refill. Hence the water level rises. 

● If the water in the injection bottle cools down, will the water level return to its initial position?
Water contracts when it cools and the water level is restored.

♦ Thermal Expansion in Liquids
Liquids expand on heating and contract on cooling.

♦ When heated, do gases undergo the same changes as in solids and liquids? Do gases also expand when heated?
Experiment 

Materials Required: An Aluminium lamp, a plastic tube, a board, coloured
water, hot water, cold water.

Procedure: Fix the plastic tube on the Aluminium lamp as shown in the figure. Fix the remaining part of the tube in U shape on a board. Pour two or three drops of coloured water into the plastic tube. Place the Aluminium lamp in hot water and observe it.
Then place the Aluminium lamp in cold water. observe it.

Observation: When placed in hot water the coloured water in the tube moves upwards.
When placed in cold water the coloured water in the tube moves back and reach its initial position

Inference: When the Aluminium lamp becomes hot, the air inside it also becomes
hot. As this hot air expands, the coloured water in the tube moves upwards.

As the Aluminium lamp cools down, the air contracts, causing the coloured water to move back in the tube and reach its initial position. 

♦ Thermal Expansion of Gases
Gases expand on heating and contracts on cooling.
♦ What is temperature?
Temperature is the term that indicates the degree of hotness. The units used to indicate temperature are degrees Celsius (°C) and degrees Fahrenheit (°F).

♦ Which instrument is used to check your body temperature at a hospital? 
A clinical thermometer is used to measure body temperature. Normal temperature of human body is 37°C (98.6°F).

♦ Which instrument is used in a laboratory to measure temperature?
Laboratory thermometer 

♦ If soil and water are placed under sunlight at the same time, which one will get heated faster? 
Experiment 

Materials Required:  Two glass tumblers, two laboratory thermometers, water, sand.

Procedure: Take sand in one glass tumbler and water in another and check the temperature with a laboratory thermometer. Place both the glasses in the sun. Then measure the temperature of sand and water with a thermometer at an interval of 20 minutes and record it. And then move both the glasses from the sun to a shade. Measure and record the temperature at an interval of 20 minutes.

Observation: Sand is heated faster than water when placed in sunlight. Sand cooled faster than water when placed in the shade.

Inference: Sand gets heated and cooled faster than water.

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♦ What is Sea Breeze?
The land is warmer than the sea during the daytime. The air just above the land gets heated. This air expands and rises up. The air over the sea is cooler compared to that over the land. As the warm air over the land rises up, the cold air over the sea moves towards this place. Thus, sea breeze is formed.

♦ What is Land Breeze?
Land cools faster than the sea. So, the air over the sea is relatively warmer. Therefore, it will be the air above the sea that is more expanded. Then the cold air above the land flows towards the sea. This causes the land breeze. 
Let’s Assess

1. Consider some situations in everyday life.
• Tightly bound electric wires get sagged in summer.
• One end of a PVC pipe is heated and the end of another pipe of the same diameter is inserted into it to join them.
a.  Which property of matter with respect to heat is evident in the above two cases?
Answer: Thermal Expansion of Solids

b.  Based on this, can you explain why a fully inflated balloon bursts when exposed to sunlight?
Answer: Air inside the balloon expands due to heat from sunlight and the balloon burst

2. Observe the arrangement of an experiment shown in the picture (Textbook Page: 125).
An injection bottle fitted with a plastic tube is placed in a beaker of hot water. The tip of this tube is inserted into the hole at the bottom of another jar filled with water.
a.  What do you observe?
Answer: Air bubbles rises from the second jar

b.  What inference can be drawn from this?
Answer: When heated, the air in the jar expands. The expanded air escapes through the pipe as it needs more space to occupy.

3. Classify and tabulate the following materials on the basis of thermal conductivity.
Iron, Paper, Bakelite, Copper, Wood, Steel, Aluminium, Cloth.
Answer:
Good conductors: Iron, Copper, Steel, Aluminium

Bad Conductors: Paper, Bakelite, Wood, Cloth.

4. Haven't you noticed the utensils used for cooking? What is the difference between the materials used to make the utensil and its handles? Explain this on the basis of thermal conductivity. 
Answer: Cooking utensils are made of good conductors of heat and their handles are made of poor conductors of heat.

5. Hot tea of same quantity is kept in an open steel tumbler and in a closed glass tumbler of same size. Tea in which tumbler stays hot longer? Explain your finding on the basis of heat transmission.
Answer: Tea in the closed glass tumbler stays hot longer.
Steel is a good conductor of heat, so the heat is lost through conduction. Since the steel tumbler is kept open heat is also lost through convection and radiation.
Glass is a poor conductor of heat so there is less heat lost by conduction. Since it is closed there is less heat loss through convection and radiation.




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