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Page No 373:

Question 1:

What is the difference between a reflex action and walking?


Reflex action shows sudden response and does not involve any thinking. The brain has no role in the creation of the response in reflex action. Walking is a voluntary action which is under the control of the cerebellum region of the brain.

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Question 2:

Name a metal which is kept immersed in kerosene oil. Why is it kept this way?


Sodium metal is kept immersed in kerosene oil. 
Sodium is a very active metal that combines with oxygen at room temperature and burns with a brilliant golden-yellow flame. It reacts violently with water to produce hydrogen gas, which may be ignited by the heat of the reaction. So, it is kept in kerosene oil.

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Question 3:

Equal amounts of sodium hydrogencarbonate are taken in two test tubes A and B. Acetic acid is added to test-tube A whereas an equal volume of hydrochloric acid is added to test-tube B. In which test-tube, effervescence will occur more vigorously and why?


Sodium hydrogen carbonate is a base which when mixed with an acid, will give brisk effervescence. Test tube A contains acetic acid which is a weak acid, so it does not give much effervescence. But test tube B contains hydrochloric acid, which is a strong acid and thus gives brisk effervescence.

Page No 373:

Question 4:

(a) What is hypermetropia?
(b) What are the causes of hypermetropia?
(c) Name the lens used to correct hypermetropia?


(a) Hypermetropia or far-sightedness is a defect where light rays are focused behind the retina. In this defect, a person is not able to see the nearby object but the farther objects are clearly visible to him.

(b) Hypermetropia is caused due to the eyeball becomes too small or the focal length of the eye lens becomes too large.

(c) Hypermetropia is corrected by a converging lens or convex lens. 

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Question 5:

What does saliva contain? What is the role of saliva in the digestion of food?


Saliva contains the enzyme salivary amylase. This enzyme helps in the digestion of starch in the mouth and also facilitates swallowing by lubricating the food.

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Question 6:

(a) The recovery of silver from silver nitrate solution involves displacement by copper metal. Write the equation of the reaction that takes place. Give the states of all the substances.
(b) Under what soil conditions do you think a farmer would treat the soil in his fields with quicklime? Why?
(c) Name one sodium compound which is used for softening hard water and another which is used in fire extinguishers.


(a) Silver can be recovered from the silver nitrate solution by the displacement by copper metal as follows:

(b) If the soil is acidic and improper for cultivation, then to increase the basicity of soil, the farmer would treat the soil with quick lime.

(c) Sodium carbonate (Na2CO3.10H2O) is used for softening of hard water.
Sodium bicarbonate (NaHCO3) is used in the fire extinguisher.

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Question 7:

(a) Fresh milk has a pH of 6. When it changes into curd (yogurt), will its pH value increase or decrease? Why?
(b) Name the gas evolved when dilute HCl reacts with sodium hydrogencarbonate. How is it recognised?
(c) On adding dilute hydrochloric acid to copper oxide powder, the solution formed is blue-green. Predict the new compound formed which imparts a blue-green colour to the solution.


(a) When fresh milk changes into curd, its pH value decreases because of an increase in the acidity of the milk. Bacteria usually change fresh milk to curd by producing lactic acid.

(b) When sodium hydrogen carbonate reacts with dilute hydrochloric acid, it gives sodium chloride, carbon dioxide, and water. When the gas liberated is passed through lime water, lime water turns milky.


(c) When dilute hydrochloric acid is added to copper oxide powder, a blue-green coloured compound called copper chloride will be obtained.

CuO+2HClCuCl2blue green+H2O

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Question 8:

(a) Balance the following chemical equation:
(b) State two characteristic features of carbon which when put together give rise to a large number of carbon compounds.
​(c) Describe an activity to show that acids produce ions only in aqueous solutions.


(a) 2PbNO32sheat2PbOs+4NO2g+O2g

(b) Feature of carbon which gives rise to a large number of carbon compounds:
1. Catenation: Is is the ability of carbon compounds to form long chains, branched chains.
2. Smaller nucleus helps it to attract a number of electrons.

