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Page No 311:

Question 1:

From amongst the metals sodium, calcium, aluminium, copper and magnesium, name the metal:
(a) which reacts with water only on boiling, and
(b) another which does not react even with steam.

Answer:

When metals reacts with water(Hot, cold or steam), they displace hydrogen from the water and Hydrogen gas is released. Metals react with water according to their reactivities. Only those metals react with water, which are above hydrogen in the reactivity series. 

(a) Out of given metals, only magnesium reacts with hot boling water and forms magnesium hydroxide and hydrogen gas.

(b) Out of all the given metals only copper is less reactive than hydrogen, so it cannot displace hydrogen from the water even if the temperature is so high. That's why copper does not react even with the steam.

Page No 311:

Question 2:

To an astronaut, why does the sky appear dark instead of blue?

Answer:

In the space, there is no atmosphere. So, the sunlight does not scatter in the space. Hence, an astronaut sees the dark sky.

Page No 311:

Question 3:

Explain with the help of a diagram, why a pencil partly immersed in water appears to be bent at the water surface.

Answer:




Refractive of water is more than the air, so the speed of the light decreases when it travels from the air into the water. That's why when light rays travel into the water, they bend towards the normal. Hence, the part of the pencil dipped inside water appears to be bent at the water surface, as shown in the above figure.

Page No 311:

Question 4:

What is an oxidation reaction? Identify in the following reaction (a) the substance oxidized, and (b) the substance reduced:

ZnO + C → Zn + CO

Answer:

The reaction which involves gain of oxygen, loss of hydrogen or loss of electrons is defined as the oxidation reaction.



(a) In the given reaction, C is gaining oxygen, so it is oxidized.
(b) Whereas zinc(Zn) is losing oxygen, so it is reduced.



 



Page No 312:

Question 5:

What is 'reproduction'? Mention the importance of DNA copying in reproduction.

Answer:

Reproduction is the process by which an organism gives rise to a new organism/offspring.
DNA copying is more commonly known as DNA replication. It is an important process which ensures that equal amount of DNA (genetic material) is passed on to the new cell as the parent cell multiplies.

Page No 312:

Question 6:

What physical and chemical properties of elements were used by Mendeleev in creating his periodic table? List two observations which posed a challenge to Mendeleev's Periodic Law.
 

Answer:

Physical and chemical properties used by Mendeleev in creating his periodic table:

1. Physical properties: Mendeleev arranged the periodic table in the increasing order of the atomic masses of the elements.

2. Chemical properties: Mendeleev distributed the elements into groups on the basis of formation of hydrides and oxides.

Challenges to Mendeleev's periodic law:

(i)  Position of the isotopes: Isotopes have different masses, so they must be assigned separate positions in the periodic table.
(ii) Different groups of similar elements: Some chemically similar elements are placed in different groups, while some chemically different elements are placed in the same group.

Page No 312:

Question 7:

(a) Show the formation of NaCl from sodium and chlorine atoms by the transfer of electron(s).
(b) Why has sodium chloride a high melting point?
(c) Name the anode and the cathode used in the electrolytic refining of impure copper metal.

Answer:

(a)  Formation of sodium chloride by electron transfer:

Electronic arrangement of sodium and chlorine:
 

Formation of ionic bond:


(b) Sodium chloride has a high melting point because of strong lattice structure formed due to the ionic bond. Interatomic attractive forces hold the two elements tightly and increase their boiling point.

(c) In the electrolytic refining of impure copper metal, the impure copper plates act as the anode while a thin sheet of pure copper act as the cathode.

Page No 312:

Question 8:

(a) What happens chemically when quicklime is added to water? Write chemical equation of the reaction involved.
(b) What is 'Baking Powder'? How does it make the cake soft and spongy?

Answer:

(a) When quicklime or calcium oxide(CaO) is added to water then it reacts vigorously with water and forms slaked lime or calcium hydroxide(Ca(OH)2). This reaction is highly exothermic and a large amount of heat is released in the process. The chemical reaction involved is as follows: 

CaO(s)+H2O(l)Ca(OH)2(aq)

(b) Baking powder is a mixture of baking soda(sodium hydrogen carbonate) and a solid, edible acid such as tartaric acid or citric acid. When baking powder is mixed with water, the sodium bicarbonate reacts with tartaric acid to produce carbon dioxide gas. The carbon dioxide gas gets trapped in the wet dough made for baking a cake or bread and then bubbles out slowly. Which makes the cake or bread soft and spongy.

