In this video we will look into the Science Questions that were asked in CDS Exam on November 18th 2018.
If you are interested only in the answers, you will find a link to download the corresponding PDF file in description below.
However, if you are interested in the detailed solution, please keep watching.
There were 31 Science questions out of total 120.
This is almost one-fourth of entire question paper, as expected.
There were 10 questions from Biology, 10 questions from Chemistry, and 11 questions from Physics.
I was glad to know that answers to most of these questions can be found in my book, either directly or indirectly.
During explanation of answers,
I will provide reference to page numbers in my book, where you can read details of that particular topic, so that you can easily follow.
Let's start with biology first.
Notice that you can see the question number highlighted on the top, and subject highlighted on right side of the screen.
This will help you in navigating if you intend to jump to a particular section.
To make it easy for you, the PDF provided for download in the description, follows the same theme and question numbers.
Let's start with first question.
Which of the following is OR are the main absorbing organ OR organs of plants?
A. Root only:
Roots absorb water and other solutes from soil. So, this is TRUE
B. Leaf only:
Leaves absorb gases from atmosphere. So, this is also TRUE
C. Root and Leaf only: This option simply means both A and B. So, this is better answer as of now.
D. Root, leaf and bark: To understand the absorption by barks, you need to know the term 'Lenticels'.
In my book, I introduced the term on Page B.44. Lenticels are small cracks in tree bark, and they do help in gas exchange.
So, essentially, bark is also an absorbing organ.
However, lenticels develop only when a tree increases its width of stem.
This is called secondary growth. Not all plants undergo secondary meristematic growth.
As we read the question again, we find that it is asking about only the main organs.
Lenticels, thus, do not qualify to be main organs. Hence, Option C is our correct answer.
Question number 2 asks which among the options is not primary function of leaves.
Option A says manufacturing food. Obviously, leaves perform photosynthesis, so this is a function of leaves.
Option B says interchange of gases.
Presence of stomata, usually on the lower side of leaves, helps gas exchange. This topic is studied in depth under 'Plant water relations'.
Option C is evaporation of water.
Again transpiration by leaves, its rate, effect of cuticle on leaves, and other related aspects are also studied in 'Plant water relations'.
Option D, that is conduction, is however preformed by xylem and phloem.
Now notice the word NOT in question. That leaves Option D as our correct answer.
Question number three. Which of the following denotes a true fruit?
In my book, on page B.40, I clearly mentioned the meaning of true fruit, as one which arise from ovary.
When we look at options, Option-D simply states this definition. It was a pretty straight-forward question.
Question four asks the name of the process, when water escapes as droplets of water.
For this, refer to Page B.46. I clearly mentioned in the definition of Guttation, that water oozes out in liquid form.
This is very different from transpiration from leaves, where water leaves in gaseous form, as discussed in question 2.
Now notice the question, how it clearly asks about the water droplets.
That leaves us with no choice
but mark option B as our correct answer.
Question 5. This questions when rephrased, simply asks the function of Xylem in plants.
Again, refer my book with page number mentioned here, and you will find that Xylem transports water and minerals.
And by minerals, it is intuitive that they are dissolved in the water.
Now let's come back to options. Option A. says water only. This can not be correct because it leaves minerals out.
Hence, we mark this as incorrect answer.
Option B says both water and solutes, which makes it our correct answer.
Coming to question number 6,
we find a question regarding plant reproduction, OR pollination to be precise.
The questions asks which of the following is not a form of pollination.
Among various means of pollination, we study pollination by wind (called as anemophily),
pollination by animals (zoophily)
and pollination by water (hydrophily).
There are also other means like pollination by bats,
bees, and other insects which are not mentioned in the options.
BUT pollination by fungus is not among those explained in textbooks, which makes option B, our desired answer.
To learn more about plant reproduction, refer to chapter B.21 in my book.
Question number 7 asks, which among the given options is a stem modification.
Plant modifications is an important topic, and it is covered under chapter "Plant morphology" in textbooks.
