Which of the following is a chemical reaction?

a. making a hydrogen bond between a water molecule and a sugar molecule.
b. the melting of ice.
c. changing a carbon atom to a nitrogen atom by radioactive decay.
d. the formation of a covalent bond between two amino acids

d. The formation of a covalent bond between two amino acids

Explanation:

In a chemical reaction, a substance transforms into another with different chemical properties and a different formula. Bonds are created and/or destroyed. It must obey the law of conservation of mass and the law of conservation of elements.

Which of the following is a chemical reaction?

a. Making a hydrogen bond between a water molecule and a sugar molecule. NO. This is a physical change in which sugar dissolves in water but both their chemical identities remain the same. The hydrogen bond is a force of  attraction and not a real bond in which electrons are gained, lost or shared.

b. The melting of ice. NO. This is a physical change. Solid water turns into liquid water but the identity of the molecules is the same.

c. Changing a carbon atom to a nitrogen atom by radioactive decay. NO. This is a nuclear reaction in which a new element appears but it is not a chemical reaction because it does not fulfill the law of conservation of elements.

d. The formation of a covalent bond between two amino acids. YES. A new bond is created and a protein dimer is formed.