CBSE Class 10 - Science

Know the difference between source and fuel

Good fuel should have the following characteristics:

a.Low ignition temperature

b.Storage and transportation of the fuel should be easy

c.The fuel should burn completely without leaving behind any residue

d.The fuel should be available at cheap cost

Know the difference between non-renewable resources of energy and renewable resources of energy.

Recall the concept of sources of energy and proceed to answer the given queston

Non-renewable resources of energy are those whose consumption rate is very much higher than the replenishment rate.

The energy resources, that are consumed at a rate faster than the rate at which they are replenished are called Non-renewable resources of energy.

Examples: Petroleum, natural gas.

Students may think valency and valence electrons both are same. But both are different.

Write the electronic configuration of an element.

Remember the number of valence electrons is equal to its group number.

Valency = 8- its group number

(i)The combining power of an atom is called it's valencyor it defined as the number of unpaired electrons in the outermost shell of the atom.

Valency = number of unpaired electrons in the outermost shell of the atom known as valence electrons if valence electrons is less than or equal to 4.

Valency = 8 - number of valence electron, if number of valence electrons is more than 4

(ii)Electronic configuration of element X = 2,7

From this we can see that valence electrons = 7

Hence valency = 8-7 =1

Some students may not relate the electronic configuration and position of element.

While finding the valency of an element there is a chance to commit a mistake

Based on the number of valence electrons we can identify the first and last element of a period.

In the period the first element has 1 valence electron and the last element in the period has 8 valence electrons.

know the difference between the laws of reflection of light and the laws of refraction of light

State the laws of reflection of light

First law:

The angle of incidence is equal to the angle of reflection.

Second law:

The incident ray, reflected ray and the normal all will lie on the same plane.

Do not confuse with refraction, scattering and dispersion of light.

Give your answer based on the refraction oflight.

It is because of refraction. The light from the stars are refracted towards the normal by the atmosphere and hence the stars appear higher from horizon than they actually are. This position too changes.

Do not confuse with refraction, scattering and dispersion of light.

Based on the knowledge of the concept of refraction of light try to answer the question.

Stars appear as small point size sources of light and they twinkle because of the refraction of the light from the stars by the atmosphere.

Planets appear as circular objects as they are nearer to the earth than the stars. The light from the planets is not be blocked completely, even if light from one point is blocked, the other parts of the planet will be visible and hence planets do not twinkle.

Similarities should not be emphasised

Asexual mode of reproduction should be known.

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

Significance of DNA replication in the process of reproduction is to be discussed.

DNA copying is important because when cells multiply to form new cells, equal quantity of DNA is passed 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.

a) While writing the difference there may arise small doubts like ethanol reacts with Bicarbonate or Ethanoic acid reacts with bicarbonate

b) While writing the equation for dehydration of ethanol student commit mistake

a)By knowing the properties of ethanol and ethanoic acid pupil write the differences.

b) Ethanol can be dehydrated to ethene by heating 140 ⁰C in presence of concentrated sulphuric acid.

a)

b)

While writing the chemical formula of soaps students may forget to write the names of ions.

There is chance to get confusion between hydrophobic and hydrophilic ends of a soap molecules.

Give the definition of soap.Then explain why soaps are not effective in hard water.

Then explain the Micelle formation in detail.

(a)Soaps are salts of sodium or potassium of higher fatty acids.

Examples: stearic acid, oleic acid and palimitic acid.

The water which donot give leather with soap is called hard water.The hardness of water is due to the presence of magnesium and calcium salts.

When soap is mixed with hard water the calcium and magnesium salts displaces the salts of sodium and potassium present in the soap and forms scum which is not soluble. Hence soaps are not effective in hard water for washing clothes.

The cleaning action of soap is due to micelle formation and emulsion formation. Inside water a unique orientation forms clusters of molecules in which the hydrophobic tails are in the interior of the cluster and the ionic ends on the surface of cluster. This results in the formation of micelle.Soap in the form of micelle cleans the (dirt) oil as the oil will be collected at the centre of micelle.This property of soap makes it an emulsifier. The dirt suspended in micelles is easily rinsed away. This is known as cleaning action of soap

Speciation should not be confused with genetic drift.

Speciation and its types are to be discussed. Natural selection is also to be discussed

(i) Speciation is an event that splits a population into two independent species which cannot reproduce among them.