There can be root modifications, leaf modifications, etc.
This question however, concerns stems. And to identify a stem, one rule of thumb is presence of nodes and inter-nodes in it.
In the book, I have explained that Potato is actually an underground stem. And this directs us to Option A, which is our correct answer.
Till now, all 7 questions discussed were related to plants.
Regarding human biology, there were only 2 questions, and both are from reproduction. This is first of them.
The question is asking about various functions in male-reproductive system.
In options, we see only three processes. The first among them is 'Spermatogenesis'.
Let us mark it in cyan.
Second process is maturation. Let us mark it red. Third process is motility. Let us note it in green.
Keeping only these three aside, I am providing with the associated parts of reproductive system.
Testis are responsible for spermatogenesis.
Maturation takes place in epididymis. Others involve in motility of sperms.
Looking at options, and marking all the parts concerned,
we see epididymis (that is in red) and others (that is in green). But we do not see Testis.
So, we eliminate all options that have Spermatogenesis in them.
This eliminates options A and C.
Now option D can not be the answer because it does not take epididymis into account. Hence, option B is our answer.
However, you must know that spermatogenesis is merely one of the types of gametogenesis that takes place in humans.
In males, it is called spermatogenesis (meaning the production of sperms).
In females, it is called Oogenesis (meaning production of ovum).
To study about Gametogenesis in short, you can refer to the page number given here. Now, let's move on to next question.
It is also related to human reproduction. This question is regarding females.
It is asking the particular term given to the milk secreted by a lactating mother.
This is also a straight-forward question. I clearly mentioned in my book that this milk is called Colostrum. So, Option C is our answer.
Coming to the last question in biology section, we find that it is regarding genetics.
It is asking why mutation is more frequent in RNA virus.
Before reading the options, you may take this question in many ways. Either the question intends to check our knowledge about virus, as compared to cellular organisms.
Or it may be trying to check our ability to distinguish between the properties of RNA and DNA.
When we look at the options, it becomes clearer.
Option A says that DNA is more stable which is actually TRUE.
Option B says that a virus needs a host cell to multiply, which is also TRUE
Option C says that there are no metabolic processes inside a virus.
Assuming it is not inside another cell, this is also a correct statement.
Option D is about the ability of virus to remain inactive for very long time, and this is ALSO correct statement.
A question with such options is one of the best ways an examiner can set up question paper.
This is testing, if the answer is actually related to what is asked in the question. The relevance of answer now depends on the keywords.
Focus on words like HIGHER MUTATION RATE and RNA VIRUS.
Mutation is the change in sequence (usually defined for DNA)
and that change is not predictable. It can be harmful for organism.
It can also be beneficial for the organism. But most mutations are neither of the two. They are not expressed.
For RNA to show more mutation that DNA, it is essential that RNA is prone to changes compared to DNA.
That is simply a question of its stability.
Now when you check my book, I have mentioned that DNA (and especially double stranded DNA) is more stable.
So, this points to our answer, Option A.
There is also an entire article about the types of virus in the book,
but that section was not relevant for this question.
It would have had its relevance, if the question intended to distinguish RNA virus from DNA virus, or their virulence.
But the question was only about RNA v/s DNA. This brings us to the end of Biology sections.
Now we will move on to the first question of Chemistry.
I found three out of ten questions in chemistry, where more than one option was correct.
It would have been easier for me to digest this fact if SSC did it,
because reputation and past question papers of SSC have always been full of errors.
But I found it hard to accept that UPSC did such a thing,
because UPSC is known to set up quality questions in my opinion.
So, I was skeptical about my understanding of chemistry at first.
My trust on UPSC experts, that they are less likely to make mistakes, than my own expertise in the subject,
forced me to dive back in a few reference books.
Eventually, I decided to go ahead and accept that there were actually more than one correct answers.
However, if you differ from my opinion in those questions, please leave your comment below, and let me know.
Okay, enough chitchat. Let's come to question number one. This question is asking how to turn lime water milky.
Now everybody knows that carbon-dioxide turns lime water milky.
