CBSE Class 10 - Science

Only vegetative propagation should be discussed and not all the asexual modes.

Listing the advantages of vegetative propagation.

Vegetative propagation is a mode of asexual reproduction observed in plants.

1.In this method, a plant can produce its offspring even during unfavourable environmental conditions from vegetative parts such as corms, tubers, bulbs etc.

2.Plants which do not produce seeds can be vegetatively propagated.

3.Pollination is not required in plants reproducing by vegetative propagation.

4.The desired characters of the parent are restored in the offspring produced by vegetative propagation as they are exact copies of the parent.

1. Marking of arrows to show the direction of light rays is important.

2. The angle of reflection should be equal to the angle of incidence.

3. The normal at the point of incidence should pass through the centre of curvature.

4. Labels should be given and explained.

5. Use a scale (or ruler) to draw lines.

• Draw the convex mirror (with shading on the interior surface) and the principal axis.
• Draw the incident ray parallel to the principal axis.
• Draw the normal at the point of incidence which passes through the center of curvature of the mirror.
• Draw the reflected ray such that the angle of reflection is equal to the angle of incidence, i.e. the normal lies in the middle of the incident and reflected rays.
• Label the diagram.

P - Pole of the convex mirror

C - Centre of curvature of the convex mirror

MPC - Reflecting surface of the convex mirror

i - Angle of incidence

r - Angle of reflection

Advantages of plastics should not be discussed at all.

Polythene and its ill-effects on the environment. Alternative materials for polythene are to be suggested.

Polythene bags are considered harmful to the environment as they do not degrade easily thereby getting accumulated in the soil. Biodegradation happens by the action of microorganisms but polythene bags are non-biodegradable substances. On burning they release toxic gases which pollute the environment.

Bags which can be used instead of polythene bags are cloth bags, jute bags, paper bags etc.

Ban on usage of polythene increases the usage of alternative bags such as cloth and jute bags. Using them will improve the environment as:-

1.They are bio-degradable and will degrade naturally.

2.As they are bio-degradable, they do not pollute the environment.

3.Jute and cloth bags can be recycled easily.

4.They last much longer and they are stronger.

Biodiversity also involves variations and diversifications. These should not be ignored.

Explanation of biodiversity is to be done. Advantages of forest and wildlife conservation are to be listed.

Biodiversity is the variety and variability on the earth brought about by evolutionary changes, changes in the environment and the changes brought about by human beings.

Advantages of conservation of forests

1.Forests provide variety of food products and raw materials for various industries such as paper, furniture, construction, medicines etc.

2.Floods and soilerosion are checked by forests.

Advantages of conservation of wildlife

1.Wildlife is necessary to maintain balance in the ecosystem.

2.Different characters of the wild species are prevented from being lost.

There is chance of writting aldehyde functional group instead of alcohol.

It is required to know about the attachment of which group imparts "-ol" at the end of name of the molecule and which group imparts "-oic acid".

In a carbon compound, the chemical property of the compound is determined by the group of atoms or the bonds in the carbon chain. This group of atoms is called functional group.

(i)

Functional group - Hydroxy ( -OH)

Structure - CH₃CH₂OH

(ii)

Functional group - Carboxylic acid( -COOH)

Structure - CH₃COOH

It is important to know definition of addition reactions other wise their is chance that student may put substitution reaction of alkanes.

It is required to know that which are hydrocarbons will prefers undergo addition reaction among saturated and unsaturated hydrocarbons. And also show the importance of catalysts which can alter the rate of reaction.

Alkene and Alkyne are the names of the hydrocarbons in which an addition reaction with hydrogen can take place. Their general formulae are :-

Alkene - C_nH_2n

Alkyne - C_nH_2n-2

Essential conditions required for an addition reaction to occur:-

1.Presence of catalyst is needed for the reaction to take place. Nickel, platinum etc are the catalysts that are generally used.

2.There should be double and triple bonds existing between the carbon atoms in the hydrocarbon

The chemical equations for the reactions:-

It is need to avoid mismatching of valence.

Knowing the valence of Oxygen, halogens and other groups can makes easy to solve.

(i) If where M is 1st group element and O is oxygen, then the formula of the oxides of group 1 is M ₂O

1st group elements have valency = 1

Oxygen has valency of 2

(ii)

If M is the group 13 element and X is halogen, the formula of the halides of group 13 is MX₃

Group 13 elements have valency = 3

Halogens have valency = 1

(iii)

If M is the 2nd group element and N is 16th group element, then formula of the compounds of group-2 and group-16 is MN

Group 2 elements have valency = 2

Group 16 elements have valency = 2

There is a chance that students may think valency and valence electrons both are same. But both are different.

