• When the force acts in a direction at right angle to the direction of displacement (cos90^° = 0), no work is done (zero work).
• Work done by the force is negative if the angle between force and displacement is obtuse (90^°<θ<180^°)  as cosθ is negative. This signifies, when the force and displacement are in opposite direction, work done is negative. This work is said to be done by the body.

Power is rate of doing work i.e. ,
p = {tex}\frac{w}{t}{/tex}
where w is work done in t seconds.
now, we know work is given by -
w = f.x
where f is the force acting for x distance.
putting this value of work in the equation of power we'll get.
p ={tex} \frac{fx}{t} \: \: \:{/tex}
and
v ={tex} \frac{x}{t}{/tex}
therefore P= f.v

(a) When heated, borax undergoes various transitions. It first loses water molecules and swells. Then, it turns into a transparent liquid, solidifying to form a glass-like material called borax bead.
Na₂B₄O₇.10H₂O → Na₂B₄O₇ + Heat → 2NaBO₂ + B₂O₃
(b) When boric acid is added to water, it accepts electrons from –OH ion. Boric acid is sparingly soluble in cold water however fairly soluble in hot water. boric acid behaves as a weak monobasic acid. It doesn't act as a proton-donor, i.e., protonic acid, however, behaves as a Lewis-acid, i.e., it accepts a pair of electrons.
B(OH)₃+2HOH→ [B(OH)₄]^- + H₃O⁺
(c) Al reacts with dilute NaOH to form sodium tetrahydroxoaluminate(III). Hydrogen gas is liberated in the process.
2Al + 2 NaOH + 6 H₂O →2Na⁺ [Al(OH)₄]^- + 3H₂

(i) Electron deficient compounds. Hydrides of group 13 (i.e. BH3, AlH3 etc.) have lesser electrons to form normal covalent bonds and hence are called electron deficient hydrides. To make up this deficiency, these hydrides generally exist in polymeric forms such as B2H6 , B4H10, (AlH3)n   etc. They act as Lewis acids i.e. electron acceptors.
(ii) Electron-precise compounds. Electron precise compounds have the required number of electrons to write their conventional Lewis structures. All elements of group 14 form such compounds (i.e. CH4 , Si H4)), which are tetrahedral in geometry. They do not act as Lewis acids or Lewis bases.
(iii) Electron rich compounds. Electron rich hydrides have excess electrons which are present as lone pairs. Elements of group 15, 16, 17 form such compounds (NH4, PH3, H2O, HF, HCl etc.). They all behave as Lewis bases i.e. electron donors.

UV rays are very harmful to living things. It can cause diseases like skin cancer and can also alter the climate drastically. The ozone layer protects us from these harmful rays and is essential for life on earth.

The ozone layer is not uniform throughout the earth; it is found in a thick layer at some places and a thin layer at others. If this layer becomes too thin, it cannot stop the UV rays from entering the earth and we say that a hole is formed in the ozone layer. Antarctica has the biggest ozone hole followed by the Arctic region and the Tibetan plateau. In these places, the layer was naturally thin, but over the years due to air pollution, these holes have grown bigger and thinner. They are a serious cause of worry across the world.

Causes of Ozone Layer Depletion

The primary reason behind the reduction of the ozone layer is the use of Hydro- Chloro-fluoro-carbons (HCFCs) and chloro-fluoro-carbons ( CFCs).  They are compounds of chlorine, fluorine, and carbon such as CF3Cl, CHCl2F etc. These are used as refrigerants in refrigerators, air conditioners, and in cooling plants. These molecules can destroy O3 molecules and therefore depletion of the O3 layer.

Nitrogen oxides such as nitrous oxide are also very reactive to O3 and are also responsible for holes in the ozone layer. These molecules are released by burning fossil fuels by cars and especially airplanes which fly near the ozone layer.

Since 1975, the hole has increased in size due to depletion in the ozone layer. Reductions of up to 70% have been found in some areas.

Prevention of Ozone Layer Depletion

All is not lost though. The depletion of the O3 layer has almost stopped today and there are signs that it can grow back. This is because countries around the world have agreed to stop the production and use of CFCs and HCFCs.

They have also introduced the application of bio-control agents for controlling the plant pests. In January 1989, the Montreal protocol was signed to limit the use of CFCs and HCFCs. 197 countries have ratified this protocol which has reduced CFC production by 98% today. It remains the most successful environmental treaty to this date.

