The Class 8 is an important year in a student’s life and Maharashtra State Board Science is one of the subjects that require dedication, hard work, and practice. It’s a subject where you can score well if you are well-versed with the concepts, remember the important formulas and solving methods, and have done an ample amount of practice. Worry not! Home Revise is here to make your Class 8 journey even easier. It’s essential for students to have the right study material and notes to prepare for their board examinations, and through Home Revise, you can cover all the fundamental topics in the subject and the complete Maharashtra State Board Class 8 Science Book syllabus.
Question 1: A. Whom should I pair with?
| Group A | Group B |
| a. Temperature of a healthy human body | i. 296 K |
| b. Boiling point of water | ii. 98.6°F |
| c. Room temperature | iii. 0°C |
| d. Freezing point of water | iv. 212°F |
Answer:
| Group A | Group B |
| a. Temperature of a healthy human body | ii. 98.6°F |
| b. Boiling point of water | iv. 212°F |
| c. Room temperature | i. 296 K |
| d. Freezing point of water | iii. 0°C |
B. Who is telling the truth?
a. The temperature of a substance is measured in Joules.
b. Heat flows from an object at higher temperature to an object at lower temperature.
c. Joule is the unit of heat.
d. Objects contract on heating.
e. Atoms of a solid are free.
f. The average kinetic energy of atoms in a hot object is less than the average kinetic energy of atoms in a cold object.
Answer:
a. This sentence is not correct as heat energy is measured in Joules.
b. This sentence is correct as heat energy flows from an object at higher temperature to an object at lower temperature.
c. This sentence is correct as Joule is the unit of heat.
d. This sentence is not correct as objects expand not contract on heating.
e. This sentence is not correct as atoms of a solid are closely packed due to force attraction among them.
f. This sentence is not correct as the average kinetic energy of atoms in a hot object is greater than the average kinetic energy of atoms in a cold object.
C. You will find it if you search:
a. A thermometer is used to measure ________________________.
b. The apparatus used to measure heat is called a ________________________.
c Temperature is the measure of the _____________ kinetic energy of the atoms in a substance.
d. The heat contained in a substance is the measure of the ___________________ kinetic energy of atoms in the substance.
Answer: a. A thermometer is used to measure temperature .
b. The apparatus used to measure heat is called a calorimeter .
c Temperature is the measure of the average kinetic energy of the atoms in a substance.
d. The heat contained in a substance is the measure of the total kinetic energy of atoms in the substance.
Question 2: Nishigandha kept a vessel containing all the ingredients for making tea in a solar cooker. Shivani kept a similar vessel on a stove. Whose tea will be ready first and why?
Answer: Shivani ’s tea will be ready and prepared first.
In Shivani ’s case, the intensity of the flame that is in contact with the vessel is extremely high due to which the flow of heat into the vessel containing water will be faster. Thus, the time taken for the water to boil will be less. Hence, preparing tea in this case will be faster and convenient.
In the case of Nishigandha, the intensity of radiation that reaches the vessel is not as high in comparison to the stove ’s flame. Thus, the flow of heat will be lower and slower in this case and the time taken to reach the boiling temperature of the tea will be more. Hence, tea prepared through this method would be at a slower rate.
Question 3: Write brief answers:
a. Describe a clinical thermometer. How does it differ from the thermometer used in a laboratory?
b. What is the difference between heat and temperature? What are their units?
c. Explain the construction of a calorimeter. Draw the necessary figure.
d. Explain why rails have gaps at specific distances.
e Explain with the help of formulae the expansion coefficients of liquid and gas.
Answer: a. Clinical thermometer: Clinical thermometers are mostly used at homes. A clinical thermometer has a glass tube with a bulb at one end to measure temperature and it is closed at the other end. This bulb and some portion of the glass tube is filled with thermometric liquid such as mercury or alcohol. The rest of the volume of the tube has vacuum in it. There is a kink towards the end of the glass tube which keeps the thermometric liquid intact once it is removed from the hot body that is under observation. Such thermometers are basically used to measure the temperature of a human body. A clinical thermometer has a temperature range of 35ºC to 42°C only.
Differences between Clinical thermometer and Laboratory thermometer
| Clinical thermometer | Laboratory thermometer |
| i. This thermometer is used at homes. It is basically used to measure the body temperature of humans. | This thermometer is particularly used for measuring the temperature of all things, excluding the human body. |
| ii. A clinical thermometer has a temperature range of only 35ºC to 42°C. | A laboratory thermometer has a temperature range of -10 ºC to 110 ºC. |
| iii. There is a kink in a clinical thermometer. | A laboratory thermometer does not have a kink. |
b. Differences between heat and temperature
| Heat | Temperature |
| i. It is a form of energy which makes us feel hot or cold. | It measures the degree of hotness or coldness of a particular object. |
| ii. It is measured in Joules (J). | It is measured in kelvin (K), Celsius (°C) and Fahrenheit (°F). |
c. Calorimeter: It is a device that is used for measuring heat.