Speciation can be categorised into 3 models:-

1.The species that will form new species are in same geographical location.

2.The species that will form new species are in nearby geographical locations and hence they can move between the locations.

3.The species that will form new species are in geographical locations which are distant from each other and hence the locations are isolated.

(ii)Natural selection is the theory proposed by Charles Darwin. Natural selection is the process by which individuals who are well adjusted in the environment can survive and reproduce better.

Somatic chromosomes should not be confused with sex chromosomes.

Information about sex chromosomes during the process of reproduction is discussed.

DNA in the nucleus of the cell stores the the genetic information which is to be transferred to the offspring during reproduction.

Of 23 pairs of chromosomes present in human beings, one pair forms the sex chromosomes. Male chromosomal pair is XY and that of female is XX.

Male gametes are haploid containing 22+ either X or Y chromosome as their sex chromosome. Female gametes are haploid containing 22+ X as sex chromosome.

On zygote formation, offspring will have 23 pairs of chromosomes in total. The sex chromosomal pair is determined to be XY or XX depending upon which male gamete is fusing with the female gamete.

Acids are good conductors of electricity.Both strong and weak acids conducts electricity.

weak acid do not dissociate completely into ions in solution and they do not conduct electricity or conduct less amount of electricity

Strong acids dissociates completely into ions in solution and conducts electricity.

Activity - Electric conductance

weak acid do not dissociate completely into ions in solution and they do not conduct electricity or conduct less amount of electricity Strong acids dissociates completely into ions in solution and conducts electricity.

CH₃COOH is a weak acid as it does not dissociate completely into ions in solution.

HCl dissociates completely into ions in solution and hence is a strong acid.

Activity:

Take two corks ,two nails and 2 beakers.now fit into each cork.Place this corks in both beakers.Then connect the nails to two different terminals of a battery(6v) to which a bulb and swich are attached.

Then pour dil.HCl in beaker one turn the switch on.

Pour CH₃COOH in the 2nd beaker turn the switch on

Observations:

The bulb glows in the beaker with dilute HCl whereas it does not glow with CH₃COOH.

It can be inferred that:-

HCl dissociates into H⁺ and Cl⁻ ions which conduct electricity when the switch is turned on. Hence the bulb glows.

CH₃COOH does not dissociate completely into ions and so do not conduct electricity. Therefore bulb does not glow.

This proves that CH₃COOH is a weak acid and HCl is a strong acid.

While writing the valency there is a possibility to commit a mistake.

While writing the formula of compounds there is a possibility to commit a mistake.

Group number equals to its valence electrons

Valency = 8- its group number

Left to right in a periodic table metallic character decreases

Left to right in a periodic table atomic size decreases

Find the valancies of elements and write the formula of compound formed when they combines with oxides and chlorides.

(i)Number of valence electrons in

X = 1 and Y = 2

(ii)Valency of

X= 1 and Y=2

(iii)Metallic character of X is more than Y

(iv)Atomic size of X is more than that of Y

(v)Oxide of X = X₂O and that of Y = YO

(vi)Chloride of X = XCl and that of Y = YCl₂

Remember to represent the light rays with arrows and follow the rules to draw the ray diagram in case of the image formed by a concave mirror when the object is placed at the centre of curvature of the concave mirror.

Draw the ray diagram by keeping the rules to draw the ray diagram in case of the image formed by a concave mirror when the object is placed at the centre of curvature of the concave mirror.

When the object is placed at the centre of curvature of the mirror, the image will have following characteristics:-

Position: at centre of curvature

Size: Magnified

Nature of image: Real and inverted

Use the meaning of the definition of refractive index

Explain the definition of the refractive index, then explain the meaning of “The refractive index of diamond is 2.42”

Give an example for a liquid whose mass density is less than that of water but it is optically denser than water.

(i)As we know that the refractive index is the ratio of sine of angle of incidence to sine of angle of refraction, we can say that the ratio of sine of angle of incidence to sine of angle of refraction of the diamond = 2.42, If refractive index of diamond is 2.42

(ii) The liquid whose mass density is less than that of water but it is optically denser than water is kerosene (Refractive index of kerosene = 1.44, refractive index of water = 1.33).

Know the difference between hypermetropia and myopia, remember hypermetropia is long sigtedness.