1. Take about 1 g of sodium chloride in a clean and dry test tube.
2. Add some concentrated sulphuric acid.
3. A gas is produced and comes out through the delivery tube.
4. Test the gas evolved with dry and wet strips of litmus paper.

1. When the gas evolved is passed through dry litmus paper, there is no change in the colour of litmus paper.
2. However, when the gas is passed through wet blue litmus paper, the paper turns red, which indicates that the gas evolved is an acid.

This activity concludes that hydrogen ions in hydrochloric acid are produced in the presence of water because only wet blue litmus turns red.

Page No 373:

Question 9:

(a) What are the common names of (a) CaOCl2, and (b) Na2CO3.10H2O?
(b) Why should plaster of Paris be stored in a moisture-proof container?
​(c) Explain why, while diluting a concentrated acid, acid should be added to water and not water to the acid.


(a) CaOCl2 is chemically known as calcium oxychloride and common name is bleaching powder.
Na2CO3.10H2O is chemically known as sodium carbonate decahydrate and its common name is washing soda.

(b) Plaster of Paris should be stored in a moisture-proof container because plaster of Paris, a powdery mass, absorbs water to form a hard solid known as gypsum.


(c) The dissociation of acid is a highly exothermic reaction. When acid is slowly added to water, the heat produced is absorbed by the water. But if the water is added to the acid, the high amount of heat produced may lead to splashing of water. Thus, when diluting, the acid is always added to the water.

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Question 10:

State the various factors on which the electrical resistance of a conductor depends. How does it depend on these factors?


The resistance of a conductor depends on the following factors:

(i) The material of the conductor: The resistance of a conductor is directly proportional to the resistivity of the material. It is the characteristic property of a material. 

(ii) The shape of the conductor: The resistance of a conductor is directly proportional to the length of the conductor.

(iii) Size of the conductor: The resistance of a conductor is indirectly proportional to the area of the cross section of the conductor. 

Page No 374:

Question 11:

(a) What is meant by the 'series combination' and 'parallel combination' of resistors? Explain with diagrams.
(b) What is the combined resistance of the three resistors of 5 Ω, 3 Ω and 2 Ω connected as shown in the figure?



(a) In a series combination, the resistances are connected with end to end in contact, such that current flow is equal in all the resistances in the combination. Whereas in the parallel combination, resistances are connected in such a manner that they get an equal voltage across their ends. 

(b) 3-ohm and 2-ohm resistors are connected in series, and their combination is connected in parallel with the 5-ohm resistor. 

Net resistance of the series combination of 3-ohm and 2-ohm resistance = 3 Ω + 2 Ω = 5 Ω

This 5-ohm resistance is in parallel combination with given 5-ohm resistance.

Hence, the total equivalent resistance of the given combination = 5 Ω×5 Ω5 Ω+5 Ω=2.5 Ω

Page No 374:

Question 12:

A person needs a lens of power, −5.5 D for correcting his distant vision. For correcting his near vision, he needs a lens of power, + 1.5 D.  What is the focal length of the lens required for correcting:
(a) distant vision?
(b) near vision?
​What type of lens is being used for correcting (i) distant vision, and (ii) near vision?


(a) According to relation, Power =1f( in meter)

For distant vision, P = -5.5 D

Focal length, fdistant=1-5.5m=-1005.5cm=18.18 cm

(b) According to relation, Power =1f( in meter)

For near vision, P = +1.5 D

Focal length, fnear=1+1.5m=1001.5cm=66.66 cm

For distant vision, the person is using a lens of negative power, so it is a concave lens. For near vision, the person is using a lens with positive power, so it is a convex lens.

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Question 13:

What are the different ways in which glucose is oxidised to provide energy in various organisms? Explain with equations. Which of these processes provides the maximum energy?


Glucose can be oxidised by one of the following three ways -

(i) In the absence of oxygen (anaerobic respiration), in muscles

(ii) In the absence of oxygen (anaerobic respiration), in yeast

(iii) In presence of oxygen (aerobic respiration), in mitochondria

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Question 14:

What are the three R's to save the environment? Explain with one example of each. Which fossil fuel is conserved when we save on electricity?