Construction of a Calorimeter
d. It is a known fact that all solids tend to expand on heating. As rail lines are made up of steel, they tend to expand during summers and contract in winters. However, these expansion and contraction can cause sagging and bending of railway tracks which could derail the trains that run on them. Thus, the rails are provided with gaps at specific distances so as to prevent this bending of rails and avoid derailing of trains. These spaces get closer in summers and become wider in winters.
e. The formulae for the expansion coefficients of liquid and gas is discussed below:
V2 = V1 (1+β∆T)
or β = (V2-V1/V1) 1/∆T
From the above formula, we can say that –
Question 4: Solve the following examples.
a. What must be the temperature in Fahrenheit so that it will be twice its value in Celsius?
b. A bridge is made from 20 m long iron rods. At temperature 18°C, the distance between two rods is 0.4 cm. Up to what temperature will the bridge be in good shape?
c. At 15°C the height of Eiffel tower is 324 m. If it is made of iron, what will be the increase in length in cm, at 30°C?
d. Two substances A and B have specific heats c and 2 c, respectively. If A and B are given Q and 4Q amounts of heat, respectively, the change in their temperatures is the same. If the mass of A is m, what is the mass of B?
e. When a substance having mass 3 kg receives 600 cal of heat, its temperature increases by 10°C. What is the specific heat of the substance?
Answer: a. Let the temperature in Celsius be T.
So, the temperature in Fahrenheit = 2T
Now,
F = 32 + 9 / 5 C
⇒ 2T = 32 + 9 / 5T
T = 160°C
⇒Temperature in Fahrenheit = 2T = 320°F
b. Length of the iron rod = 20 m = 2000 cm at 18°C
Distance between the length of two rods = 0.4 cm
Temperature coefficient of linear expansion of iron = 11.5 × 10−6 °C−1
The bridge will be in proper and good shape until both the rods expand by 0.2 cm due to increase in temperature. Let the temperature be T, both the rods expand by 0.2 cm i.e. the total expansion of rods is 0.4 cm.
Using formula for linear expansion of solids, we have
∆l / l = α1 ∆T
⇒0.4 / 2000 = 11.5 × 10−6 × (T−16)
T = 18 + 0.4 /2000 × 11.5 × 10−6 = 35.4°C
c. Height of Eiffel tower = 324 m = 32400 cm at 15°C
Temperature coefficient of linear expansion of iron = 11.5 × 10−6 °C−1
Change in temperature = 30°C − 15°C = 15°C
Change in length = Δl
Using formula for linear expansion of solids, we have
∆l / l = α1 ∆T
⇒Δl / 32400 = 11.5 × 10−6 × 15
Δl = 32400 × 11.5 × 10−6 × 15 = 5.6 cm
d. Let the mass of B be M.
Let the change in temperature be T for both the bodies, A and B.
The amount of heat in a body is described as
Q = m × c × ΔT
For body A,
Q = m × c × T
⇒T = Q / mc …..(i)
For body B,
4Q = M × 2c × T
⇒M = 4Q / 2c × T
From (i), T= Q / mc
⇒ M = 4Q / 2c × Q / mc = 2m
e. Let the specific heat capacity of the substance be c.
We know, Mass of the substance, m = 3 kg = 3000 g
Heat given to the substance, Q = 600 cal
Increase in temperature of the substance = 10°C
Now, the amount of heat in a body is given as
Q = m × c × ΔT
⇒c = Q / m × ΔT = 600 / 3000 × 10 = 0.02 cal g−1 °C−1
= 0.0033 cal /(gm°C)
Question 1: Define the unit of heat and one calorie heat.
Answer: The unit of heat in the SI system is Joule while that in the CGS unit, it is denoted in calories. One calorie is equivalent to 4.18 Joule. One calorie heat is the heat that is required to increase the temperature of 1 gm of water through 1°C.
Question 2: Name the sources of heat.
Answer: Some of the rich sources of heat are listed below:
1. Sun
2. Earth
3. Chemical energy
4. Electrical energy
5. Atomic energy
6. Air
Question 3: Which is the biggest source of heat for earth?
Answer: The Sun is the biggest source of heat that is received by the earth. A large amount of heat is produced and generated due to the nuclear fusion that takes place at the core of the sun. Here, hydrogen nuclei fuse together to form helium nuclei, thereby generating heat in the process. Some of the sun rays reach the earth in the form of light and heat.
Question 4: What is geothermal energy?
Answer: The earth is a rich source of heat as the temperature at its core is also extremely high. This heat is also referred to as geothermal energy.
Question 5: How is heat derived from chemical energy?
Answer: When fossil fuels such as wood, coal, petrol etc. burn, a chemical reaction occurs between the fossil fuel and oxygen and heat is generated in this reaction. This results in the evolution of chemical energy.