But which of the given reactants will produce carbon-dioxide?
I stated in the book, that combustion of organic materials produces CO2.
So, that directs us to option C. Methane plus Oxygen gives us water and CO2.
Eventually, CO2 produced will turn lime water milky The second question of chemistry is about limiting reagent.
It gives us a reaction between methane and oxygen. The balanced reaction says that 1 mole of methane requires half mole of oxygen.
However, the question is asking to react 2 moles of methane with 2 moles of oxygen. So, let's try to analyse it.
1 mole of methane requires half mole of oxygen. This means, 2 moles of methane will need only 1 mole of oxygen.
And any more amount of oxygen will be in excess, and it will not be utilized.
Also, to consume full 2 moles of oxygen, we need at least 4 moles of methane.
This is not the case, as our question clearly specifies that we are limited with only 2 moles of methane.
In this case, methane is the limiting reagent, and oxygen is in abundance.
So, we will have to revert back to second case, and ignore the last case.
Now, we know that 1 mole CO will be produced if 1 mole CH4 is combusted.
For two moles of CH4, we will get 2 moles of CO.
It will not use all the oxygen, that was given in the question,
but it will consume all the methane, and the reaction will cease to proceed any further.
Thus, 2 mole is the solution, and Option A will be our correct answer.
Third question is asking how to produce hydrogen.
This is one of those questions, where there are two correct answers.
A simple rule of thumb is when a metal reacts with an acid, it produces hydrogen gas.
This may not be always true, like option B in this question. But generally, it is true. Here, both A and D are correct answers.
Next question is about four gases. It says "which of the following characteristics is common to –
Hydrogen
Nitrogen
Oxygen And Carbon Dioxide
We know that a molecule of hydrogen has 2 atoms with a single bond, Nitrogen has two atoms with triple bond
Oxygen has two atoms with double bond And CO2 has one atom of carbon and two atoms of oxygen,
and its molecule has a linear structure.
Now coming to first option, it says that they all are diatomic. This is obviously wrong as CO2 has three atoms.
Second option says that normally they are gases. This is true.
Next option says that they are coloured. However, all of them are transparent, which makes it a wrong statement.
Last option says that they all have same reactivity.
This is also not correct. It is evident by the fact that Nitrogen has a triple bond
which makes it much more stable and less reactive than Oxygen and Hydrogen.
So, that brings us to Option B as our correct answer.
Question number five provides us with this compound which has
C-Seven H-Seven One atom of nitrogen And two atoms of oxygen
The first option says that one such molecule has 17 atoms. Let us add all these atoms.
7+7+1+2 gives us 17. So, first option is correct.
Option 2 says that mass of Carbon and Hydrogen is same. We know that each carbon atom weighs 12 amu.
So, total mass of carbon is 84amu.
Similarly, each hydrogen atom weighing one-AMU
give us total 7amu hydrogen. So, this option is wrong
Third option compares mass of nitrogen and oxygen.
Mass of 14 for nitrogen,
and mass of 32 for oxygen do not differ by a factor of 2. Hence, Option C is also wrong.
Coming to the last option, we see that hydrogen has mass of 7 and nitrogen has mass of 14.
So, nitrogen is double the mass of hydrogen atoms.
It makes this option, a TRUE statement.
Now we encounter another such question with two correct answers. Both A and D are correct answers.
Sixth question asks what is the general formula for saturated hydrocarbons.
In my book, I have explained that a saturated molecule is one which does not have a double or triple bond.
Also, I have explained that there are only single bonds in Alkanes, which signifies that alkanes are saturated molecules.
Then, we have general formula for alkanes given on Page C.72. So, we can pin point our answer as option A. It was pretty straight forward question.
Now, we are on 7th question of chemistry.
This question is asking to calculate equivalent weight of Barium Hydroxide, and the weight of Barium is provided in the question to be 137.3
We know weight of Oxygen to be 16, and hydrogen atom to be 1. So, weight of OH becomes 17.
Let's note this value of 17.
