10 Science Solved Sample Paper 3 – Page 2
Question: 5. What is meant by a homologous series of carbon-compounds? Select two homologous compounds from the following carbon-compounds: CH4O, C2H6O2, C3H6O (2 marks)
Hint: CH4O and C2H6O
Question: 6. Given are three resistances of 3 ohms each. How can they give a total resistance of 9, 1 and 2.0 ohm? Draw suitable diagram for each case. 
Case I: When 3 resistors are connected in series. Then Total R = 9Ω
R = R1+ R2+ R3 = 3 + 3 + 3 = 9 Ω
Case II: When 3 resistors are connected in parallel combination. Then Total R = 1 Ω
R = R1+ R2+ R3 = 1/3 + 1/3 + 1/3 = 1 Ω
Case III: When out of 3 resistors, 2 are connected in series and 3rd is connected in parallel to them. 1/R = 1/6 + 1/3 = 3/6 = 1/2; Then Total R = 2 Ω
The resistivity of three metals namely iron, silver and mercury are 10 × 10-8 Ω m, 1.6 × 10-8 Ω m and 98 × 10-8 Ω m respectively. Answer the following using given data:
(1). Which among iron, silver and mercury is the best conductor?
(2). If the length of the iron conductor is doubled, what will happen to its resistivity?
Give reasons for your answers.
- Silver is the best conductor, as its resistivity is the least among these metals.
- The resistivity of Iron will remain the same, as resistivity of the material is a constant value for that material.
Question: 7. A student has set up a solar cooker in a box using a black painted aluminium sheet, a black cooking vessel, some glass wool, a transparent glass sheet and a mirror plate. What is the role of black paint, transparent glass sheet and a mirror used in the solar cooker? 
Answer: Black Paint absorbs the heat energy quickly and thus helps in increasing the inner temp. Glass sheet causes green-house effect to retain the reflected infrared rays of heat, within the solar cooker.
Mirror reflects the solar rays so that these rays reach inside the solar cooker.
Question: 8. 2g of lead nitrate powder is taken in a boiling tube. The boiling tube is heated over a flame. Now answer the following: 
(1). State the colour of the fumes evolved and the residue left.
(2). Name the type of chemical reaction that takes place stating its balanced chemical equation.
- Brown colored fumes of NO2 gas are evolved; residue left is solid white lead oxide.
- It is a thermal decomposition reaction.
2Pb (NO3)2 Heat → 2PbO + 4NO2↑ (Brown fumes) + O2↑
Question: 9. What is biomass? Name the reaction that takes place in biogas plant. Write the main constituent of biogas. List two reasons for considering biogas an ideal fuel for domestic use. 
Answer: Anaerobic decomposition reaction takes place; Main constituent of biogas is CH4 (methane).