Answer: For safety
Explanation: One of the biggest reason why the water we use are treated is so that it is safe for us to use.
Answer:- B- 
Explanations:- Sharing of electrons takes place between non metals and the bond formed by the sharing of electrons is known as covalent bond. In first choice we only have a zinc metal and so no sharing of electrons would be possible here.
In third choice we have NaCl. Na is a metal and Cl is a non metal. The property of metals is to transfer its valence electrons to the non metal and the bond formed is known as ionic bond. So, third choice is also not correct.
In choice B we have sulfate ion that has sulfur and oxygen atoms and both of these are non metals. So, sharing of electrons is present here between the S and O atoms and covalent bonds are formed.
So, choice B-
is the right answer.
Answer:
is the density of solid calcium.
Explanation:
Formula used :

where,
= density
Z = number of atom in unit cell
M = atomic mass
= Avogadro's number
a = edge length of unit cell
We have:
r = 197 pm = 



M = 40 g/mol
Z = 4
On substituting all the given values , we will get the value of 'a'.


is the density of solid calcium.
First we need to find the number of moles of both K and O reacted
K - 0.779 g / 39 g/mol
= 0.02 mol
the mass of O₂ reacted = 1.417 g - 0.779 g = 0.638 g
O₂ moles = 0.638 g / 32 g/mol
= 0.02 mol
the number of both K and O₂ moles reacted are equal
therefore stoichiometry of K to O₂ reacted are 1:1
then the formula of potassium superoxide is KO₂
It's b. calcium
because the outer electron structure in all of that element is similar, they have the same chemical and physical properties.<span />