2019 QUESTIONS

Question1. What is a homologous series of carbon compounds ? Give an example and list its three characteristics. [3]
Answer:
A homologous series is a group of organic compounds having similar structures and similar chemical properties in which the successive compounds differ by CH₂ group.
Example: Alkanes with general formula C_nH_2n+2
Characteristics:

·         All the members of a homologous series can be represented by the same general formula.

·         Any two adjacent homologues differ by 1 carbon atom and 2 hydrogen atoms in their molecular formulae.

·         The difference in the molecular masses of any two adjacent homologues is 14 u.

Question2. Write the essential function performed by ozone at the higher levels of the Earth’s atmosphere ? How is it produced ? Name the synthetic chemicals mainly responsible for the drop of amount of ozone in the atmosphere. How can the use of these chemicals be reduced? [3]
Answer:
Ozone layer absorbs most of the harmful ultraviolet radiations from the sun to the earth. It is formed high up in the atmosphere by the action of ultraviolet radiation on oxygen gas. Chlorofluorocarbons are the synthetic chemicals responsible for the drop of amount of ozone in the atmosphere.
The use of these chemicals can be reduced by:

·         Replacement of chlorofluorocarbons with hydrochlorofluorocarbons because it breaks down more quickly.

·         Safe disposal of old appliances such as refrigerators and freezers.

 

Question3. (a) List any three observations which posed a challenge to Mendeleev’s Periodic law.
(b) How does the metallic character of elements vary on moving from
(i) left to right in a period,
(ii) from top to bottom in a group of the Modem Periodic Table?
Give a reason for your answer.
OR
The electrons in the atoms of four elements A, B, C and D are distributed in three shells having 1, 3, 5 and 7, electrons respectively in their outermost shells. Write the group numbers in which these elements are placed in the Modem Periodic Table. Write the electronic configuration of the atoms of B and D, and the molecular formula of the compound formed when B and D combine. [5]

Answer:
(a) Three observations which posed a challenge to Mendeleev’s Periodic law are:

·         The position of isotopes could not be explained.

·         Wrong order of atomic masses of some elements could not be explained.

·         A correct position could not be assigned to hydrogen in the periodic table.

(b) (i) On moving from left to right in a , period, the metallic character of elements decreases because electropositive character decreases.
(ii) Ongoing down in a group of the periodic table, the metallic character of elements increases because of electropostive character of elements increases.
OR
A – 1st group.
B – 13th group.
C – 15th group.
D – 17th group.
Electronic configuration
B → Atomic number = 13.
K L M
2 8 3
D → Atomic number = 17
K L M
2 8 7
The molecular formula of the compound when B and D combine is BD₃.

 

Question 4. (a) What are homologous structures ? Give an example.
(b) “The sex of a newborn child is a matter of chance and none of the parents may be considered responsible for it.” justify this statement with the help of a flow chart showing sex-determination in human beings. [5]
Answer:
(a) The structures which have the same basic design but different functions are called homologous structures or homologous organs.
Example: Forelimbs of a man, a lizard, a frog they have same basic design of bones but perform different functions.
(b) The sex of a newborn depends on what happens at the time of fertilization.
1. If a sperm carrying X chromosome fertilizes the ovum carrying X chromosome, then the girl child will be born and the child will have XX combination of sex chromosomes.
2. If a sperm carrying Y chromosome fertilizes the ovum carrying X chromosome, then the child born will be a boy and the child will have XY combination of sex chromosome.





The above presentation clearly shows that it is matter of chance whether the newborn will be boy or girl and none of the parents may be considered responsible for it.

Question5. (a) How will you infer with the help of an experiment that the same current flows through every part of a circuit containing three resistors in series connected to a battery?
(b) Consider the given circuit and find the current flowing in the circuit and potential difference across the 15 Ω resistor when the circuit is closed. [5]


OR
(a) Three resistors R₁, R₂ and R₃ are connected in parallel and the combination is connected to a battery, ammeter, voltmeter and key. Draw suitable circuit diagram and obtain an expression for the equivalent resistance of the combination of the resistors.
(b) Calculate the equivalent resistance of the following network:

Answer:
(a) Let three resistors R₁, R₂ and R₃ are connected in series which are also connected with a battery, an ammeter and a key as shown in figure.