Today there are better CFC free refrigerants available that do not pollute the atmosphere. Almost all the air-conditioners and refrigerators you buy today do not contain these harmful pollutants.

Vapour pressure is the pressure exerted by vapours when they are in equilibrium with the liquid phase at a given temperature. It depends on the nature of the liquid and temperature.
Vapour pressure of a liquid increases with increase in temperature due to increase in kinetic energy of molecules.

Real gases deviate from ideal behaviour because their particles (atoms for inert gases or molecules) occupy some finite space and do exert interactive forces among them. Completely ideal behaviour is hypothetical because of the reasons above. At low pressure and high temperature, real gases behave approximately as ideal gases. In ideal behaviour, gas particles don't occupy space and do not have any interaction, as assumed in the kinetic theory of gases. But in reality this is not the case: we get errors by applying the ideal gas law. That's why van der Waals corrected it by introducing suitable constants.

Wurtz reaction: Alkyl halides react with sodium in dry ether to give hydrocarbons containing double the number of carbon atoms present in the halide. This reaction is known as Wurtz reaction.

RX + 2Na + XR’ → R—R’ + 2NaX

A shiver of horror passed over Andrew as he gazed at the still form of the newborn baby. Though it was a perfectly formed boy, its limp warm body was white. The whiteness meant suffocation caused by lack of oxygen. Andrew remembered the treatment given to such a case in the Samaritan. Before the hot and cold water came he had asked for, he laid the child upon a blanket and gave it artificial respiration. Then he dipped the child alternately in hot and cold water. Now, the child was quite slippery. He rubbed it with a rough towel. Then he pressed and released his chest till it heaved up. It was followed by other heaves. Andrew redoubled his efforts. The child started gasping. A bubble of mucus came from one tiny nostril. The pale skin turned pink. The limbs were no longer boneless. His head did not lie back spinelessly. The child gave a cry. It came alive.

The tissues of a plant are organized into three tissue systems: the dermal tissue system, the ground tissue system, and the vascular tissue system.  Plant cells form plant tissue systems that support and protect a plant. There are three types of tissue systems: dermal, vascular, and ground. Vascular tissue is composed of xylem and phloem. These tube-like structures transport water and nutrients throughout the plant.  

There are mainly three types of vascular bundles:

(i) Radial: Those in which the xylem and the phloem lie radically side-by-side (for example, in roots of seed plants). This is the most primitive type.

(ii) Conjoint: Those in which the two types of tissues are separated from one another. Here xylem and phloem together form a bundle. The two sub-types are collateral and bicollateral.

(a) Collateral: The xylem and phloem lie together on the same radius in such a position that xylem lies inwards and the phloem outwards. Here the phloem occurs on one side of the xylem strand. In the dicotyledonous stem, the cambium is found to be present in between xylem and phloem, such bundles are called open (for example, in Helianthus) and when the cambium is absent, it is called closed (for example, in monocotyledonous stems).

(b) Bicollateral: In such bundles, the phloem is found to be present on both sides of xylem. Simultaneously two cambium strips also occur. Various elements are arranged in the following sequence-outer phloem, outer cambium, xylem, inner cambium and inner phloem. Such bundles are commonly found in the members of Cucurbitaceae. Such bundles are always open.

(iii) Concentric: Those in which one type of tissue surrounds or ensheaths the other. The concentric bundles may be of two subtypes, amphivasal bundle as found in Dracaena., Yucca and other monocots and some dicots. If the phloem surrounds the xylem, it is amphicribal as found in many ferns. Such bundles are always closed.

Electronegativity is defined as the tendency of an element to attract the shared pair of electrons towards itself in a covalent bond. There is no specific unit for electronegativity. Electron gain enthalpy is the energy released when one mole of electron are added to gaseous atoms of an element. It can be negative or positive depending upon whether the electron is added or removed. An element has a constant value of the electron gain enthalpy that can be measured experimentally.

In sigma bond linear overlapping occurs while in pi bond parallel overlapping occurs.
Explanation:
In sigma bond the bonding occurs by the linear overlap of the orbitals whereas in the pi bond parallel overlapping occurs.
Electron density is maximum between the nuclei of bonded atoms in sigma bond as compared to the pi bond.