The three R’s that need to be practiced in order to save the environment are:

1. Reduce: Reducing the usage of resources will help to save them for the future. E.g.: reducing usage of electricity by switching off fans and lights when not in use.

2. Recycle: Plastics, paper, and glass can be recycled to make other objects.

3. Reuse: Using things again and again. For example, jam bottles made of glass can be used to store salt, sugar, oil, etc.

If we save electricity, we can save coal which is a type of fossil fuel.

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Question 15:

Describe the various steps involved in the sexual reproduction in plants.


In sexually reproducing plants, flowers function as the reproductive organs.

  • In flowers, male organ is the stamen and female organ is the carpel.
  • Flowers which have both the male and female organs i.e. stamens and carpels are called bisexual flowers whereas flowers which have either male or female organs are called unisexual.
  • Male gametes called pollen grains are produced by stamen, and carpels produce female gametes called ovules or egg cells inside ovaries.
  • Fertilization takes place in the ovule where the egg cell and pollen grain fuse.
  • This fertilized egg cell later develops into an embryo and the entire ovule gets converted into a seed.
  • Under favourable conditions, the seed germinates to give rise to a new plant.

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Question 16:

(a) What is reactivity series of metals? What can you say about the reactivity of metals which are placed:
(i) in the middle of reactivity series?
(ii) towards the bottom of reactivity series?
(iii) towards the top of reactivity series?
(b) A metal is placed in the middle of reactivity series. Suggest a chemical process to obtain this metal from its oxide. Support your answer with one example.
​(c) A metal is placed towards the top of the reactivity series. Suggest a chemical process to obtain this metal from its oxide. Support your answer with one example.


(a) When elements are arranged in the increasing order of reduction potential with respect to hydrogen, the series obtained is called the reactivity series.
(i) Hydrogen is placed in the middle of the series. It can lose an electron to form a positive ion and combine with non-metals which are electron attracting species.
(ii) In the bottom of the series elements are very less reactive they are even available in their elemental state. For eg. gold, silver, etc.
(iii) Elements at the top of the table are very reactive, they have a strong tendency to lose their electrons and form bonds.

(b) For obtaining metal that is in the middle of reactivity series, oxide of such metal can be reduced with coke which acts as a reducing agent.


(c) For obtaining metal that is high in the reactivity series their oxide is reduced to metal by the process of electrolysis.
Electrolysis of sodium chloride:  

At cathode: NaNa++e-At anode: Cl+e-Cl-

Page No 374:

Question 17:

(a) Describe the importance of pH in everyday life. Illustrate your answer with examples.
(b) A farmer has found that the pH of soil in his fields is 4.2. Name any two chemical materials which he can mix with the soil to adjust its pH. Give reason for your choice.
(c) Why are detergents also called 'soap-less soaps'? Give the various advantages of detergents over soaps.
​(d) A person put some universal indicator paper onto wet soap. The pH is found to be 8. What does this tell you about soap?


(a) pH has great importance in our daily life:

(i) pH and plants:
Plants require a specific pH for growth. It cannot grow in too acidic or too basic soils. If soil is too acidic then its pH can be increased by adding lime:

And on the other hand, if the soil is too alkaline then its pH can be decreased by adding gypsum.

(ii) pH and blood or digestive system:
The pH of blood is tightly regulated in the range of 7.2-7.4 and medium in our stomach is highly acidic (pH = 1.2). Sometimes stomach produces too much hydrochloric acid which causes irritation or pain.

(iii) pH and skin:
The acidic pH of the skin prevents colonization by bacteria. Thus, it acts as a defense mechanism against infection.

(b) As the acidity of the soil increases, it becomes improper for cultivation. To reduce the acidity of the soil, the farmer will have to add quick lime or slaked lime his fields.

(c) Soaps are sodium or potassium salts of fatty acids, however, detergents are not salts but are simply surface active chemicals showing soap-like properties, hence they are called as soapless detergents.
Advantage of detergents over soap: Detergents are often preferred over soap as they are more effective at cleaning and have a wider range of applications. They can also be used in hard water.

(d) pH of the soap was found to be 8, which means the soap is of basic nature. Universal indicator show solutions with pH greater than 7 are bases.