Double of it becomes 34.
Let's note this new value.
Adding these two values gives us weight of Barium Hydroxide to be 171.3
Now pay attention, this is molecular weight, but it is not what is asked in the question.
Equivalent weight is Molecular weight divided by valency.
We know valency of Barium is two. Thus, we come to a value of 85.65.
Though, rounding off must be to even number at the end as per rules and its round off should be 85.6,
but this is a minor issue, and we can ignore this. The nearest value to our answer is Option A.
Now we are on Question 8.
This question is about oxidation states. In the book, I have explained that the sum of oxidation states of all atoms in a species
will be total charge on that particular molecule. As, all these options are electrically neutral,
thus sum of all oxidation states must be zero for all of these.
I have also explained in the book that oxidation state of Oxygen is almost always minus two. Using this information,
let us come to option A.
Let oxidation state of nitrogen atom be n.
Also, sum of n for nitrogen and minus two for oxygen must be zero. We solve for n and get n is equal to plus two.
Let us note it along with the option.
Coming to Option B, we do similar calculation, and get n is equal to plus four.
In option C, we repeat the calculation for two nitrogen atoms and single oxygen atom.
We find n is equal to plus one.
However, in reality both the atoms of nitrogen in this case have different oxidation states.
One of those atoms has plus three, and other has minus one.
There is no way you can find out merely by calculation method, unless you know the molecular structure of this particular molecule.
The answer of plus one, that we arrived on, was actually the average of both oxidation states.
I am writing down the actual values of minus one and plus three with option C.
Coming to option D, we repeat calculation for n for 2 nitrogen atoms each, and minus two for each of five oxygen atoms.
We find n equals plus five. As five is maximum, this becomes our correct answer.
Second last question of chemistry is regarding properties of diamond, as an allotrope of carbon. We have to find out the wrong statement.
I have discussed many relevant facts about diamond in my book. Equipped what we know about it, let us look at the first option, which says that is it harder than graphite.
Obviously, it is the hardest natural substance known to humanity as of now. So, this option is true.
For second option, we know that diamond is nothing but carbon atoms. Thus, it has 100% carbon.
Same is true for graphite also. For both, the percentage of carbon is 100. This renders the statement to be true.
Third option is about electrical conductivity.
We know that graphite can conduct electricity because each carbon atom is bonded with only 3 other atoms,
and thus it has free electrons.
But this is not the case for diamond.
Diamond is bonded in tetrahedral shape, and there are no free electrons. The statement provided in Option C says otherwise.
Hence, Option C is a FALSE statement.
Last option says that the distance between carbon atoms in all directions vary in diamond.
This is also an incorrect statement, as the bonding in Diamond is tetrahedral, and there is a perfect symmetry in all directions.
So, now we end up with two true, and two false statements. This is another such question with two answers.
I opine that option C and D, both are correct answers.
The last question of chemistry section is calculation intensive.
You also need to know the molecular masses of molecules concerned.
Also, the statement of question is not easy to comprehend.
In simple words, the questions is, that if one gram of compound for respective options is taken,
and reacted with either water or oxygen, which will produce more grams of hydrogen?
You also need some understanding of mole concept to be able to solve such questions.
For better understanding,
this concept with solved examples, has been explained in chemistry section of my book.
Let us begin with first option.
16 grams of methane produces 6 grams of hydrogen.
So, I gram methane will produce three-over-eight gram hydrogen.
For second option,
28 gram carbon-monoxide gives 2 gram hydrogen. So, I gram will produce one-over-forteen.
For option C,
16 gram methane produces 4 gram hydrogen. So, 1 gram methane will give one-fourth
Coming to Last option,
we find that 168 gram C-12 H-24 produces 24 gram hydrogen.
Thus one gram of such molecule will produce one-seventh gram of hydrogen.
We see that hydrogen produced in A is maximum. Hence Option A is the correct answer.
This brings us to end Chemistry, and we will begin with Physics section now.
Physics portion has 11 questions.
