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.
Correct answer: (a) Calcium
Explanation: Calcium reacts with oxygen to form calcium oxide, with a high melting point, and is soluble in water.
Extra information: Calcium is the fifth most abundant element in the earth's crust, it is never found in a free state in nature since it is a highly reactive metal. Calcium easily reacts with oxygen and water readily available in our environment and forms corresponding compounds. Calcium reacts with oxygen to form calcium oxide, with a high melting point, and is soluble in water. Ionic bonding exists between calcium and oxygen as calcium loses 2 electrons and oxygen gains it, to fulfill their octet.
Calcium (solid) + Water (liquid) → Calcium hydroxide (solution) + Hydrogen (gas)
$Ca\ ( s) \ +\ 2H_{2} O\ ( l) \ \rightarrow \ Ca( OH)_{2} \ ( aq) \ +\ H_{2} \ ( g)$
This is because Calcium is a highly reactive metal and is found at the top of the Reactivity series. 
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