Why is hollow box provided in a guitar?

The vibrating srings force air molecules in the box to vibrate with greater amplitude to produce loud sound.

Anamolous expansion of water

It's a general property of any substance that it expands on heating and contract on cooling but certain substances behave just different way at certain range of temperature. Water  is an example.
Water expands on cooling from 4°C to 0°C or you can say water contracts on heating from 0°C to 4°C.
This unusual behaviour of water is called Anomalous expansion  of water

Why does bar magnet align itself along North and South direction?

Due to the continuous flow of molten magma and lava inside the earth, there is the magnetic field of the earth as if there is a huge bar magnet inside the earth along its axis little different from geographic axis.

    The earth's magnet has it north pole towards the geographic south and vice versa.The magnet has the bar magnet suspended is affected by this field and hence shows the N-S direction.

GREEN HOUSE EFFECT

Infrared radiations from the Sun have short wavelength (high frequency) s they are emitted from the surface of the sun at high temperature.These radiations have more energy due to high frequency and hence they can penetrate through a glass or plastic house. But the radiations reflected or emitted from the plants in the house are of longer wavelength so they cannot penetrate through the house. So heat is retained inside the house and temperature increases.
This rise in temperate is called green house effect

What is relative density?

We compare the density of different substances with the density of water at 4°C.

Hence "the ratio of the density of a substance to the density of water at 4°C is called Relative density of the substance".

4°C should be mentioned because water has maximum density of 1g/cm^3 at 4°C only.

There is no unit of relative density as it is  the ratio of two similar quantities.

The value of density of any substances in g/cm^3 is equal to its relative density. For example if the density of a substance in g/cm^3 is 7.7gcm-3, then the relative density is 7.7.
It means that the substance is 7.7 times heavier than water.

LAW OF FLOATATION

"For a floating body, the upthrust experienced by it is equal to its weight."
Net force  on the floating  body is zero

What is Regelation?

The melting point of ice is 0°C at normal pressure but when you increase the pressure on ice it melts at the temperature below 0°C( i.e. melting point of ice decreases with pressure).
When the pressure applied is released then water formed from ice starts freezing. This phenomenon is called REGELATION.

Water seeks it's own level.( Hydrostatic paradox)

Irrespective of shape of vessels or containers, a connected liquid maintains same level in all containers.
This happens to maintain same pressure at a given depth. As the pressure depends on depth of a point from the liquid surface.

MIRAGE. HOW IS IT FORMED ?

MIRAGE is formed by the phenomenon of total INTERNAL reflection of light. Mirage is formed in extremely high or low temperature conditions.
For example: When a sand surface (desert) or road surface is heated, the air above this surface is also heated and rises up.
As a consequence, the density of air in the lower level is less than that of upper level.
  Now , the light from any object starting from the upper layers (denser medium) travels to lower layers ( rarer medium), total internal reflection takes place i.e, the light is reflected back towards the denser medium. When these light rays reflected back, it seems that the light rays are refected from some bright reflecting surfaces on the ground.  Hence the  image is seen below the object.

What is SMOG?

Literally,  smoke + fog= smog.
Smoke is the collection of unburnt fuel particles suspended in the air.
Fog is the suspended water particles in the air.
The combination of these suspended particles gives the smog.

A flute has a number of holes. Why?

The sound produced in a flute is due to the vibration of air column in the flute. The frequency of sound produced depends on length. The frequency is inversely proportional to the length of the air column. By closing or opening the holes , the length of the air column is changed and hence the frequency can be changed.

Plant leaves appear green. Why?

Sunlight contains seven colours, viz violet, indigo ,blue, green, yellow, orange and red. The leaves appear green because they absorb all the colours and reflect green as green colour is least used in photosynthesis. This green colour reaches our eyes. Hence leaves appear green.

Can unstable hydrogen isotope release alpha particles ?

Unstable hydrogen isotope contains
1 proton
2 neutrons

Now, alpha particles are the particles with
2 protons
2 neutrons

What do you think?
As there are insufficient particles in the hydrogen isotope, no alpha particles can be released.