First of them is about uniform accelerated motion in a straight line.
The question asks that if a particle has uniform acceleration,
then by what percentage, distance traveled in 6th second is more than distance traveled in 5th second.
You will find the ratio of distances travelled in nth second on Page A.26 in the book.
If object starts with zero speed, the ratio goes to be one, three, five, seven,
nine, eleven and so on. We need to focus on 5th second and 6th second.
From 9 to 11, there is an increase of 2.
This is 22% increase, and thus option B becomes our desired answer.
Question 2 tries to mix two liquids and find the final density of mixture. We know that Density is Mass over volume.
This can be abbreviated as D equals M over V. Or alternatively, M equal D times V.
Now, for the first liquid, we have M1 equals D1V1 and for the second liquid, M2 equals D2V2.
On pouring them to same container, we add mass as well as volume.
Now, we can replace M1 and M2 with their respective values.
Pay attention to the question that says both liquids have same volume.
This means V1 and v2 are equal. Let is replace both V1 and V2 by a common variable V.
This makes denominator 2V, and V cancels out.
Thus, we arrive at our answer D1 plus D2 divided by 2. Or P1 and P2 in this case.
Third question presents us with a vector equation.
To answer this question, we need to know a bit of calculus.
Although, I have introduced some idea regarding it in my book,
but I never expected CDS to actually come-up with a question that requires you to solve equations like this.
Nevertheless, I can not explain differentiation in this video. That would deviate us from the intention behind this video.
However, I have made some other videos about "basic mathematics needed for physics". You can check them on my Youtube channel.
I will simply proceed assuming you know how to differentiate an algebraic equation.
The first derivative of position vector will give us velocity.
Differentiating it further will give us acceleration. Now, we see that velocity has non-zero values only for X and Y.
The Z component of velocity is zero.
So, we narrow our options to only A or D.
Now, looking at equation for acceleration,
we see that it has non-zero values only in X. Hence, for it, only Option B or D would be possible answers.
Combining both of these, we get Option D as correct answer.
Students who never had science background in 11th and 12th might find this question difficult.
But, as a matter of fact,
now you know that you can't filter topics as per your liking. I have seen students making statements like
"this is beyond the level of exams and there is no need to study in so much depth".
That would make sense if you can tolerate to skip a few questions.
However, this was an easy question and it doesn't take more than few minutes to learn
basic differentiation. I would advise you to sit with a friend who knows differentiation and learn the basic method at least.
It really wouldn't take more than 5 minutes.
However, if you do not have any such person in your circle, you may learn it from my Physics Course on Youtube.
Question four appears to be calculation intensive. However, this is a common concept of Newtonian Physics.
Any person who has gone through a Physics textbook will immediately recognize the answer without even reading the question completely.
But for the sake of explanation, I will provide with proper solution.
In addition, there is also a shortcut to solve such questions, which I will introduce a little later.
So, let's look at the question.
There is a ground. And there are two men pulling on a rope.
There is also a mass hanging in the middle.
Let us indicate the forces involved with a red arrow, a blue arrow, and the weight as yellow arrow.
We name them T-Red, T-Blue and MG.
Now we can resolve the red-tension in X and Y axis.
How to take resolution of vectors, is explained in detail in my book.
This turns out to be T Cos theta AND T Sine theta.
Similarly, we resolve the blue-tension into vertical and horizontal components. The angle theta here is taken between rope and Y axis.
For the object to be stable, forces in Y axis should be balanced,
and X should be balanced separately.
We will focus mainly on Y-Axis
Total force in positive Y is sum of Red and Blue. This gives us 2T Cos theta.
Thus, total force in positive Y must be equal to force in negative Y.
Hence, we write 2T Cos theta equal to mg.
Now look at the question. It wants the rope to completely straighten.
For this to happen, the angle theta must be 90 degrees. So, we proceed with calculation, replacing theta with 90 degrees.
Cos 90 degrees is zero. Thus T is infinite. Hence we get Option D as correct answer.
Now, it is time to look into an easier method. This is using Lami's theorem.