Page No 312:

Question 9:

(a) The refractive index of diamond is 2.42. What is the meaning of this statement in relation to speed of light?
(b) Describe an activity to draw a magnetic field line outside a bar magnet from one pole to another pole.

Answer:

(a) Refractive index of a medium is the measure of the speed of light in that, medium with respect to the vacuum. It is defined as the ratio of the speed of the light in a vacuum to the speed of the light in the given medium. So, the term "The refractive index of diamond is 2.42" means that the speed of light in diamond is 12.42 times the speed of the light in vacuum.

(b) We can put a magnetic compass nearby a bar magnet. The needle of the compass will show deflection in the direction of magnetic field lines. If we move the compass around the magnet, we can get the exact path of magnetic field lines around the bar magnet. 

Page No 312:

Question 10:

For the circuit shown in the diagram given below:



Calculate:
(a) the value of current through each resistor.
(b) the total current in the circuit.
(c) the total effective resistance of the circuit.

Answer:

(a) All the resistors are connected in parallel. They are getting an equal voltage across their ends, which is equal to the voltage of the battery.

Hence, the current flowing through each resistor = Voltage across the batteryResistance of the resistor

The voltage across the battery = 6 V

Current flowing through 5-ohm resistor = 6 V5 Ω=1.2 A


Current flowing through 10-ohm resistor = 6 V10 Ω=0.6 A

Current flowing through 30-ohm resistor = 6 V30 Ω=0.2 A

(b) The total current in the circuit = Sum of the current through all the resistors = 1.2 A + 0.6 A + 0.2 A = 2 A

(c)The total resistance of the circuit=Total voltageTotal current=6 V2 A=3 Ω
 

Page No 312:

Question 11:

(a) Draw a diagram to show the formation of image of a distant object by a myopic eye. How can such an eye defect by remedied?
(b) State two reasons due to which this eye defect may be caused.
​(c) A person with a myopic eye cannot see objects beyond a distance of 1.5 m. What would be the power of the corrective lens used to restore proper vision?

Answer:

(a) A diagram to show the formation of an image of a distant object by a myopic eye is shown below:



The defect of a myopic eye can be corrected by using a concave lens. A concave lens helps in diverging the rays to form an image on the retina of the eye.

(b) Causes of myopia: (i) Elongation of the eyeball (ii) Decrease in the focal length of the eye lens.

(c) As it is a case of myopia one has to use a concave lens.
The far point is 1.5 m, it means that the image should be formed at 150 cm or before on the same side as the object. The distance of the object is taken as infinite, as the problem is nearsightedness.

Now, u = ∞, v = –150 cm and f = ?

Using the lens formula, 1v-1u=1f

1-150-1=1f1f=1-150f=-150 cm

The maximum focal length to be used = -150 cm

Hence, the power of the lens required = P = 1f( in meter)=11.5 m=0.67 D

Therefore, a concave lens with power 0.67 D or more is required for the correction in vision.

Page No 312:

Question 12:

(a) Distinguish between the terms 'overloading' and 'short-circuiting' as used in domestic circuits.
​(b) Why are the coils of electric toasters made of an alloy rather than a pure metal?

Answer:

(a) Overloading: When too many electrical appliances of high power voltage are switched on at the same time, a large current from the circuit is drawn. This is called overloading of the circuit. Overloading also occurs due to accidental hike in the supply voltage.

Shortcircuiting: When the live wire and neutral wire come into direct contact, a large amount of current flows through the circuit due to very small resistance, this is called short circuiting. The heat produced during short-circuiting is so high that it may cause a fire.

(b) The coil of the electric toaster is made up of an alloy because alloys have better properties than the pure metals. Such as,

1.  They have a high melting point. This is necessary for the electric toaster as it is a heating device.
2.  More resistant to oxidation. This is another requirement in a toaster.
3.  Mechanically stronger than that of pure metal.

Page No 312:

Question 13:

"Damage to the ozone layer is a cause for concern". Justify this statement. Suggest any two steps to limit this damage.

Answer:

“Damage to the ozone is a cause of concern”; this is because ozone depletion has created a hole in the ozone layer over the Arctic region.