Students may think Atomic number increases from left to right so atomic size also increases. But this is not correct.

Students may think while moving left to right in a periodic table the group number increases so valency also increases. But this is not correct.

Knowledge regarding the variation of atomic size and valence in periods of the periodic table will helps to solve.

(a)

(i) Valency is defined as the combining capacity of an atom or the number of unpaired electrons in the outermost shell of the atom.

(ii) Atomic size is the distance measured from the outermost shell of an atom to the centre of the nucleus. It is also known as the atomic radius.

(b)

While going from left to right along a period in the modern periodic table, Valency increases till 4 and after that decreases.

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

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

Atomic size decreases as you move from left to right. The number of the valence shells do not vary along a period. Electrons are added by one unit which increases the nuclear charge. This decreases atomic size

Only the process of regeneration is to be explained and any other mode of asexual reproduction.

Regeneration process in Planaria is to be represented diagrammatically. Another example of the organism exhibiting regeneration is to be quoted.

Regeneration is a type of asexual reproduction. For example in Planaria, if the organism is cut up, its pieces can grow into separate individuals.

Regeneration can be observed in Hydra also.

Contraception should not be confused with conception.

Methods of contraception are to be listed. Effects of contraceptives on health should be discussed.

Natural method, Oral contraceptives, Barrier method, Implants and surgical methods are 4 methods of contraception used by humans.

(a)Natural method - Sexual act is avoided during the period of 10 - 17 days of the menstrual cycle to avoid sure fertilisation of the egg by sperm.

(b)Oral contraceptives - Fertilisation is prevented by preventing the release of eggs. This is achieved by using oral drugs or tablets.

(c)Barrier method - Fertilisation is prevented by using barriers to avoid contact between the penis of males and vagina of females . Condoms act as barriers to prevent fertilisation.

(d) implants and surgical methods - Contraceptive devices like copper-t and loop placed in the uterus can prevent fertilisation. Surgical methods like vasectomy and tubectomy also prevent fertilisation.

Effects of contraceptives can be both advantageous and disadvantageous.

1.Prevention of frequent pregnancies by contraceptives helps in maintaining health of females in the society.

2.Over usage of contraceptives may lead to permanent sterilty.

3. Usage of contaceptives helps in limiting the number of children in a family which in turn helps in economic development of the family.

4.Contraceptives help in reducing the chances of transmitting sexually transmitted diseases.

Evolutionary phenomena should not be confused. Clear identification should be done.

Evolutionary relationships should be discussed. Phenomena described in the form of examples are to be identified.

(a) Fossils serve as tools for tracing evolutionary relationships.

Archaeopteryx is a fossil bird which is thought to have existed around 150 million years ago forms a connecting link between reptiles and birds. It possesses both reptilian characters which include long and bony tail, strong jaws with teeth and bird like wings with feathers.

The phenomenon of adaptation can be explained by the fact that birds evolved from reptiles.

Dinosaurs were reptiles and some of them had feathers which were not used for flight but used them to withstand cold. Post evolution, these feathers were used by birds for flight.

(b)Though insects, octopus, planaria and vertebrates possess eye as characteristic feture they cannot be placed in the same group. These serve an example of biological convergence. The similarities in structure, despite of different origins. Hence, invertebrates (insects, octopus and planaria) cannot be placed along with vertebrates.

Gametes produced should be clearly represented in the cross.

Dihybrid cross should be explained. Representation of the results in the form of ratios is to be done.

(a)

The cross represented here is the dihybrid cross. The cross is between the pea plants producing round, green seeds and wrinkled, yellow seeds.

In this case, their dominant characteristics are round and yellow over wrinkled and green colour.

Green colour and wrinkled shape get suppressed and we obtain Yellow coloured, round seeds in the F₁ generation as they are dominant.

(b) On selfing the F₁ generation plants we have following results.

The phenotypic ratio obtained is 9:3:3:1

9 Round yellow, 3 Round green, 3 Wrinkled yellow and 1Wrinkled green seeded plants.

Follow sign convention. Object distance is always taken as negative. While drawing ray diagram don't forget to mark the arrows for light rays and label the diagram.