Page No 375:

Question 18:

(a) What is myopia?
(b) What are the causes of myopia?
(c) Draw three labelled ray-diagrams to show the defect of vision called myopia and how it is corrected by the use of lenses.
​(d) The far point of a myopic person is 80 cm in front of the eye. What is the nature and power of the lens required to correct the defect?


(a) Myopia or short-sightedness is a common defect of vision in which a person is unable to see the farther objects but the nearby objects are clearly visible to him. 

(b) Myopia occurs when the eyeball is too long or the focal length of the eye lens is too short to focus the image on the retina. 


(d) To correct the myopic eye, a concave lens is required. 

Far point of the myopic eye, v = 80 cm

Object distance = 

Using lens formula,

1-80-1=1f1f=1-80f=-80 cm

Therefore, the focal length of required to correct the defect is -80 cm. As the focal length is negative, so the lens is a concave lens.

Power of the lens = P=1-80×10-2 m=-10080=-1.25 D

Page No 375:

Question 19:

A resistor of 4-ohm resistance and an electric lamp of 20-ohm resistance are connected in series across a 6-volt battery. Draw a labelled circuit diagram by using the symbols for ohm and volt. Calculate:
(a) total resistance of the circuit.
(b) current flowing through the circuit.
(c) potential difference across the resistor.
​(d) potential difference across the electric lamp.


(a) 4-ohm resistance and 20-ohm lamp are connected in series. Hence, the total resistance of the circuit = 4 Ω+20 Ω = 24 Ω

(b) Current flowing in the circuit = VoltageTotal resistance=6 V24 Ω=0.25 A

(c) The potential difference across the resistor = Current through the resistor × Resistance = 0.25 A × 4 Ω=1 V

(d) The potential difference across the lamp = Current through the lamp × Resistance = 0.25 A × 20 Ω=5 V

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Question 20:

(a) What is menstruation? Why does menstruation occur?
(b) Write the various steps involved in the sexual reproduction in animals.
​(c) Write the differences between aerobic respiration and anaerobic respiration in tabular form. Name one organism which uses aerobic and another which uses anaerobic mode of respiration.


(a) Menstruation is the regular discharge of blood and mucosal tissue from the uterus through the vagina. It is a method to remove the unfertilized eggs from the body at regular intervals.

(b) The events observed during sexual reproduction in animals are -

(i) Formation of gametes
(ii) Fusion of male and female gametes to produce zygote
(iii) Development of zygote and embryogenesis
(iv) Development of an embryo


Aerobic Respiration

Anaerobic respiration
It involves the complete breakdown of glucose into CO2 and H2O. It involves the partial breakdown of glucose.
A net gain of 36 molecules of ATP occurs. A net gain of only 2 molecules of ATP occurs.
Here, oxidation of NADH to NAD+ is a vigorous reaction. Here, oxidation of NADH to NAD+ is a slow reaction.

Organism which use aerobic respiration - humans
Organism which use anaerobic respiration - yeast

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Question 21:

(a) Name the three types of methods usually used for birth control. Describe each method briefly.
(b) What is meant by sexually transmitted diseases? Name any two sexually transmitted diseases and their causative organisms.
​(c) Which method of contraception can prevent sexually transmitted diseases?


(a) Three types of methods for birth control are -

  • Barrier method: In this method, the fertilisation of the ovum and the sperm is prevented with the help of barriers such as a condom.
  • Oral contraceptive method: In this method, tablets or drugs are taken orally to prevent fertilisation.
  • Implants and surgical methods: Contraceptive devices, such as a loop and a copper-T rod, are placed in the uterus to prevent pregnancy. However, they can cause side effects to the uterus.

(b) Diseases that get transferred from one individual to another through sexual contact are called sexually-transmitted diseases.
Gonorrhoea - Neisseria gonorrhoeae
AIDS - Human immuno virus (HIV)

(c)  The spread of sexually transmitted diseases can be prevented by using condoms and avoiding multiple partners.

