Class 8 IMP SCIENCE

Important questions of Science class 8

Chapter 1: Crop Production And Management

1. What are the primary practices involved in crop production?

2. Explain the importance of crop rotation in agriculture.

3. Discuss the role of organic farming in sustainable crop production.

4. How does the use of pesticides impact the environment?

5. Describe the different methods of weed control in crop fields.

Chapter 2: Microorganisms: Friends and Enemies

1. How do microorganisms contribute to the nitrogen cycle in soil?

2. Explain the process of fermentation and its industrial applications.

3. Discuss the role of microorganisms in sewage treatment.

4. What are the benefits and drawbacks of using antibiotics?

5. How can we prevent food spoilage caused by microorganisms? 

Coal and Petroleum

1. Compare the extraction processes of coal and petroleum.

2. Discuss the various uses of coal in industries.

3. Explain the concept of refining in the petroleum industry.

4. What are the environmental challenges associated with fossil fuel consumption?

5. Describe the formation of fossil fuels over geological time.

Combustion And Flame

1. How is combustion different in a candle flame and a forest fire?

2. Explain the role of oxygen in the combustion process.

3. Discuss the importance of fire safety measures in households.

4. Differentiate between a luminous and a non-luminous flame.

5. What is the significance of controlling the combustion process in engines?

 Conservation of Plants And Animals

1. Explain the role of national parks in biodiversity conservation.

2. Discuss the impact of deforestation on the ecosystem.

3. How can individuals contribute to the conservation of endangered species?

4. Describe the importance of wildlife corridors.

5. Why is it essential to conserve water for plant and animal life?

Apologies for the oversight. Let's continue with the questions for the remaining chapters:

**Chapter 10: Reproduction In Animals**

1. Compare internal and external fertilization in animals.

2. Explain the significance of reproductive health education.

3. Discuss the adaptations in animals for different modes of reproduction.

4. How does the process of fertilization occur in humans?

5. Describe the life cycle of a butterfly, focusing on the reproductive stages.

**Chapter 6: Towards Adolescence**

1. Enumerate the physical changes during adolescence in boys and girls.

2. Discuss the importance of emotional well-being during adolescence.

3. Explain the role of hormones in the development of secondary sexual characteristics.

4. How can a balanced diet support the growth spurt during adolescence?

5. Describe the concept of peer pressure and its impact on adolescents.

**Chapter 7: Force and Pressure**

1. Differentiate between contact and non-contact forces.

2. Explain the effect of force on the shape and size of an object.

3. Discuss the practical applications of pressure in everyday life.

4. How does force affect the motion of an object?

5. Describe the relationship between force, mass, and acceleration (Newton's second law).

**Chapter 5: Friction**

1. Explain the factors affecting friction.

2. Discuss the role of lubricants in reducing friction.

3. How does friction influence the efficiency of machines?

4. Describe the difference between static and kinetic friction.

5. Provide examples where friction is advantageous in daily life.

**Chapter 6: Sound**

1. How does sound travel through different mediums?

2. Explain the concept of pitch in relation to sound waves.

3. Discuss the applications of ultrasonic waves in various fields.

4. Describe the factors influencing the speed of sound.

5. What is resonance, and how is it related to sound?

**Chapter 7: Chemical Effects of Electric Current**

1. Discuss the role of electrolytes in conducting electricity.

2. Explain the process of electroplating and its applications.

3. How do chemical effects of electric current contribute to corrosion?

4. Define resistance and its factors in an electrical circuit.

5. Describe the safety measures while working with electric circuits.

**Chapter 12: Light**

1. Explain the concept of refraction of light. Provide examples.

2. Discuss the formation of an image by a concave lens.

3. How does dispersion of light lead to the formation of a spectrum?

4. Describe the working principle of a periscope.

5. Explain the difference between regular and diffuse reflection.

**Chapter 13: Some Natural Phenomena**

1. Describe the process of lightning formation.

2. Explain the characteristics of earthquakes and their measurement.

3. Discuss safety measures during a cyclone.

4. How is a tornado formed, and what are its effects?

5. Explain the phenomenon of auroras and their occurrence.

FACTORISATION IMPORTANT CLASS 8

 FACTORISATION IMPORTANT

BY AARISH SIR

1. Express the following as in the form of (a+b)(a-b)

(i) a² – 64

(ii) 20a² – 45b²

(iii) 32x²y² – 8

(iv) x² – 2xy + y² – z²

(v) 49x² – 1

Solution:

