CBSE Class 10 - Science

Students may confuse about basic changes in chemical reaction from physical change

Notice the changes involved in a chemical reaction

We can know that a chemical reaction has taken place if:-

(a) Any gas has evolved

(b) Change in temperature has taken place

Example:-

In the above reaction, CO2 is evolved which shows that a reaction has taken place

A large amount of heat is produced in the above reaction. Since there is a change in temperature, it confirms that a chemical reaction has taken place

Fossil fuels are advantageous in some cases but these advantages should not be discussed.

Explanation of fossil fuels. Effects of combustion of fossil fuels on the enviroment.

Fossil fuels are hydrocarbons formed by plants and animals which have been buried in the earth for a long time under special temperature and pressure. e.g. Natural gas, coal, petroleum etc.

Disadvantages of burning fossil fuels:

1.Combustion of fossil fuels release lot of smoke.

2.Burning of fossil fuels releases lot of gases such as nitrogen dioxide and sulphur dioxide which combine with rain water to form acid rain.

Know the diffference between electro magnetic induction and mutual induction.

As the galvanometer ( connected to the coil A) shows a deflection if current passes through it we can say that, if there is a change of current flow in coil B, it produces a change in magnetic field around coil B as well as coil A which is kept near coil B.

In (i), current through B changes suddenly from 0 to maximum, whereas in (ii), it changes from maximum to zero at once. In both the cases, there is a change in magnetic field around coil B and hence this causes current flow in coil A.

(i)The galvanometer shows a deflection as current passes through coil A due to the mutual induction.

(ii)In this case also galvanometer shows deflection, but it would be in the opposite direction as current passes through coil A in the opposite direction.

The phenomenon is called electromagnetic induction

When there is a change of current flow in coil B, it produces a change in magnetic field around coil B as well as coil A which is kept near coil B.

In (i), current through B changes suddenly from 0 to maximum, whereas in (ii), it changes from maximum to zero at once. In both the cases, there is a change in magnetic field around coil B and hence this causes current flow in coil A.

Remember the length of a solenoid is more longer than its diameter.

Draw the pattern of magnetic field lines of a solenoid through which a steady current flows and compare it with that of a bar magnet

A solenoid is a coil made up of either insulated or enamelled wire which can conduct electricity (usually copper) wound in the shape of a cylinder. It is used to produce magnetic fields when electricity is passed through the wire

The pattern of the field lines inside the solenoid indicate that the magnetic field has the same strength in all places inside the solenoid.

Remember a person with hypermetropia or longsightedness can see only the distant objects clearly but not the nearby objects, as the image is formed behind the retina of the eye.

Remember to mark the arrows to represent the light rays.

Draw the suitable ray diagram to illustrate hypermetropia, mention the possible causes for it, Draw a ray diagram to show how this defect may be corrected using a lens of appropriate focal length.Write the importance of eye donation.

Hypermetropia is an eye defect where the person can see distant objects clearly but cannot see the nearer objects clearly.

To correct the defect:

Hypermetropia can be corrected by using a suitable convex lens as shown in the figure.

Remember wind energy is a renewable energy.

Write the limitations in harnessing wind energy on a large scale.

1.Land should be available at a large scale to set up wind mills

2.Initial cost to set up the wind mills is very high

3.To harness wind energy, the speed of the wind must be minimum 15 km/hr

4.Wind mills and plants to produce wind energy can be set up only where there is high speed wind blowing for majority of the year.

Chance of finding less reactive metals as high reactive metals.

Knowledge of metal activity series and reactivity of metals with halogens will makes easy to solve

Sodium and Potassium are the two most reactive elements in group 1.

Sodium- Na

Potassium - K

Reaction with Halogen:

The above reaction takes place between Na and Cl as Na ion is positively charged and will attract the negatively charged Cl ions.

Bond formed between these elements is electrostatic bond otherwise known as ionic bond

The compound formed is sodium chloride. It is ionic compound.

Ionic compounds have following physical properties:-

1.High boiling point and high melting point.

2.They are solids.

