In adolescent boys, sometimes the muscles of the growing voice box go out of control and the voice becomes hoarse

CNG: Compressed Natural Gas.
LPG: Liquefied Petroleum Gas.

Yes, the destruction of habitat, increasing population and introduction of new species may affect the natural habitat of endemic species and endanger their existence.

Lead and arsenic are two water pollutants which are toxic for both plants and animals.

Since, no chemical changes take place when electricity is allowed to pass through the solids while in liquids chemical changes take place when electricity is pass through it.

Frictional force helps to write by chalk on blackboard.

Since eyelid is also known as eye-shutter. So, it prevents any object entering the eye.

No, we cannot draw the solar system to scale.

The tremors produce waves on the surface of the earth. These waves are called seismic waves.

Yes, both the liquids and gases also exert pressure.

When we touch a sound producing bell, we feel the vibration in it.

The phenomenon by which female gametes produce off spring without fertilisation is called parthenogenesis.

Cell membrane or plasma membrane gives shape to a cell.

Metals are generally solid, hard, ductile, malleable, sonorous, have metallic lustre and are good conductors of heat and electricity.

It is because CNG produces harmful products in very small amount, so it is a clean fuel.

The raw materials used in making nylon are coal, water and air.

Spherical bacteria are called as cocci.

Yes, In drip irrigation method, water is released drop by drop near the roots of plants. It is thus considered advantageous over other methods as there is no wastage of water and it is also economical.

The decreasing amount of air decreases loudness of sound. When air is removed completely from a vessel, it is said that there is a vacuum. If all air is sucked from the vessel the sound would stop completely. The sound cannot travel through a vacuum.

Invention of microscope brought many advantages in study of science, such as regarding microscopic animals and internal structure of various animals and plants. Microscope is an instrument which makes an object look many times bigger than its original size. It has one eye piece , draw tube, body tube, objective lens, stage and mirror as main parts. Tiny structures are kept on stage under objective lens. Light is adjusted by mirror . Then structure is watched through eye piece . Much bigger structure is visible. So students can easily and clearly understand the microscopic structures.

Halley's comet appears periodically after every 76 years. It was last seen in 1986. So it will be visible again in the mid of 2061.

The 4Rprinciple means Reduce, Reuse, Recycle and Recover. One should develop these habits, which are environment friendly.

Disadvantages of fertilizers:
(i) They become source of water pollution.
(ii) They make soil less fertile.
(iii) They do not provide any humus to the soil.

Global warming is an increase in the average temperature of the earth's surface. It occurs as a result of increased concentration of greenhouse gases in the atmosphere. The greenhouse gases such as CO₂, CH₄, etc are increasing due to human activities including vehicular emissions, burning of fossil fuels, etc. Global warming has become a major concern for governments worldwide. This is because even as little as 0.5⁰C Crise in temperature can have serious implications.

As Boojho is not in the range of reflecting surface rather, he is standing away from the mirror, so he cannot see himself in the mirror. But he can see the image of P which is in the reflecting area but not of Q and R.

There are two factors that affect the effect of changes, they are
(i) Magnitude usually expresses the strength of force.
(ii) When the force is applied on the body, then at that moment the force acts, has direction as well.
So, if the magnitude or direction of force or both, Change, then its effect also changes.

Three precautions during earthquake inside the house are as follows:
(i) Instead of running away, we should get under a desk, table or bench or sit against a nearby interior wall.
(ii) Pick a safe place where things will not fall on you, away from windows, bookcases or tall heavy furniture.
(iii) Wait at your safe place until the shaking stops.

Zygote Single celled structure formed by the fusion of male and female gamete.
Larva The active form that emerges from the eggs.
Placenta The specialised tissue that attaches the developing foetus to the uterine wall and provides it nourishment, etc.
Foetus The multicellular structure formed after the repeated cell divisions in the zygote.

Red Data Book is a source book that maintains an international list of all endangered animals and plant species.

Menstruation is the monthly reproductive cycle that occurs in women once in about 28-30 days. It is the release of unfertilised egg along with blood.
If the egg released from ovary does not get fertilised, the released egg and the thickened lining of uterus along with its blood is released out from body.

Aluminium is a good conductor of electricity. When it is exposed to air, its surface gets covered with thin layer of oxides which prevents metal underneath from further corrosion.