When key is closed, the current starts flowing through the circuit. Take the reading of ammeter. Now change the position of ammeter to anywhere in between the resistors and take its reading. We will observe that in both the cases reading of ammeter will be same showing same current flows through every part of the circuit above.
(b) Given,
R₁ = 5 Ω, R₂ = 10 Ω, R₃ = 15 Ω, V = 30 V
Total resistance, R = R₁ + R₂ + R₃ [∵ 5 Ω, 10 Ω and 15 Ω are connected in series]
= 5 + 10 + 15
= 30 Ω
Potential difference, V = 30 V
Current in the circuit, I = ?
From Ohm’s law.
I = V = 30 = 1 A
Current flowing in the circut = 1 A
Potential difference across 15 Ω resistors = IR3 = 1 × 15 = 15 V
OR
(a) Let R1, R2 and R3 are three resistance connected in parallel to one another and R is the equivalent resistance of the circuit. A battery of V volts has been applied across the ends of this combination. When the switch of the key is closed, current I flows in the circuit such that,








10 Ω and 10 Ω are connected in series.
Equivalent resistance of the circuit = 10 Ω + 10 Ω = 20 Ω

Question6. Draw the pattern of magnetic field lines produced around a current-carrying straight conductor passing perpendicularly through horizontal cardboard. State and apply the right-hand thumb rule to mark the direction of the field lines. How will the strength of the magnetic field change when the point where magnetic field is to be determined is moved away from the straight conductor ? Give reason to justify your answer. [5]
Answer:
Maxwell’s Right Hand Thumb rule states that if the current-carrying wire is imagined to be held in the right hand so that thumb points in the direction of current, then the direction in which fingers encircle the wire will give the direction of magnetic field lines around the wire. If we hold the current-carrying straight wire so that thumbs Magnetic field pattern due to a straight current-carrying wire in upward direction points the direction of current, the direction of magnetic field lines will be anticlockwise. The strength of the magnetic field is inversely proportional to the distance of the point of observation from the wire. So, as we move away from the wire the strength of magnetic decreases.

 

Question 7.  Should the resistance of a voltmeter be low or high? Give reason. [1]
Answer:
The resistance of a voltmeter should be high because voltmeter is connected parallel to the component of a circuit and it also takes negligible current from the circuit in order to measure the potential difference

Question 8. Distinguish between pollination and fertilisation. Mention the site and the product of fertilisation in a flower. [3]
Answer:

1.    The transfer of pollen grains from anther of a stamen to the stigma of a carpel is called pollination whereas fertilisation is the process when the male gamete present in pollen grain joins the female gamete present in ovule.

2.    Pollination is an external mechanism whereas fertilisation is an internal mechanism which takes place inside the flower.
Site of fertilisation in flower is ovary. Product of fertilisation in flower is zygote.

 

Question 9. Which compounds are called (i) alkanes, (ii) alkenes and (iii) alkynes? C₄H₁₀ belongs to which of these? Draw two structural isomers of this compound. [3]
Answer:
(i) The hydrocarbons in which carbon atoms are connected by only single covalent bonds are called alkanes.
(ii) The hydrocarbons in which carbon atoms are connected by the double bond are called alkene.
(iii) The hydrocarbons in which carbon atoms are connected by the triple bond are called alkynes.
C₄H₁₀ belongs to alkane

 

 

Question 10.
(a) What are the dominant and recessive traits?
(b) “Is it possible that a trait is inherited but may not be expressed in the next generation ?” Give a suitable example to justify this statement. [5]
Answer:
(a) The trait which can express its effect over contrasting trait is called dominant trait whereas the trait which cannot express its effect over contrasting trait or which gets suppressed by the contrasting trait is called recessive trait. The inherited trait which is not expressed will be a recessive trait.

(b) In Mendel’s experiment, when pure tall pea plants were crossed with pure dwarf pea plants, only tall pea plants were obtained in F₁ generation. On selfing, the pea plants of F₁ generation both tall and dwarf pea plants were obtained in F₂ generation. The reappearance of the dwarf pea plants in F₂ generation proves that the dwarf trait was inherited but not expressed in F₁ generation. The recessive trait does not express itself in the presence of the dominant trait. So, it is possible that one trait may be inherited but may not be expressed in an organism.

 

Question 11. Define a food chain. Design a terrestrial food chain of four trophic levels. If a pollutant enters at the producer level, the organisms of which trophic level will have the maximum concentration of the pollutant in their bodies? What is this phenomenon called? [3]
Answer:
It is the sequence of arrangement of a living organism in a community in which one organism consumes another organism to transfer food energy.


Grass → Insect → Frog → Bird
The organism at a higher tropic level will have a maximum concentration of pollutants. This phenomenon is called biological magnification.