Damage in the ozone layer allows the harmful UV rays to reach the earth surface. UV rays can cause -

  • skin cancer, ageing and corneal cataracts in human beings
  • death of many phytoplanktons leading to the increase in the rate of global warming
  • increase in the global warming rate, in turn, leads to the melting of Arctic ice cap which may further result in the increase in sea level
Steps which can limit the damage to the ozone layer are -
  • Not buying products in aerosol cans and maintaining air-conditioning filters and units. In order to halt the depletion of the ozone layer, countries around the world have banned the use of chlorofluorocarbons and other ozone-depleting substances. These compounds produce chlorine and bromine atoms which reach the atmosphere react with ozone, thus destroying it. 
  • By minimising the use of vehicles to limit the emission of harmful gases which cause damage to the ozone layer.



Page No 313:

Question 14:

Define 'hormones'. Name the hormone secreted by thyroid. Write its function. Why is the use of iodised salt advised to us?

Answer:

Hormones are intercellular chemical messengers secreted by specific endocrine glands of the body. They travel throughout the body to reach the target organs or cells and initiate a process. Chemically, hormones are proteins. Once the hormone is secreted from the gland, it is directly poured into the blood stream, which transports it to the site of action. They maintain the balance of biological activities in the body.

The hormone secreted by thyroid gland is thyroxine hormone. It regulates carbohydrate, fat, and protein metabolism in the body. 

It is advised to use iodised salt because iodine is required for the synthesis of thyroxine hormone. Deficiency of iodine would result in deficiency of thyroxine hormone and in turn lead to a disease called goitre.

Page No 313:

Question 15:

(a) State any one difference between pollination and fertilization.
​(b) A man with blood group A marries a woman with blood group O and their daughter has blood group O. Is this information enough to tell you which of the traits - blood group A or O is dominant? Why?

Answer:

(a) 

Pollination Fertilisation
Pollination is the process of transfer of pollen from the anther of one flower to the stigma of the same flower or another flower. Fertilisation is the process of fusion of male and female gametes.

b. No. The information provided above is not enough to tell us whether the trait of blood group A or O is dominant. Either of the two can be possible because each individual carries two alleles. Recessive traits can occur only when the two similar alleles are present.

There are two possibilities -

Possibility I - Blood group A is dominant and blood group O is recessive
In this case, the daughter can have either blood group A or O, if the mother and father have one allele of A and other of O blood group.

Possibility II - Blood group O is dominant and blood group A is recessive
In this case, the daughter can have blood group O, if the father carries both the alleles of blood group A while the mother has either both the alleles of blood group O or one of A blood group and other of O blood group.
 

Page No 313:

Question 16:

(a) Why does carbon form compounds mainly by covalent bonding?
(b) List any two reasons for carbon forming a very large number of compounds.
(c) An organic acid 'X' is a liquid which often freezes during winter time in cold countries, has the molecular formula C2H4O2. On warming it with ethanol in the presence of a few drops of concentrated sulphuric acid, a compound 'Y' with a sweet smell is formed.
(i) Identify 'X' and 'Y'.
​(ii) Write a chemical equation for the reaction involved.

Answer:

(a) The electronic configuration of carbon is 2, 4.  The electron present in its outer most shell is 4 but for the stability, the number of electrons has to be 8. So, carbon can neither lose 4 electrons nor gain 4 electrons to complete its octet. For losing or gaining electrons from the outer-most shell, we require an enormous amount of heat/energy, which is not possible in nature. Hence, the four electrons present in the outer-most shell will try to share with other four electrons in order to fulfill their octet. That is why it forms covalent bonds.

(b) Two of the main reasons because of which carbon forms a large number of organic compounds are:
1. Tetravalent nature of carbon.
2. Greater C-C bond strength.

(c) Organic compound "X" freezes in colder countries must be a carboxylic acid containing molecule. So, the molecular formula will be CH3COOH.
When a carboxylic acid is treated with alcohol in the presence of sulphuric acid, it gives a sweet-smelling compound ester. So, when ethanoic acid treated with ethanol, it will give ethyl ethanoate.
Hence, X will be ethanoic acid and Y will be ethyl ethanoate which is a sweet smelling compound.

(ii) The reaction involved is as follows: 
CH3COOHEthanoic acid+CH3CH2OHEthyl aclocholH2SO4CH3COOCH2CH3Ethyl ethanoate+H2O

 

Page No 313:

Question 17:

(a) What is a homologous series of compounds? List any two characteristics of a homologous series.
(b)  (i) What would be observed on adding a 5% alkaline potassium permanganate solution drop by drop to some warm ethanol in a test tube?
      (ii) Write the name of the compound formed during the chemical reaction.
(c) How would you distinguish experimentally between an alcohol and a carboxylic acid on the basis of a chemical property?

