Question

An element reacts with oxygen to give a compound with a high melting point. This compound is also soluble in water. The element is likely to be:
A.Calcium
B.Carbon
C.Silicon
D.Iron

Answer 1

The answer of this problem is a metal and the compound formed when this metal reacts with oxygen has a high melting point because it is ionic in nature. So, the element belongs to either group I or II which form ionic compounds. The element combines with water to form its hydroxide while the other three elements do not dissolve in water.

Complete step-by-step answer:

To solve such problems, we need to consider every option. So, let us react to each element given in the options with oxygen and write the chemical equation of each.

$$2Ca + {O_2} \to 2CaO \\\ C + {O_2} \to C{O_2} \\\ Si + {O_2} \to Si{O_2} \\\ 2Fe + 2{O_2} \to F{e_2}{O_4} \\\$$

But we can clearly see from these equations that each element is forming oxide on reacting with oxygen, so we can’t predict the further two things such as melting point and solubility with the help of these equations. Therefore, the next thing we need to look at is their melting points because then only we will be able to compare which one has the highest.

$$CaO:{2614^o}C \\\ C{O_2}: - {56^o}C \\\ Si{O_2}:{1650^o}C \\\ F{e_2}{O_4}:{1597^o}C \\\$$

Now, we can clearly see that calcium oxide has the highest melting point out of the four oxides formed above. Also, the oxides of alkaline earth metals have highest melting points as compared to nonmetals or transition metals. Thus, we can say that the element is calcium.
We are also given that the compound formed is soluble in water i.e. calcium oxide is soluble in water and produces its hydroxide, so we can write the chemical reaction as:
$CaO + {H_2}O \to Ca{(OH)_2}$
This can be clearly understood as ionic compounds have high melting and boiling points in general such as calcium oxide. Also, we know that highly reactive metals cannot be obtained from their oxides by heating them with carbon and iron vessels get a brown coat when left exposed to air in the rainy season due to rusting of iron.

Hence, the correct option is (A).

Note: These questions require prior knowledge of physical and chemical properties of metals, nonmetals and metalloids. Metalloids such as silicon from silicon oxide which dissolves slightly in water and is weakly acidic in nature. Whereas metal oxides are basic in nature because they form salt and water upon reaction with acids. The case is just opposite with oxides of non-metal which are acidic.