For your reference, this theorem is explained in my book Page A.21.
However, I will explain it in summary here too. Let is assume three vectors.
Let us name them A, B and C.
Now, let the angle opposite to A is alpha,
the one opposite to B is beta and angle opposite to C is gamma.
Now for all these forces to balance, following equation applies.
A over Sine alpha equals B over Sine beta equals C over sine gamma.
Using this equation in our problem, let us name the forces as A, B and C.
The angle opposite to C is gamma,
opposite to A is alpha and opposite to B is beta.
To completely straighten the rope, angle gamma must be 180 degrees, and the angles alpha and beta must be 90 degrees each.
Using these values in the equation,
we get A equals B equals C over zero.
Thus, we gets A and B to be infinite which is much easier than calculating it with full-fledged method.
Moving on to 5th question,
which is a very easy question, the question simply asks which of the following does not require electricity to produce light.
A candle, B LED, C laser, D TV.
Obviously, Option A is our answer.
Next question is asking about the visible portion of EM Waves.
This is also an easy question.
However, it is important to learn uses and discoveries of other waves as well.
A list of them is given in my book. As far as this question is concerned, option D is our answer.
The seventh question is about image formation by a convex lens. It says that the image formed is real.
And then asks about the position of object.
Refer to Page A.142 in my book. You will find that for real images in convex lens,
the object can be anywhere but not between Focus and Pole.
Hence, the object must be at least at focus or beyond for this to happen.
This brings us to option A as our correct answer.
Very easy and straightforward question.
I have explained a trick to remember all positions, nature, orientation and size of image in my video course.
See Chapter 'Optics' for that.
The next question is about earth's magnetism.
It is explained in my notes as well in my videos. Let us have a brief look.
Let us say, the magnetic field of earth is diving into the surface of earth obliquely.
However, when we hold a compass, we intend to look for direction horizontally.
This angle between green and red arrow is called Angle of Dip, also called angle of inclination.
Thus, it is our answer. Option B is the correct Option.
Antepenultimate question is about electricity.
This is a standard type of CDS question.
They have given a fuse which allows up to 5 amps of current through it.
Also, rating of a bulb is provided with 100 watts and 220 volts.
Using the formula, we can find the current requirements of this bulb.
Simply substituting the values give us current to be 5 over 11 amps.
Now there are N such bulbs, and they all are connected in parallel.
Given that each bulb consumes 5 over 11 ampere of current,
we can find number of bulbs, as total current has to be maximum 5 amperes.
A simple calculation provides us with answer n = 11.
Hence, option C is our correct answer. The concerned formulas for other similar questions are given on Page A.122
Now we are on second last question of this video.
The question is asking best means to extinguish fire quickest.
If you check my book, I have clearly mentioned a fact that evaporation causes cooling.
Water at 100 degree celsius is at the verge of evaporation,
so it will absorb the heat earliest and escape the site of incident quickly. Hence, option B is our correct answer.
This brings us to the last question.
The question asks which is not due to convection.
Again, I would ask to refer my book, Page A.101 and you will find that I clearly mentioned the convection to take place
only in liquids and gases. So, first option, boiling water is clearly a liquid.
Second option, breeze is a gas. In fact, breeze is such a common example of convection,
it is invariably given in almost all textbooks, and my book is no exception. Third option is circulation of air,
again example of gas.
But heating of bulb-glass is not mainly by gas.
We like to believe that there is vacuum in bulb. Well, not exactly.
Actually, there are gases inside the bulb. But, refer to radiation in my book and you will find
that radiation need not be in perfect vacuum.
Second, radiation is similar to light. So, any heating along with light is radiation.
Options A, B, C are not light producing examples. Whereas Bulb is primarily an example of heating by radiation.
So, Option D is correct answer.
This ends our discussion on science questions.
Download link to PDF of the questions discussed in this video is given in the description below.
The URL of my website is displayed here on the screen, if you want to download notes, watch videos OR buy my book.
See you next time. Good bye.
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