3.When ionic compounds are in molten state, they conduct electricity

4.They dissolve in polar solvents and do not dissolve in non-polar solvents.

Improper notation of anode and cathode and direction of transfer of electrons

It is important to know to keep impure metal as anode and pure metal will be obtained at cathode

Refining is the process used for extracting pure metals from impure metals

Most widely used method :Electrolytic refining

The diagram given above shows the electrolytic refining of copper

Cathode = pure metal, thin strip

Anode = impure metal, big block

Electrolyte = copper sulphate solution(acidified)

Pass electricity through the electrolyte. The positive metal ions in the copper sulphate solution, Cu²⁺ ,are attracted towards the cathode and hence move towards the cathode. They gain electrons from cathode and as a result Cu gets deposited on the cathode. As the process keeps repeating for some time, the size of the cathode block increases.

Meanwhile the anode discharges ions into the electrolyte solution when the positive charges move towards cathode. The anode keeps becoming thinner as the process keeps repeating for some time.

Thus the pure metal is deposited on the cathode. The soluble impurities from anode move into the solution and insoluble impurities, known as anode mud, settle down in the container and are found in the bottom of the container.

It should be clear that the stimulus responsible for the movement of plant is light.

Phototropism should be defined along with an activity.

The movement by growth exhibited by plants towards light as stimulus is called as phototropism.

Activity:

Phototropism can be explained by placing a potted plant outside in sunlight. We can observe that the shoot of the plant exhibits growth in the direction of sunlight. i.e. phototropism.

Fertilisation concept in plants should not be confused with that of fertilisation in animals.

Two main differences between pollination and fertilisation are to be represented.

Pollination is the transfer of pollen grains from anther to stigma of the carpel whereas fertilisation is the fusion of male germ cell with the female germ cell to form a zygote, future embryo.

Pollination requires external agents to help in the process whereas fertilisation just requires the presence of male and female germ cells.

Pollination in plants can be cross-pollination or self-pollination whereas fertilisation in plants is called as syngamy taking place inside the ovary.

Heredity and inheritance are related. But acquired and adaptation are not completely the same.

Inherited characters are to be differentiated from acquired characters along with examples.

Example of inherited character - Shape of nose.

Example of acquired character - Malfunctioning of kidney affected by disease.

Inherited characters are those which are transmitted from one generation to another. i.e Parents transmit certain characters to their offspring.

Acquired characters are the changes produced in an individual as a response to the environment. These acquired characters are not purposefully inherited.

Identify the lens used and use the proper formula.

Calculte the object distance, in case of a lens of focal length 25 cm is used to obtain its image onto the desired screen placed on the other side at a distance of 50 cm from the lens and magnification produced in this case.

Data:

Image distance (v) = 50cm

Focal length (f) = 25 cm

Using lens formula:

$\frac{1}{f}=\frac{1}{v}-\frac{1}{u}$

⇒$\frac{1}{u}=\frac{1}{v}-\frac{1}{f}$

$\Rightarrow \frac{1}{u}=\frac{1}{50}-\frac{1}{25}$

$\Rightarrow \frac{1}{u}=-\frac{1}{50}\Rightarrow u=-$50

Using this we can find the magnification

$\mathrm{Magnification}=\frac{v}{u}$

$\Rightarrow \mathrm{Magnification}=\frac{50}{-50}=-1$

Therefore we get magnification as -1, so the object should be placed 50 cm away from the lens.

It is important to learn the properties of elements of different groups otherwise it is difficult to get solution. And not knowing the valency of elements will make difficult to write the formula.

Knowledge regarding the properties and valencies of elements of different groups will make easy to solve

(i)All of these belong to the 3rdperiod.

They belong to the following groups:-

A - group 1

B - group 2

C - group 3

D - group 14

E - group 15

F - group 16

G - group 17

H - group 18

(ii)Elements B and F will form ionic compounds

(iii)A and B belong to group 1 and 2 respectively and are definitely metals

(iv)Element H

(v)CG3

Student confuses in identifying the metal oxide

Manganese dioxide is reduced to manganese by heating with aluminium powder.