(a) Given that the magnification is −1. A negative magnification indicates that the image is real and inverted. A real image can be formed only by a concave mirror. Hence the mirror used is a concave mirror. The numeral 1 indicates that the image is of the same size as the object. By recalling the ray diagrams for the various positions of the object in front of a concave mirror, an image of the same size is formed only when the object is kept at the center of curvature. Now you can draw the ray diagram (d). (b) Using the magnification formula find the image distance and hence the distance between the object and image. (c) Using the mirror formula find the focal length of the concave mirror.

(a)

Magnification = −1, which is negative. A negative magnification indicates that the image is real and inverted. A real image can be formed only by a concave mirror. Hence the mirror used is a concave mirror.

(b)

Magnification m = −v/u = −1

Object distance u = −50 cm

∴ Image distance v = −50 cm

Distance of object from image = |u| + |v| = 100 cm

(c)

Focal length can be calculated with the help of mirror formula:

$\frac{1}{f}$ = $\frac{1}{v}$ + $\frac{1}{u}$ = $\frac{1}{-50}$ + $\frac{1}{-50}$ = $-\frac{1}{25}$

∴ f = −25 cm

(d) The ray diagram is drawn such that the object is at the centre of curvature. The real image formed is also at the centre of curvature. The size of the image is same as that of the object.

Write the formula correctly and proceed. Remember that the speed of light in a medium is always less than that in air or vacuum.

First state the two laws of refraction. Then using the formula for refractive index, find the speed of light in a medium of refractive index 1.5.

First law of refraction or Snell’s law: It states that the ratio of the sine of the angle of incidence i to the sine of the angle of refraction r is constant for the light of a given colour and for the given pair of media.

If n₁₂ is the refractive index of medium 1 with respect to medium 2, then the law is given by

$\frac{\sin i}{\sin r}$ = n₁₂

Second law of refraction: As per this law, if there are two media, the incident ray, the refracted ray and the normal to the interface of the two media at the point of incidence - all lie on the same plane.

If a ray of light travels from medium 1 to medium 2, then the refractive index of medium 1 with respect to medium 2, is given by

$n_{12}=\frac{v_1}{v_2}$ ,

where v₁ is the speed of the light in medium 1 and v₂ is the speed of the light in medium 2.

Given: Refractive index of the mediumn_medium = 1.5

Speed of light in vacuum c = 3 × 10⁸ ms⁻¹

$n_{\mathrm{medium}}=$$\frac{c}{v}$, where v is the speed of light in the medium.

⇒$\frac{3\times {10}^{8}m{s}^{-1}}{v}$

⇒ v = $\frac{3\times {10}^8{\mathrm{ms}}^{-1}}{1.5}$

= 2 × 10⁸ ms⁻¹

Therefore speed of light in the medium = 2 × 10⁸ ms⁻¹ .

It is scattering and not refraction which causes the blue colour of the sky.

Remember that we cannot see sunlight if it is not scattered.

Recall the reason for the blue colour of the sky. Recall what will be the colour of the sky in the absence of atmosphere. Give reason.

The scattering of blue component of the white sun light by air molecules present in the atmosphere causes the blue colour of sky. The air molecules and other fine particles in the atmosphere have size smaller than the wavelength of visible light. When sunlight passes through the atmosphere, the fine particles in air scatter the blue colour (shorter wavelengths) more strongly than red. The scattered blue light enters our eyes. That is why sky appears blue.

For an astronaut staying in the international space station orbiting the Earth, the colour of the sky will be black because there is no atmosphere containing air molecules in space to scatter sunlight. As no light enters our eyes, the sky appears dark and black.

The phenomenon discussed in the question should not be mistaken.

The phenomenon of biomagnification is to be identified from the question. Reasons for the same are to be discussed.

Chemical pesticides sprayed over crop get settled on the food they produce and enter the food chain.

Pesticides enter the body of animals (primary consumers) and human beings who consume the affected food.

Pesticides enter higher animals and human beings when they consume primary consumers.

Pesticides are absorbed by aquatic organisms when the affected soil is washed away into water resources.

On consumption of aquatic food also leads to accumulation of these chemical pesticides in higher organisms.

Larger amount of pesticides are accumulated in higher organisms leading to biomagnification.

To summarise, we can say that plant products have less amount of pesticide residues than primary consumers which in turn possessless amount of pesticides than secondary consumers.