For representing the expressions in (a+b)(a-b) form, use the following formula

a² – b² = (a+b)(a-b)

(i) a² – 64 = a² – 8² = (a + 8)(a – 8)

(ii) 20a² – 45b² = 5(4a² – 9b²) = 5(2a + 3b)(2a – 3b)

(iii) 32x²y² – 8 = 8( 4x²y² – 1) = 8(2xy + 1)(2xy – 1)

(iv) x² – 2xy + y² – z² = (x – y)² – z² = (x – y – z)(x – y + z)

(v) 49x² – 1 = (7x)² – (1)² = (7x + 1)(7x – 1)

2. Verify whether the following equations are correct. Rewrite the incorrect equations correctly.

(i) (a + 6)² = a² + 12a + 36

(ii) (2a)² + 5a = 4a + 5a

Solution:

(i) (a + 6)² = a² + 12a + 36

Here, LHS = (a + 6)² = a² + 12a + 36

Now, RHS = a² + 12a + 36

Hence, LHS = RHS.

(ii) (2a)² + 5a = 4a + 5a

Here, LHS = (2a)² + 5a = 4a² + 5a

Now, RHS = 4a + 5a

So, LHS ≠ RHS

Correct equation: (2a)² + 5a = 4a² + 5a

3. For a = 3, simplify a² + 5a + 4 and a² – 5a

Solution:

Substitute the value of a = 3 in the given equations.

a² + 5a + 4 = 3² + 5(3) + 4 = 9 + 15 + 4 = 28

And,

a² – 5a = 3² – 5(3) = 9 – 15 = -6

Long Answer Type Questions:

4. Find the common factors of the following:

(i) 6 xyz, 24 xy² and 12 x²y

(ii) 3x² y³, 10x³ y² and 6x² y² z

Solution:

(i) 6 xyz = 2 × 3 × x × y × z

24 xy² = 2 × 2 × 2 × 3 × x × y × y

12 x²y = 2 × 2 × 3 × x × x × y

Thus, the common factors are common factors of 6 xyz, 24 xy² and 12 x²y are 2, 3, x, y and, (2 × 3 × x × y) = 6xy

(ii) 3x² y³ = 3 × x × x × y × y × y

10x³ y² = 2 × 5 × x × x × x × y × y

6 x² y² z = 3 × 2 × x × x × y × y × z

Now, the common factors of 3x² y³, 10x³ y² and 6x² y² z are x², y² and, (x² × y²) = x² y²

5. Factorize the following expressions:

(i) 54x³y + 81x⁴y²

(ii) 14(3x – 5y)³ + 7(3x – 5y)²

(iii) 15xy + 15 + 9y + 25x

Solution:

(i) 54x³y + 81x⁴y²

= 2 × 3 × 3 × 3 × x × x × x × y + 3 × 3 × 3 × 3 × x × x × x × x × y × y

= 3 × 3 × 3 × x × x × x × y × (2 + 3 xy)

= 27x³y (2 + 3 xy)

(ii) 14(3x – 5y)³ + 7(3x – 5y)²

= 7(3x – 5y)² [2(3x – 5y) +1]

= 7(3x – 5y)² (6x – 10y + 1)

(iii) 15xy + 15 + 9y + 25x

Rearrange the terms as:

15xy + 25x + 9y + 15

= 5x(3y + 5) + 3(3y + 5)

Or, (5x + 3)(3y + 5)

6. Factorize (x + y)² – 4xy

Solution:

To solve this expression, expand (x + y)²

Use the formula:

(x + y)² = x² + 2xy + y²

(x + y)² – 4xy = x² + 2xy + y² – 4xy

= x² + y² – 2xy

We know, (x – y)² = x² + y² – 2xy

So, factorization of (x + y)² – 4xy = (x – y)²

7. Factorize x² + 6x – 16

Solution:

To factorize, it should be checked that the sum of factors of 16 should be equal to 6.

