✅ GRADE 8 — 10-MARK TOPPER ANSWERS

MATHEMATICS + SCIENCE (IPS + IPC Format)

📘 MATHEMATICS

CH–1 Numbers & Calculations

Q1 (10M): Explain BODMAS rule and solve an expression using it.

Answer (IPS+IPC)

I – Introduction

In mathematics, expressions often contain more than one operation.

To get the correct answer, we follow a standard order called BODMAS.

P – Procedure / Explanation

BODMAS stands for:

• B → Brackets

• O → Orders (powers, roots)

• D → Division

• M → Multiplication

• A → Addition

• S → Subtraction

Example

Solve:

8+4×(6–2)28 + 4 × (6 – 2)^28+4×(6–2)2

Step 1: Brackets

(6–2)=4(6 – 2) = 4(6–2)=4

Step 2: Orders

42=164^2 = 1642=16

Step 3: Multiplication

4×16=644 × 16 = 644×16=64

Step 4: Addition

8+64=728 + 64 = 728+64=72

C – Conclusion

Thus, by applying BODMAS correctly, the value of the expression is:

72\boxed{72}72

⭐ Keywords: Order of operations, brackets, powers, correct calculation

CH–4 Equations & Inequalities

Q2 (10M): Solve the equation and verify the solution:

3x–7=2x+53x – 7 = 2x + 53x–7=2x+5

Answer

I – Introduction

An equation is a statement showing equality between two expressions.

Solving means finding the value of the variable.

P – Stepwise Solution

Given:

3x–7=2x+53x – 7 = 2x + 53x–7=2x+5

Step 1: Bring variable terms together

Subtract 2x2x2x from both sides:

3x–2x–7=53x – 2x – 7 = 53x–2x–7=5x–7=5x – 7 = 5x–7=5

Step 2: Bring constants together

Add 7 to both sides:

x=12x = 12x=12

Verification

LHS:

3(12)–7=36–7=293(12) – 7 = 36 – 7 = 293(12)–7=36–7=29

RHS:

2(12)+5=24+5=292(12) + 5 = 24 + 5 = 292(12)+5=24+5=29

Since LHS = RHS, solution is correct.

C – Conclusion

Therefore, the value of xxx is:

12\boxed{12}12

⭐ Keywords: Transposition, balance method, verification

CH–11 Ratio & Proportion

**Q3 (10M): A recipe uses flour and sugar in ratio 3:2.

If flour is 450g, find sugar required.**

Answer

I – Introduction

A ratio compares two quantities of the same kind.

P – Solution

Flour : Sugar = 3 : 2

Flour = 450 g

So,

3 parts=450g3 \text{ parts} = 450g3 parts=450g1 part=450÷3=150g1 \text{ part} = 450 ÷ 3 = 150g1 part=450÷3=150g

Sugar = 2 parts:

2×150=300g2 × 150 = 300g2×150=300g

C – Conclusion

Thus, sugar required is:

300g\boxed{300g}300g

⭐ Keywords: Ratio, unitary method, proportional reasoning

🔬 SCIENCE

⚡ PHYSICS

Unit 3.1 Density

Q4 (10M): Define density and describe an experiment to find density of a solid.

Answer (IPS+IPC)

I – Introduction

Density is a physical property that tells how much mass is packed into a given volume.

P – Explanation

Definition

Density=MassVolumeDensity = \frac{Mass}{Volume}Density=VolumeMass

Unit: g/cm³ or kg/m³

Experiment to Find Density

Apparatus

• Measuring cylinder

• Water

• Solid object

• Weighing balance

Steps

Step 1: Measure mass

Suppose mass = 200 g

Step 2: Measure volume using water displacement

Initial water level = 50 cm³

Final water level = 80 cm³

Volume of solid:

80–50=30cm380 – 50 = 30 cm³80–50=30cm3

Step 3: Calculate density

Density=20030=6.67g/cm3Density = \frac{200}{30} = 6.67 g/cm³Density=30200=6.67g/cm3

C – Conclusion

Thus, density depends on both mass and volume and can be found using displacement method.

⭐ Keywords: Mass, volume, displacement, physical property

Unit 9 Electricity

Q5 (10M): Explain parallel circuits and why they are used in homes.

Answer

I – Introduction

A parallel circuit has components connected in separate branches, providing multiple paths for current.

P – Explanation

Features

• Voltage is same across each branch

• Current divides among branches

• Devices work independently

Diagram

   Bulb 1
---( )---|
         |
---( )---| Bulb 2
         |
---( )---| Bulb 3

Advantages

1. If one bulb fails, others continue working

2. Each appliance gets full voltage

3. Safe and reliable for wiring

C – Conclusion

Therefore, parallel circuits are preferred in household wiring for efficiency and safety.

⭐ Keywords: Branches, same voltage, household wiring

🧪 CHEMISTRY

Unit 5 Reactivity Series

Q6 (10M): Explain the reactivity series and its importance.

Answer

I – Introduction

The reactivity series is an arrangement of metals in decreasing order of their chemical reactivity.

P – Reactivity Order

K>Na>Ca>Mg>Al>Zn>Fe>Cu>Ag>AuK > Na > Ca > Mg > Al > Zn > Fe > Cu > Ag > AuK>Na>Ca>Mg>Al>Zn>Fe>Cu>Ag>Au

Importance

1. Predict displacement reactions

   Example:

Zn+CuSO4→ZnSO4+CuZn + CuSO_4 → ZnSO_4 + CuZn+CuSO4→ZnSO4+Cu

1. Metal extraction

   Highly reactive metals need electrolysis.

2. Corrosion prevention

   Less reactive metals resist rusting.

C – Conclusion

Thus, reactivity series helps in understanding metal behaviour in reactions.

⭐ Keywords: Displacement, extraction, reactivity order

🌱 BIOLOGY

Unit 1 Photosynthesis & Carbon Cycle

Q7 (10M): Describe photosynthesis with equation and its role in carbon cycle.

Answer

I – Introduction

Photosynthesis is the process by which green plants prepare food using sunlight, CO₂ and water.

P – Process Explanation

Occurs in chloroplasts containing chlorophyll.

Equation:

6CO2+6H2O→lightC6H12O6+6O26CO_2 + 6H_2O \xrightarrow{light} C_6H_{12}O_6 + 6O_26CO2+6H2OlightC6H12O6+6O2

Role in Carbon Cycle

• Plants absorb CO₂

• Produce oxygen

• Animals consume plants

• Respiration returns CO₂

Flow:

CO₂ → Plants → Animals → CO₂

C – Conclusion

Thus, photosynthesis is essential for life, oxygen supply and carbon balance.

⭐ Keywords: Chlorophyll, glucose, carbon cycle, oxygen

Unit 7 Genes & Inheritance

Q8 (10M): What are genes? Explain inheritance with example.

Answer

I – Introduction

Genes are the basic units of heredity present on chromosomes.

They control traits passed from parents to offspring.

P – Explanation

• Genes carry instructions for features like height, eye colour

• Offspring inherit genes from both parents

Example:

Parents: Tall trait dominant

Child likely inherits tallness

C – Conclusion

Thus, genes ensure continuity of characteristics across generations.

⭐ Keywords: Heredity, chromosomes, traits, inheritance