There is a chance that all the covalent compounds are bad conductors of electricity and possesing low melting and boiling points. But covalent compounds also posses high melting and boiling points in diamond form and even good conductors of electrity in graphite form.

It is important to know valence electrons in Carbon, electronegativity and electropositivity of carbon. And properties of covalent compounds.

When we study the periodic table, we know that carbon has an atomic number of 6. That means the outer most shell of the carbon has got 4 electrons. Removing the 4 electrons to form C⁴⁺ cation, a noble gas would require huge amounts of energy and at the same time the cation formed hence would be highly unstable due to the presence of six protons and two electrons. Similarly, carbon cannot form the anion C⁴⁻ due to the fact that the nucleus being unable to hold 6 protons and ten electrons. It is much easier for carbon to achieve a stable noble gas configuration by sharing electrons to form covalent compounds.

(i)Covalent compounds don’t have free electrons and for this reason they are bad conductors of electricity.

(ii)The intermolecular attraction forces associated with covalent bond are weak and hence compounds which posses covalent bond have low boiling point and melting point.

The phenomenon of reproduction in plants should be exactly demarcated from that of reproduction in animals.

Listing the examples of unisexual and bisexual flowers. Post fertilisation changes are explained. Restoration of amount of DNA after the process of reproduction is explained.

(a)Water melon, papaya, cucumber are some of the examples of unisexual flowers.

Rose, hibiscus, lily are some of the examples of bisexual flowers.

(b)Changes in a flower after fertilisation are termed to be post fertilisation changes.

i)A fertilised ovule transforms into a seed with embryo enclosed in it. The nourishing material present in the ovule is retained by cotyledons of the seed. The integuments of the ovule form the thick seed coat.

ii)Mature ovary develops into a fruit enclosing the seed in most cases.

iii)The accessory appendages like sepals, petals, stamens, styles and stigma usually fall off after fertilisation except in certain rare cases.

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

Male gametes are haploid containing exactly half amount of DNA. Female gametes are also haploid containing exactly half amount of DNA.

During fertilisation, the nuclei of male and female fuse to form a diploid zygote thereby combining the amount of DNA present in them on a total. Hence the amount of DNA remains constant.

The phenomenon of reproduction in animals should be exactly known in order to name the parts of the reproductive system.

Labelled diagram of female reproductive system is to be given. Concept of pregnancy and the processes after fertilisation are to be discussed.

(a)

(i) The organ that produces eggs - Ovary

(ii) The place where fusion of eggs and sperm takes place - Fallopian tube

(iii) The place where zygote is implanted - Uterus

Diagram of the female reproductive system

(b)

The zygote after it gets fertilised in the fallopian tube repeatedly divides to form an embryo which travels down to get implanted in the walls of the uterus or endometrium by the process of implantation. The placenta is a connective tissue established between foetus and the mother. It provides a large surface area for the nutrients and oxygen to pass from mother to the embryo.

This is the beginning of pregnancy.

As the months proceed, there are different changes observed in the embryo which gets itself transformed into foetus by giving rise to organs. Fully formed foetus is delivered out by the proces of parturition.

Only convex lens can form real images; concave lens cannot form real images.

While drawing ray diagrams, don't forget to label the diagrams and mark the arrows on light rays.

(a) Recall which lens can produce real images on a wall. So it is convex lens.

(b) Recall to get magnified and diminished images what will be the positions of the object on the principal axis.

(c) Draw the respective ray diagrams.

(a) To project the image of a candle flame the student should use a convex lens as only a convex lens can form real images.

(b) For a magnified image the object should be placed between F and 2F of the lens while it should be placed beyond 2F to get a diminished image, where F is the focal length of the lens.

(c) Magnified image

Do not list defects like cataract or night blindness. List only refractive defects.

List refractive defects of the eye and their correction; state in what ways you can create awareness among people about eye donation.

(a) The three common refractive defects of vision and the ways to correct them are the following:

1. Myopia or short-sightedness: It is corrected by using spectacles having concave lenses of appropriate power.
2. Hypermetropia or far-sightedness: It is corrected by using spectacles having convex lenses of appropriate power.
3. Presbyopia: It is corrected using bifocal lenses of appropriate power in which the upper part consists of a concave lens and the lower part consists of a convex lens.

(b) Students can easily influence their family members to donate their eyes after their death. By making people register to donate their eyes, students can play a very active role in reducing corneal blindness in the world.