Define hypermetropia and explain is it corrected by using asuitable convex lens by drawing a suitable ray diagram.

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

Causes of Hypermetropia:

The eyeball has become too short in length

The focal length of the eye lens has become long.

Correction of Hypermetropia:

As shown in the above figure, a suitable convex lens is used to correct hypermetropia.

Normal bacterial and viral diseases should not be mentioned instead of STDs.

Name of STDs caused by bacteria and viruses. Preventive measures are to be stated.

(a) (i)Gonorrhoea and Syphilis are the two diseases caused by bacteria in human beings.

(ii)Genital Herpes and AIDS are the two diseases caused in by viruses human beings.

(b)Transmission of sexually transmitted diseases can be prevented by:

1.Avoiding sexual act with multiple partners and infected people.

2.Using condoms.

3.Preventing the sharing of needles and syringes between people.

4.Testing blood before transfusion.

5.Sterilisation of the equipments used in surgery and dental care.

6.To provide additional medical treatment to an unborn baby residing in the womb of a diseased mother.

Gametes produced should be clearly represented.

Dihybrid cross should be explained with an example.

Mendel’s law of independent inheritance states that when two pairs of characters such as, round and yellow as one pair and wrinkled and green as another pair, are combined to form a hybrid, one pair of characters will get separated and will be independent of the other pair.

Example:

In a dihybrid cross, Mendel crossed two pea plants with Round, Yellow seeds(RRYY) and Wrinkled, Green seeds(rryy).

They produced 4 types of gametes => RY, Ry, rY, ry in F1 progeny.

He observed that all of these are separate and each of them will have a frequency of 25% of the total gametes produced.

F2 progeny exhibited new combinations Round yellow, Wrinkled yellow, Round green, and Wrinkled green in 9:3:3:1 ratio.

Use the proper formulae, Know all the terms related to the rfraction of light through the shperical lenses.

To solve the question use the proper formulae, Know all the terms related to the rfraction of light through the shperical lenses and rules to draw the ray diagrams.

(a) Concave lens

(b) Pole

(c) $\frac{1}{f}=\frac{1}{v}-\frac{1}{u}\Rightarrow v=\frac{\mathrm{uf}}{u+f}\Rightarrow v=\frac{-30\times 20}{-30+20}$

$v=\frac{-600}{-10}\Rightarrow v=60\mathrm{cm}.$

(i) Position = 60 cm on the other side of the optical centre.

(ii) To find magnification:-

$m=-\frac{v}{u}\Rightarrow m=-\frac{60}{-30}\Rightarrow m=2$

(iii) Nature of image = Inverted.

OR

(a) Since a number of light rays are coming from the object, even if one half of lens is covered with black paper, image will be formed, although the brightness will be lesser.

(b)

(i)

(ii)

$\frac{1}{f}=\frac{1}{v}-\frac{1}{u}\Rightarrow v=\frac{\mathrm{uf}}{u+f}\Rightarrow v=\frac{-25\times 10}{\mathrm{-}25+10}$

$\Rightarrow v=\frac{-250}{-15}\Rightarrow v=16.66\mathrm{cm}$ $$

$m=-\frac{v}{u}=-\frac{h_i}{h_0}$

$h_i=h_0\times \frac{v}{u}$

$h_i$ = $\frac{16.66\times 5}{-25}$

$h_i=3.33\mathrm{cm}.$

(iii) Real, inverted image is obtained.

Sexual reproduction should not be confused with asexual reproduction.

Reproductive parts of a plant should be mentioned.

OR

Two types of asexual reproduction are explained.

(a)

(b)

The stamen is the male reproductive part that consists of filament and anther. The anther produces male gametes in the form of pollen grains.

The carpel is the female reproductive organ located at the centre of the flower. It consists of the ovary, style and stigma. The ovary is the swollen part at the bottom of the carpel. Ovary contains the female gametes in the form of eggs.

OR

(a) Fragmentation is one of the asexual method of reproduction. This is the process in which the organism breaks up into smaller pieces on maturation. Each fragment grows into a new individual. e.g. Spirogyra.

(b) Regeneration is a type of asexual reproduction. If the organism is cut up, its pieces can grow into separate individuals.This is possible by specialised cells which rapidly divide to form new individuals. It occurs in some fully differentiated organisms. Regeneration is also called morphallaxis. e.g. Hydra, Planaria.