Questions

Hydrogen is the common element to all Acids. But every element which contains Hydrogen is not an Acid. Most of the acids also contain Oxygen.

A 40% solution of Formaldehyde known as Formalin is used in the preservation of biological specimens because of its hardening action on proteins. 

The person one who is fond of fighting is known as Bellicose.

The Length of the Day is determined in Astronomical Units.

The length of a day corresponds to the time the Earth takes to turn around on its axis, which is approximately 24 hours. But it's not exactly 24 hours. 

This phenomenon is due to tides raised by the Moon which slow Earth's rotation. Because of the way the second is defined, the mean length of a day is now about 86 400.002 seconds, and is increasing by about 1.7 milliseconds per century.