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.

Does specific resistance/resistvity change on stretching a wire?

Specific resistance or resistivity is the resistance of a wire of unit length and unit area of cross-section.

It depends on two factors.
1. Nature of the substance.
2.  Temperature of the substance

It is not effected by
1. Length of the wire.
2. Area of cross-section of the wire.

So., when you stretch a  wire its RESISTIVITY remains constant.

Can a solenoid be used to find the direction of earth's magnetic field ( magnetic North and magnetic south)?

A solenoid is a cylindrical/helical coil whose length is more than its diameter. When we pass current through the solenoid , a magnetic field is set up in the space around the solenoid similar to that of a bar magnet.
There is ,in fact, a strong and uniform field in the core of the solenoid.At the two ends of the solenoid, poles are developed as in a bar magnet.
So the current carrying solenoid if freely can show the direction of the magnetic field of a magnet.The north end of the magnet shows the geographic north and South end shows south .

Why is electromagnet made of soft iron?

Electromagnet is a temporary magnet made of soft iron. Soft iron is magnetised by the magnetic field of a coil or solenoid.
Soft iron has high magnetic permeability due to which it gets magnetised quickly and it loses magnetic properties easily due to the fact that it has low magnetic retentivity .

Temperature doesn't change during phase change( melting, boiling etc..). Why?

The heat added to the solid is absorbed by the particles and it is utilised in overcoming the intermolecular force of attraction between the particles. So heat absorbed in solids is changed to potential energy without any change in kinetic energy of the molecules. As temperature in Kelvin scale is the measure of kinetic energy of particles, there is no change in temperature during melting.