CNG has replaced diesel and petrol as fuel in automobiles because petrol and diesel produced a lot of unburnt carbon particles and emitted carbon dioxide, nitrogen oxides and sulphur dioxide. These all gases are poisonous gases and cause various environmental hazards. The main causes of acid rain are nitrogen oxides and sulphur dioxide. But CNG is safe because it produces these substances in very small amounts. Thus, the use of CNG is reducing pollution in our cities.

Scale along X -axis, 1 unit = 1 year and along Y-axis, 1 unit = 1%

The microbial diseases that can spread from an infected person to a healthy person through air, water, food or physical contact are called communicable diseases, e.g. common cold, cholera, TB.

Factors affecting friction:
(i) It depends on the nature of surfaces in contact.
(ii) It is more between rough surfaces and less between smooth surfaces.
(iii) It depends on how hard the two surfaces press together.
(iv) It is independent of the area of contact.

(i) Nitrogen, oxygen.
(ii) Carbon monoxide, smoke
(iii) Methane, green house gases.
(iv) Global warming.
(v) Sewage, Polluted

Cloning is the production of an exact copy of a cell any other living part, or a complete organism. Ian Wilmut and his colleagues at the Roslin Institute, Edinburgh, Scotland successfully cloned a sheep named Dolly.
Following steps were involved in cloning:
(i) A cell was collected from the mammary gland of a female sheep.
(ii)An egg was obtained from a Scottish blackface and its nucleus was removed.
(iii)The nucleus from the mammary gland cellof Dorset sheep was introduced in the egg.
(iv)This egg was then implanted in the uterus.

Deforestation is a major cause which leads to the change in soil properties. Physical properties of the soil get affected by plantation and vegetation. Trees also prevents soil erosion; fewer trees result in more soil erosion. Removal of top fertile layer of soil exposes the lower, hard and rocky layers. This soil has less humus and is less fertile. So, it is not good for plantation. Gradually the fertile land gets converted into deserts, it is called desertification. This way deforestation leads to desertification.

Plastic is non-biodegradable substance, so it cannot be decomposed. It is creating a great problem for environment. While burning it gives lots of poisonous gases and takes a long time to degrade. This pollution is a major threat to our environment.
(i) Polythene bags are also a big threat to our environment.
They are non-biodegradable substances. When people throw peels of vegetables and left over food items in plastic bags they are eaten by animals such as cows. The plastic material chokes the respiratory system of these animals or forms a lining in their stomach and can be the cause of their death.
(ii) The polybags carelessly thrown here and there are responsible for clogging the drains. Carelessly the wrappers of chips, biscuits etc. are thrown here and there on picnic spots and roads which all create pollution in our environment.
(iii) Plastic wast, when dumped in water,r causes water pollution. Aquatic animals consume these plastics and die.
Hence, the plastics are not environment friendly.

The supply of water to crops at different intervals is called irrigation. Various systems of irrigation are:
(i) Traditional methods: The water available in wells, lakes and canals is lifted up by
(a) Moat (Pulley system);
(b) Chain pump
(c) Dhekli and;
(d) Rahat (Lever system)
Pumps are commonly used for lifting water.
(ii) Modern methods:
(a) Sprinkler system: This system is more useful on uneven land. The perpendicular pipes, having rotating nozzles on top, are jointed to the main pipeline at regular intervals. When water flows through main pipes, it gets sprinkled on the crop as if it is raining.
(b) Drip system: In this system water falls drop by drop just at the position of the roots. It is the best technique for watering fruit plants, garden and trees

Copper will not be deposited on the carbon rod because carbon rod is a poor conductor of electricity. For electroplating, the circuit must be completed and that can be completed only by using the conducting material. Instead of carbon rod, it should be any metal, then only it can be electroplated.

Kaleidoscope is a cylinder with mirrors containing loose, coloured objects such as beads or pebbles and bits of glass. As the viewer looks into one end, light entering the other creates a colourful pattern due to reflection. Its applications are given below:
(i) It works on the principle of multiple reflections, where several mirrors are placed at an angle (usually 60°) to one another. Typically, these are three rectangular mirrors set at 60° to each other so that they form an equilateral triangle.
(ii) The 60° angle creates seven duplicate images of the objects, 5 at 60° and 2 at 90°. As the tube is rotated, the tumbling of the coloured objects presents varying colours and patterns.
(iii) It is used in decoration purposes, toys, etc.

(a) Pituitary gland It releases hormones that control the release of sex hormones, i.e. testosterone in males and oestrogen in females.
(b) Testis It releases testosterone which is responsible for the secondary sexual character in boys.
(c) Pancreas It releases insulin which prevents diabetes.
(d) Adrenal gland It releases aldosterone that maintains the correct salt balance in the blood

Difference between a natural and an artificial satellite.