3MnO₂(s) + 4Al(s) → 3Mn(l) + 2Al₂O₃(s) + Heat

(i)

'A' can be assumed to be MnO₂ and 'B' can be assumed to be Al

In this reaction, Mn is product C and is produced in molten state because large amount of heat is released during reaction. Product D is Al₂O₃ which floats over the Mn.

(ii)

Above chemical reaction can be classified as:-

Displacement reaction.

Redox reaction.

There is a chance of confusion in identification of saturated compounds for hydrogenation process that is students misguide to alkanes from alkenes and alkynes.

In general molecules which contains unsaturation will prefer to undergo hydrogenation

Alkenes and Alkynes are the functional group of organic compounds which can be hydrogenated.

When hydrogen is added with Ni as catalyst to unsaturated hydrocarbons, they become saturated hydrocarbons.

Change in physical property:-

Unsaturated hydrocarbons:- Liquid state.

Unsaturated hydrocarbons become saturated hydrocarbons.

Saturated hydrocarbons :- Solid state

Natural source of organic compound - Vegetable oil

Mechanism of ozone formation should not be confused with that of ozone depletion.

An interrelationship between environmental issues like ozone depletion and global warming are to be discussed.

Ozone molecule is the combination of three oxygen atoms. Oxygen is split into nascent oxygen on exposure to UV- rays, part of sun's radiation. At higher level of atmosphere, nascent oxygen reacts with oxygen molecule to form ozone.

The ozone layer over Antarctica, in the South pole and over Arctic, at the North pole is getting depleted every year by the increased usage of ChloroFluoroCarbons(CFCs).

This damage to the ozone layer is allowing harmful UV rays to reach the earth’s atmosphere.

1) These UV rays are dangerous for mankind as they may cause skin cancer, cataracts and ageing.

2) UV rays can also cause the death of phytoplanktons which in turn cause global warming.

3) Global warming results in the melting of ice cap in the polar regions thereby rising the sea level.

This is the reason why it is said that, “Damage to the ozone layer is a cause of concern.”

One should not get confused between DNA, genes and the chromosomes.

Expansion of the term DNA is to be done. Its location in the cell is to be specified. Significance of DNA in the process of cell division is to be discussed.

The acronym of DNA can be expanded as Deoxyribonucleic Acid.

Maximum amount of DNA is found in the nucleus of the cell. Small amount of DNA is also found in mitochondria.

DNA contains the genetic information that is to be carried on to the new cell after the process of cell division.

DNA is the hereditary material that stores information for next generation in the form of code made up of different combinations of four nitrogenous bases (Adenine, Thymine, Guanine and Cytosine). Hence, it plays an important role in the process of reproduction of the cell, the cell division. Replication of DNA results in the formation of two copies of DNA to be transferred to two daughter cells. Three types of DNA replication are semi-conservative, conservative and dispersive.

Let us assume that current ‘I’ is flowing through a resistor with resistance ‘R’ and potential difference ‘V’ across it.

Let the ‘Q’ quantity of charge flow in time ‘t’ through it.

Let W be the work done to move charge Q

W = VQ ... (1)

As we know current = charge/time

Q = I × t ----(2)

Replacing (2) in (1)

W = V × I × t

As heat is produced during the work,

If ‘H’ is the heat produced

H = W = VIt -----(3)

As per Ohm’s law,

V = IR.

Putting this in equation (3),

H = IR × It

We get H=I²Rt .

The above relation is called Joule’s law of heating.

If P is the power,

We know that P = 12 W and V =12 V

Let t be the time duration for the flow of current

t = 1 min = 60 s

Heat generated H= P × t

= 12 W × 60 s

= 720 J

OR

If I₁, I₂ and I₃ are the currents flowing through the resistors R₁, R₂ and R₃ respectively and I is the total current, then

I = I₁ + I₂ + I₃

Ohm’s law can be applied to all the resistors independently

If Req is the equivalent resistance of the whole circuit, applying Ohm’s law to the whole circuit, we get.