Answer:

(a) A homologous series is a series of organic compounds, which differ by a same repeating unit and have similar chemical properties due to the presence of the same functional group. 

Characteristics of homologous series:
(i) Two consecutive members of a homologous series differ by a (-CH2) unit and a mass of 14 amu. 
(ii) Homologs of a series have similar chemical properties.

(b) (i) When 5% alkaline potassium permanganate is added to warm ethanol, ethanol gets oxidize to form ethanoic acid.

CH3CH2OHalk.KMnO4CH3COOH

(ii) The compound formed during the reaction is ethanoic acid.

(c) Carboxylic acids react with sodium bicarbonate to produce brisk effervescence due to the release of carbon dioxide gas, whereas alcohols do not react with sodium bicarbonate.

Page No 313:

Question 18:

(a) What is meant by saying that the potential difference between two points is 1 volt? Name a device that helps to measure the potential difference across a conductor.
(b)Why does the connecting cord of an electric heater not glow hot while the heating element does?
(c) Electrical resistivities of some substances at 20°C are given below:

Silver 1.60 × 10−8 Ωm
Copper 1.62 × 10−8 Ωm
Tungsten 5.20 × 10−8 Ωm
Iron 10.0 × 10−8 Ωm
Mercury 94.0 × 10−8 Ωm
Nichrome 100 × 10−6 Ωm
Answer the following questions in relation to them:
(i) Among silver and copper, which one is a better conductor? Why?
(ii) Which material would you advise to be used in electrical heating devices? Why?

Answer:

(a) It means that the 1 joule of energy is used up in transferring a charge of 1-coulomb between two points. A voltmeter is used to measure the potential difference across a conductor and it is always connected in parallel.

(b) The heating element of the heater is made up of an alloy which has very high resistance. So, when current flows through the heating element, it becomes too hot and glows red due to the heating effect of current. But the resistance of cord which is usually of copper or aluminium is very low so it does not glow.  

(c) The conductivity of a material is indirectly proportional to its resistivity.
(i) Out of silver and copper, silver has less resistivity, that's why it is a good conductor as compared to copper. 
(ii) The material used in electrical heating devices must have high resistance. So, a material with high resistivity should be used from the table. Nichrome has the highest resistivity, it would have the highest resistance among all the given materials. Hence, it should be used for electrical heating devices. 

 



Page No 314:

Question 19:

(a) Name an instrument that measures electric current in a circuit. Define the unit of electric current.
(b) What do the following symbols mean in circuit diagrams?
     (i)  (ii)  
(c) An electric circuit consisting of a 0.5 m long nichrome wire XY, an ammeter, a voltmeter, four cells of 1.5 V each and a plug key was set up.
     (i) Draw a diagram of this electric circuit to study the relation between the potential difference maintained between the points 'X' and 'Y' and the electric current flowing through XY.
    (ii) Following graph was plotted between V and I values:



​What would be the values of VI ratios when the potential difference is 0.8 V, 1.2 V and 1.6 V, respectively? What conclusion do you draw from these values?

Answer:

(a) An ammeter is used to measure the current in a circuit. Unit of electric current is ampere. 

(b) (i) Variable resistance or Rheostat (ii) A closed switch

(c) (i)


(ii) VI for 0.8 V, 1.2 V and 1.6 V is the same as the graph between the voltage and the current is a straight line. So, the VI ratio is constant for all the values of current and voltage.

Page No 314:

Question 20:

(a) Draw a diagram depicting Human Alimentary Canal and label on it Gall Bladder, Liver and Pancreas.
(b) State the roles of Liver and Pancreas.
(c) Name the organ which performs the following functions in humans:
    (i) Absorption of digested food.
   ​ (ii) Absorption of water.

Answer:

Ans 20. (a)
 
(b) (i) Liver −It is the largest gland in the digestive system. It secretes bile, which helps in the digestion of fats into fatty acids and glycerol.

(ii) Pancreas − It secretes enzymes, such as trypsin and lipase for the digestion of proteins and fats respectively.