Applications of artificial satellites are
(i) These kinds of satellites are used for weather for casting such as rainfall, snowfall, etc.
(ii) For communication purpose such as long distance transmission of telephone calls, TV programmes, internet the artificial satellites are used.
(iii) The natural sources of earth are usually surveyed by the artificial satellites.

It is difficult to cut cloth using a pair of scissors with blunt blades because blunt blades have more area and due to which applied force produces very less pressure.
As we know that pressure is inversely proportional to area of cross-section, so it is difficult to cut cloth from blunt blades. \((\therefore \rho=\frac{F}{A})\).

As, we all hear sound through our ears. The moment like eardrum when sound enters in outer part of ear then it travels down a canal at the end of which a thin membrane is stretched tightly which is called the eardrum. It performs an important function. The stretched rubber sheets like eardrum sound vibrations which make the them vibrate (see figure). In this way the eardrum sends vibrations to the inner ear and the signal goes to the brain.

(a) In the clouds, accumulation of charges causes, lightning.
(b) As we know that the top end of stretched umbrella is a metal part. Since, the metal is a good conductor of electricity, so if lightning strikes to the ground, then it can take the path towards umbrella. Therefore, to avoid herself from danger, Priya folded the umbrella and went to a closed building.
(c) The immediate response and the scientific method is opted by Priya here.

The size of the cells of an organism is not related to the size of organism. This can be understood with the example of an elephant and a rat.
The cells in elephant are not big in size as compared to the cells of small animal like rat. Hence, cells are not related to the size of an organism 
However, the size of a cell is related to the function that a cell performs. The nerve cells in both rat and elephant are long and branched and perform the function of transferring and receiving messages and also help in coordination of different functions of body parts 

Take about 50 mL of water in a beaker and dissolve 5 g of copper sulphate in it to obtain copper sulphate solution (which is blue in colour). Put a clean iron nail in this solution and keep the beaker undisturbed for sometime.
We will find that the blue colour of copper sulphate solution starts fading gradually and the iron nail gets covered with a red-brown layer of copper metal. It is because iron is more reactive than copper, so it displaces copper metal from its solution of copper sulphate. It is the copper metal set free from its compound which forms a red-brown layer on the surface of iron nail. This shows that iron is more reactive than copper.
Copper sulphate + Iron\(\rightarrow \) Iron sulphate + Copper
(Blue solution)      (Grey) (Greenish solution) (Red-brown)

Three essential requirements to produce fire are as follows:
(i) Fuel
(ii) Air and
(iii) Heat to acquire the ignition temperature. The job of fire extinguisher is to cut off the supply of air or to bring down the temperature of fuel or both. The most common fire extinguisher is water. But water works only when things like wood and paper are on fire. For fires involving electrical equipment and inflammable materials like petrol, carbon dioxide (C0₂) is the best extinguisher. CO₂ being heavier than oxygen, covers the fire like a blanket. Since, the contact between the fuel and oxygen is cut-off, so the fire can be controlled. The added advantage of CO₂ is that in most cases, it does not harm the electrical equipment.

Burning of fuels is a major cause of global warming because they produce a lot of greenhouse gas 'carbon dioxide' on burning.
The earth's atmosphere allows most of the sunlight that falls on it to pass through and heat the surface. But greenhouse gases such as carbon dioxide, methane, water vapours trap the heat radiated from earth. This leads to an increase in earth's temperature. This is known as greenhouse effect.
When concentration of greenhouse gases such as carbon dioxide increases, greenhouse effect also increases. This is known as global warming. It may cause melting of ice caps and glaciers resulting the rise in sea levels.

(a) Fresh milk is boiled before consumption to kill the microorganisms in it. But packed milk is pasteurised and does not contain any microorganisms. Thus, it can be consumed without boiling.
(b) Raw vegetables and fruits get easily infected by microorganisms and get spoiled. They are kept in refrigerator as low temperature inhibits growth of microbes. Jams and pickles contain sugar and salt as preservatives. They do not get infected by microbes as these preservatives do not allow the growth of microbes.
(c) Beans and peas are leguminous plants and have Rhizobium in their root nodules. These bacteria can fix atmospheric nitrogen to enrich the soil with nitrogen and increase its fertility.
(d) Though mosquitoes live on land, their larvae grow in water. If water stagnation is prevented, their larvae cannot survive, thus reducing mosquito population