Question 6: What are the electrical appliances from which heat is produced?
Answer: In your daily lives, there are several equipments which produce heat such as electric press, electric heaters, etc. Therefore, electricity is a rich source of heat.
Question 7: Define atomic energy.
Answer: A huge amount of heat is produced or generated in a very short span of time when the nuclei of some elements like uranium, thorium, etc undergo fission. This energy that is generated from a nuclear reaction is referred to as atomic energy. This is extensively used in atomic energy projects.
Question 8: Define temperature.
Answer: Temperature of an object indicates the hotness or coldness of that object. The temperature of a cold object is said to be lower than the temperature of a hot object. Due to this reason, the temperature of ice cream is lesser than the temperature of tea.
Question 9: What is the boiling point of water in temperature units of kelvin (K), Celsius (°C) and Fahrenheit (°F)?
Answer: The boiling point of water in kelvin (K), Celsius (°C) and Fahrenheit (°F) is listed below:
1. 212°F (Fahrenheit)
2. 100°C (Celsius)
3. 373.15 K (kelvin)
Question 10: What is the freezing point of water in temperature units of kelvin (K), Celsius (°C) and Fahrenheit (°F)?
Answer: The freezing point of water in kelvin (K), Celsius (°C) and Fahrenheit (°F) is listed below:
1. 32°F (Fahrenheit)
2. 0°C (Celsius)
3. 273 K (kelvin)
Question 11: What is the boiling and freezing point of mercury in Celsius (°C)?
Answer: The boiling point of mercury is 356.7°C and the freezing point is -38.8°C.
Question 12: What is the room temperature in units of kelvin (K), Celsius (°C) and Fahrenheit (°F)?
Answer: The room temperature in units of kelvin (K), Celsius (°C) and Fahrenheit (°F) is listed below:
1. 72°F (Fahrenheit)
2. 23°C (Celsius)
3. 296 K (kelvin)
Question 13: What are the different types of thermometers?
Answer: Different thermometers are used for different purposes such as:
1. Ordinary thermometer
2. Clinical thermometer
3. Digital thermometer
4. Maximum-minimum thermometer
Question 14: Define specific heat.
Answer: The specific heat of an object is the amount of heat that is required to increase the temperature of unit mass of that particular substance through one degree. This is represented by the symbol ‘c’. Its unit in SI is Joule /(kg°C) and in CGS is cal/(gm°C).
Question 15: What is the specific heat of the substances such as Aluminium, Iron, Copper, Mercury, Water and Hydrogen?
Answer: The specific heat of the substances are listed as below:
1. Aluminium –0.21 cal/(gm°C)
2. Iron –0.11 cal/(gm°C)
3. Copper –0.09 cal/(gm°C)
4. Mercury –0.03 cal/(gm°C)
5. Water –1.0 cal/(gm°C)
6. Hydrogen –3.42 cal/(gm°C)
Question 16: What are the effects of heat on matter?
Answer: The effects of heat on matter are given below:
1. expansion and contraction
2. change of state
Question 17: What happens in expansion of heat?
Answer: When heat is given to any substance, the temperature of the object increases and it expands. Its expansion depends on the increase in its temperature. Solids, liquids and gases all expand on receiving heat.
Question 18: What is linear expansion of solids?
Answer: The linear expansion of a solid is the increase in length of a wire or a rod of a solid due to increase in its temperature.
Question 19: Define λ (lambda).
Answer: λ (lambda) is the constant of proportionality and it is called the coefficient of linear expansion of a solid substance.
Question 20: What is areal expansion of solids?
Answer: The area of a sheet of a solid material increases upon heating just like linear expansion. This is called the areal expansion of solids. σ is the coefficient of areal expansion of a solid substance.
Question 21: What is volumetric expansion of solids?
Answer: When the three dimensional piece of solid expands on all sides when heated and its volume increases just like a sheet. This is called the volumetric expansion of a solid. β is the volumetric expansion coefficient of a solid substance.
Question 22: How is the effect of heat on water different from other liquids?
Answer: The effect of heat on water is somewhat different from that of other liquids. This is known as anomalous behaviour of water.
Question 23: What is the purpose of a digital thermometer?
Answer: Digital thermometers are used for clinical purposes to measure body temperature between 35°C and 42°C. This thermometer does not use the expansion of liquid due to heat like an ordinary thermometer. Instead, it has a sensor which measures the heat coming out from the body directly and from that it measures the temperature of the body.
Question 24: What is the function of a maximum-minimum thermometer?
Answer: Maximum-minimum thermometers are special types of thermometers which are used to measure the minimum and maximum temperatures in a day. These are mainly used in the laboratory as they can measure large spanning temperatures of 40°C to 110°C or even larger.
Question 25: What are potential and kinetic energies?
Answer: Potential energy is the energy in a particular body which is due to its position. While kinetic energy is the energy in a body which is due to its motion.