(c) (i) Absorption of digested food - small intestine
(ii) Absorption of water - large intestine

Page No 314:

Question 21:

(a) Draw a sectional view of the human heart and label on it Aorta, Right Ventricle and Pulmonary Vein.
(b) State the functions of the following components of transport system:
      ​(i) Blood      (ii) Lymph

Answer:

(a)

(b) (i) Functions of blood -

  • transports hormones and nutrients throughout the body
  • transports oxygen to cells and removes carbon dioxide
  • maintains the body temperature

(ii) Functions of lymph -

  • contains lymphocytes that are involved in immune response
  • carries nutrients, hormones, etc.
  • absorbs fats in lacteals (found in intestinal villi)

Page No 314:

Question 22:

A metal is treated with dilute sulphuric acid. The gas evolved is collected by the method shown in the figure. Answer the following:

(a) Name the gas.
(b) Name the method of collection of the gas.
​(c) Is the gas soluble or insoluble in water?
(d) Is the gas lighter or heavier than air?

Answer:

When a metal(M) is treated with dilute sulphuric acid, metal will displace hydrogen and form metal sulphate, along with colourless and odourless hydrogen gas. Hydrogen gas produced in the reaction will evolve with bubbles.

M+H2SO4MSO4+H2

(a) Hydrogen gas will evolve.

(b) The method of collection of the gas shown in the figure is known as "downward displacement of water".

(c) Hydrogen gas is insoluble in water.

(d) Hydrogen gas is lighter than air.



Page No 315:

Question 23:

Ashwini has been given four test-tubes containing the following:
(a) Universal indicator
(b) Distilled water
(c) Sodium hydrogencarbonate
(d) Phenolphthalein
​What will be observed by Ashwini when he adds a few drops of acetic acid to each test-tube, one by one?

Answer:

(a) When Ashwini adds acetic acid in the universal indicator, he will observe a colour either orange or yellow, depending upon the concentration of acetic acid. Because acetic acid is a weak acid, having a pH range of 3-6, so it will change the colour of universal indicator to orange or yellow.

(b) When Ashwini adds acetic acid in distilled water, he will observe acid and water mix uniformly.

(c) When Ashwini adds acetic acid in the sodium hydrogen carbonate, he will observe effervescence due to the formation of carbon dioxide.

CH3COOH+NaHCO3CH3COONa+CO2+H2O

(b) When Ashwini adds acetic acid in the phenolphthalein, he will observe the pink colour of phenolphthalein changes to colourless, because phenolphthalein turns colourless in acids.
 

Page No 315:

Question 24:

A beam of white light falling on a glass prism gets split up into seven colors marked 1 to 7 as shown in the diagram.



Which two positions correspond closely to the color of:
(a) a solution of potassium permanganate?
​(b) 'danger' or stop signal lights?

Answer:

White light gets split up into seven colors, which are as follows:

(7) Violet (6) Indigo (5) Blue (4) Green (3) Yellow (2) Orange (1) Red

(a) Potassium permanganate is an odorless dark purple colored solid. Hence, color position 7 corresponds to potassium permanganate closely. 

(b) The red color is a mark of danger or symbol for danger in general. Hence, color position 1 suits closely for danger signal. 
 

Page No 315:

Question 25:

Rajiv connected two exactly similar heating elements (or resistances) across a mains supply in two different ways to heat some water.
(a) Is more heat obtained when the heating elements are connected in series or when they are connected in parallel?
​(b) Give reasons for your answer.

Answer:

(a) More heat will be obtained when the heating elements are connected in parallel.

(b) Heat energy produced, H = I2 R t

In the parallel combination of resistances, current increases due to a decrease in effective resistance.
As the heat energy produced is directly proportional to the square of the current, it can be concluded that the parallel combination gives more heat energy.

Page No 315:

Question 26:

A student observed a permanent slide showing asexual reproduction in yeast.
(a) Draw diagrams of the observations the student must have made from the slide.
​(b) Name the process of asexual reproduction of yeast.

Answer:

Ans 26. (a) 


(b) The process of asexual reproduction in yeast is called budding.

Page No 315:

Question 27:

Given below is a sketch of a leaf partially covered with black paper and which is to be used in the experiment to show that light is compulsory for the process of photosynthesis:


(a) At the end of the experiment, which of the leaf part/leaf parts labelled I, II and III will become blue-black when dipped in iodine solution?
(b) Which substance will be missing in the leaf part/leaf parts which will not become blue-black when dipped in iodine solution?

Answer:

(a) Parts I and III will appear blue black in colour which indicates the presence of starch in these regions.
(b) Part II will not appear blue black in colour because starch will be missing from that part. The black paper in region II will prevent the sunlight from reaching that region, as a result of which no starch synthesis would occur.



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