Here, 16 = -2 × 8 and 8 + (-2) = 6

So,

x² + 6x – 16 = x² – 2x + 8x – 16

= x(x – 2) + 8(x – 2)

= (x + 8) (x – 2)

Hence, x² + 6x – 16 = (x + 8) (x – 2)

8. Solve for (4x² – 100) ÷ 6(x + 5)

Solution:

= ⅔ (x – 5)

TRY THESE ALSO

1. Factorise:
   (а) 14m5n4p2 – 42m7n3p7 – 70m6n4p3
   (b) 2a2(b2 – c2) + b2(2c2 – 2a2) + 2c2(a2 – b2)
2.  The area of a rectangle is 6a2 + 36a and 36a width. Find the length of the rectangle.
3. What are the common factors of the following terms?
   (a) 25x2y, 30xy2
   (b) 63m3n, 54mn4

EXPONENTS AND POWERS IMPORTANT QUESTION CLASS 8

EXPONENTS AND POWERS IMPORTANT QUESTIONS

 1. Find the value of (40 + 4-1) × 22

Solution:

(40 + 4 -1) × 22 = (1 + ¼) × 4

= 5/4 x 4

= 5

2. Solve 3-4 and (½)-2

Solution:

We know, b-n = 1/bn

So, 3-4 = 1/34 = 1/81

And, (½)-2 = 1-2/2-2 = 22/12 = 4

3. Simplify the following expression and express the result in positive power notation:

(−4)5 ÷ (−4)8

Solution:

Using am ÷ an = am-n

(−4)5 ÷ (−4)8 = (-4)5/(-4)8

⇒ (-4)5-8 = 1/ (-4)3

4. Evaluate a2 × a3 × a-5

Solution:

a2 × a3 × a-5 = a2+3-5

= a5-5

= a0 = 1

5. Express 4-3 as a power with base 2.

Solution:

4-3 can be written as:

4-3 = (22)-3

Now, by using exponential law i.e. (am)n = amn

4-3 = 2-6 (which is in base 2 form).

6. Evaluate (√4)-3

Solution:

(√4)-3 = (4½)-3

= 4-3/2 = 1/ 43/2

= 1/(43)½ = 1/(64)½

= 1/(82)½ = 1/8

7. Find the value of x for which 2x ÷ 2-4 = 45

Solution:

Given,

2x ÷ 2-4 = 45

Now, 2x × (½)-4 = (22)5

Or, 2x × (½)-4 = 210

Thus, 2x+4 = 210

⇒ x + 4 = 10

Hence, x + 4 = 10

So, x = 6

8. Calculate the missing value of “x” in the following expression: (11/9)3 × (9/11)6 = (11/9)2x-1

Solution:

Given: (11/9)3 × (9/11)6 = (11/9)2x-1

The multiplier of L.H.S of the equation can be written as:

(11/9)3 × (11/9)-6 = (11/9)2x-1

⇒ (11/9)3-6 = (11/9)2x-1

Therefore, -3 = 2x – 1
2x = -3 + 1
x = -2/2
x = -1

9. 5 books and 5 paper sheets are placed in a stack. Find the total thickness of the stack if each book has a thickness of 20 mm and each sheet has a thickness of 0.016 mm.

Solution:

Given,

Thickness of 1 book = 20 mm

And,

Thickness of one paper = 0.016 mm

So, thickness of 5 books = 20 x 5 = 100 mm

And,

Thickness of 5 papers = 0.016 × 5 = 0.08 mm

Now, the total thickness of a stack is:

= 100 + 0.08 = 100.08 mm

= 100.08 102 / 102 mm

= 1.0008 × 102 mm

10. If a new-born bear weighs 4 kg, calculate how many kilograms a five-year-old bear weigh if its weight increases by the power of 2 in 5 years?

Solution:

Given,

Weight of new-born bear = 4 kg

Rate of weight increase in 5 years = power to 2

Thus, the weight of the 5-year old bear = 42 = 16 kg

Q.11: Simplify [25 x t-4]/[5-3 x 10 x t-8]

Solution:

We can write the given expression as;

[52x t-4]/[5-3 x 5 x 2 x t-8]

= [52 x t-4+8]/[5-3+1 x2]

= [52+2 x t4]/[2]

= [54 x t4]/[2]

= [625/2] t4

Q.12: Express 0.00000000837 in standard form.

Solution:

0.00000000837

= 0.00000000837 x 109 / 109

= 8.37 ×10-9

Q.13: Write 3.61492 x 106 in usual form.

Solution: 3.61492 x 106

= 3.61492 x 1000000

= 3614920

TRY THESE QUESTIONS ALSO

1. Evaluate: (-4)-3
2. Simplify: (𝟑-7÷ 𝟑-9) × 𝟑-4
3. Find the value of (37 + 4-3 + 53)0
4. Evaluate: [{1/2}-1+{1/3}-1]-1
5. Express 31860000000 in standard form.
6. Find x so that (-5)x+1 × (-5)5 = (-5)7
7. Solve the following: (81)-4 ÷ (729)2-x = 94x