The time required for molecules in a solution to move from a to b is proportional

Diffusion is rapid for short distances but slow for long distances

Explanation:

Let's call t the time required for molecules in a solution to move from a to b, and d the distance between a and b. For what the sentence says

t ∝ √d where ∝ means proportional.

So, as the distance increases, the time of diffusion increases too, and the process is slow. So, diffusion is rapid for short distances but slow for long distances.

radical

Explanation:

A radical is group of atoms. Some common examples of radicals are the OH- (hydroxide ion), NO₃⁻ (nitrate ion) and more.  

Cu + 2HNO₃ →Cu(NO₃)₂  + H₂

In this single replacement reaction, the nitrate ion (NO₃⁻) which is the radical is replaced from nitric acid to form copper nitrate.