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Question 22:

A student takes about one metre long insulated copper wire and winds it round and round closely on a large iron nail. He then connects the two ends of the wire to a battery. When a steel paper clip is brought near the iron nail, the iron nail attracts it due to which the steel clip sticks to the iron nail.
(a) Which effect of electric current is demonstrated by this experiment?
(b) Name one common appliance used in our houses which works on this effect of electric current.


(a) Magnetic effect of current is demonstrated in the experiment. This effect can be described as when an electric current is passed through a conductor it produces a magnetic field around it. 

(b) An electric motor works on the magnetic effect of the electric current. It converts electrical energy into mechanical energy. 

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Question 23:

A person connects too many electrical appliances to a single socket in his house. When all these electrical appliances connected to the single socket are switched on at the same time, the electric fuse of circuit blows off.
(a) What is the process of connecting too many electrical appliances to a single socket known as?
(b) Which effect of electric current is responsible for the blowing off of fuse wire?


(a) It is known as overloading. When too many appliances are connected in the same socket, they draw a large amount of current. Which exceeds the maximum current rating of the fuse wire. That's why the fuse wire blows off.

(b) Heating effect of electric current is responsible for the blowing off the fuse wire. When a large amount of current passes through the circuit, it creates a large amount of heat. If the current exceeds the maximum current rating of the fuse, then the heat is enough to melt the fuse wire. 


Page No 376:

Question 24:

When a blue coloured copper salt is heated strongly, it forms a white copper salt. The white copper salt can be reconverted into blue copper salt by adding a liquid X.
(a) Write the formula of blue copper salt.
(b) Write the formula of white copper salt.
(c) What substance is lost when blue copper salt is converted into white copper salt?
​(d) Name the liquid X.


(a) The formula for blue copper salt is CuSO4.5H2O.

(b) The formula for white copper salt is CuSO4.

(c) Copper sulphate crystals are blue in colour, which on heating lose all the water of crystallization and become anhydrous.


(d)When anhydrous copper sulphate gets few drops of water it becomes hydrous, and imparts blue colour to copper sulphate crystals.


So, X must be water.

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Question 25:

A student took a sodium salt and dissolved it in water. When this aqueous sodium salt solution was tested with universal indicator paper, its pH was found to be 9.
(a) What does pH 9 tell us about the sodium salt solution?
(b) Name one sodium salt which can produce such a solution.


(a) Sodium salt with universal indicator shows pH = 9 which means that the salt is basic in nature.

(b) Sodium carbonate (Na2CO3) can produce such a solution.

Page No 376:

Question 26:

In the F2 generation of a cross, progeny having different traits are produced in the ratio 3:1. State whether it is a monohybrid cross or a dihybrid cross. Give one example of such a cross.


This is an example of monohybrid cross. In this type of cross, the phenotypic ratio is obatined as 3:1 whereas the genotypic ration is obtained 1:2:1.

Example: A cross between true breeding tall (TT) and true breeding dwarf (tt) plants.

In this cross, all the F1 progeny produced would be tall (Tt) since the allele for tallness is dominant over the allele for dwarfness. When the F1 generation is selfed, the F2 generation so produced contains both tall and dwarf plants in the ratio 1 : 2 : 1. The cross involved is as follows:

The phenotypic ratio is 3:1 i.e. 3 tall plants and 1 dwarf plant.

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Question 27:

In an experiment, a destarched, long and narrow green leaf (still attached to a potted plant) is introduced into a wide-mouthed glass bottle through an airtight cork. The glass bottle contains some potassium hydroxide solution in its lower part (but the leaf remains above this solution). The whole set-up is placed in sunlight for 2-3 days. The leaf is then taken out. After removing chlorophyll, the leaf is treated with iodine solution. No blue-black colour is produced in the leaf.
(a) Which substance could not be synthesised in the green leaf?
(b) What is the function of potassium hydroxide solution inside the glass bottle?
(c) Which condition necessary for photosynthesis is not fulfilled in this experiment?


(a) Starch could not be synthesized in the green leaf.
(b) Potassium hydroxide absorbs the CO2 of the air present in the glass bottle.
(c) CO2 is required for the synthesis of starch and photosynthesis to occur. In the absence of carbon dioxide, photosynthesis will not occur as a result of which no starch will be synthesized.

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