In order that current measured in ammeter is the same as the total current through the circuit, the ammeter has to be connected in series with the combination of the individual resistors.

To measure the potential difference of one resistor, voltmeter should be connected parallel to it.

Emphasis should not be given in explaining the parts of digestive system.

OR

Reactions of photosynthesis are just to be listed not explained.

Chemical process of digestion should be explained.

OR

Events of photosynthesis are to be elucidated.

Digestion is the process of breaking down complex food substances into simple absorbable molecules.

The process of digestion mainly takes place in three regions namely, mouth (buccal cavity), stomach and small intestine.

Digestion in mouth - The process of digestion starts in the mouth and is brought about by three pairs of salivary glands. The salivary glands secrete saliva, a mixture of water and hydrolytic enzymes. The main enzyme present in the saliva is the salivary amylase. It breaks down starch in the food into simple sugars.

Digestion in stomach - Food is mixed with gastric juice during its storage in the stomach. Gastric juice is a combination of HCl, mucus and enzymes like pepsinogen, gastric lipase.

HCl - It provides an acidic medium for certain enzymes to get activated and act upon food molecules.

Mucus - It protects the inner lining of the stomach from being damaged by the action of HCl.

Pepsinogen - The inactive enzyme gets converted to pepsin by acidic medium provided by HCl. Pepsin aids in digestion of the proteins into peptides.

Digestion in small Intestine - Digestion in this part is brought about by enzymes like invertase, maltase, lipase etc.

Small intestine receives the bile secreted by liver. Bile converts acidic food, received from stomach into alkaline to be acted over by enzymes. Bile also aids in emulsification of fats.

Small intestine also receives pancreatic juice from pancreas that digests emulsified fats as well as proteins.

Intestinal glands secrete intestinal enzymes which bring about digestion of carbohydrates, proteins and fats.

Carbohydrates, proteins and fats are simplified into glucose, amino acids, fatty acids and glycerol respectively.

Post digestion, this food is absorbed by the walls of the intestine.

OR

(a)

Photosynthesis is the physico-chemical process by which plants can convert light energy into chemical energy, in the form of food using atmospheric carbon dioxide and water as raw materials.

Photosynthesis involves a series of photochemical reactions in two phases - Light reactions and Dark reactions.

1)Light reaction occurs in the grana of chloroplasts. Using light energy, water molecules split out to release oxygen.

2)The chlorophyll pigments trap light energy and excite an electron. This excited electron converts light energy into chemical energy. This chemical energy is stored as ATP (Adenosine Tri Phosphate) and NADPH (Nicotinamide Adenine Dinucleotide Phosphate).

3)Dark reaction takes place in the stroma of the chloroplasts by reducing carbon dioxide to carbohydrates, utilising energy from ATP.

Stomata are tiny pores on the surface of the leaves. Stomata help in exchange of gases during photosynthesis. The opening and closing of stomata are brought about by guard cells.

(b)

Experiment:

Place a healthy, green potted plant in a dark room for a couple of days. This ensures that there is no starch left on the green leaves as the plant uses up all its stored food.

After two days, take a strip of black paper and cover a small portion of one green leaf on both sides using a clip.

Take this potted plant outside and keep it in bright sunlight. After keeping it outside for some hours, remove the leaf which is covered with a black strip. Boil it in alcohol for 10 minutes to get rid of the green colour. Pour little of iodine solution on this leaf.

Observation:

We can see that uncovered parts of the leaf as shown in the diagram, turned blue-black when iodine was used whereas the covered part was left white or had no colour change.

Inference:

The iodine solution helps in confirming the presence of food. After photosynthesis, food prepared is stored as starch. This starch reacts with iodine to give a blue-black colour.

The uncovered portions of the leaf had starch suggesting that photosynthesis has taken place as these portions received sunlight. Thus, these portions turned blue-black when iodine was used.

No photosynthesis occurred in the covered portion to prepare starch. Hence, there is no colour change when iodine is applied here.

This experiment shows that sunlight is essential for photosynthesis.