Why is it important to keep th naoh solution covered at all times?

 It is important to keep the NaOH solution covered at all time because sodium hydroxide is a very good remover of Carbon dioxide from the air means sodium hydroxide absorbs the carbon dioxide from the air react with that so the concentration of your solution will also change if you uncover the NaOH.
The following reaction occurs when sodium hydroxide reacts with carbon dioxide;
2 NaOH(aq) + CO2(g) --> Na3CO3(aq) + H2O(l) 

They can measure  the amount of energy gained or lost

Answer : option a) 84explanation :

considering the question as per the attachment

here we can see that the radon nuclide has 222 as the atomic mass number and 86 as the atomic number.

when it undergoes nuclear fission it gives polonium which has atomic mass number as 218 and atomic number will be 84 along with a helium nucleus which has 4 atomic mass number and 2 as its atomic number.

222 = 218 + 4 (atomic mass number)

86 = ? + 2; therefore ? = 86-2 = 84.

the atomic mass number of polonium will be 84.