Types of reactions

In chemistry, there are various types of reactions that can occur between different substances. Some of the common types of reactions include:

- Combination reaction: This type of reaction occurs when two or more substances combine to form a single product. For example, the reaction of magnesium with oxygen to form magnesium oxide is a combination reaction.
- Decomposition reaction: This type of reaction occurs when a single substance breaks down into two or more simpler substances. For example, the decomposition of water into hydrogen and oxygen gas is a decomposition reaction.
- Redox reaction: This type of reaction involves the transfer of electrons from one substance to another. The substance that loses electrons is said to be oxidized, while the substance that gains electrons is said to be reduced. For example, the reaction of copper with silver nitrate to form copper nitrate and silver metal is a redox reaction.
- Acid-base reaction: This type of reaction occurs when an acid and a base react to form a salt and water. For example, the reaction of hydrochloric acid with sodium hydroxide to form sodium chloride and water is an acid-base reaction.

Reaction of H2 gas with oxygen gas

The reaction of hydrogen gas with oxygen gas to form water is a chemical reaction that can be represented by the following equation:

2H2(g) + O2(g) → 2H2O(l)

This reaction is an example of a combination reaction, as two substances (hydrogen gas and oxygen gas) combine to form a single product (water). It is also an exothermic reaction, meaning that it releases heat energy as the reaction proceeds.

However, this reaction is also a redox reaction because the hydrogen atoms in the hydrogen gas are oxidized by the oxygen atoms in the oxygen gas to form water molecules. The oxygen atoms gain electrons from the hydrogen atoms, while the hydrogen atoms lose electrons to the oxygen atoms.

Therefore, the correct answer to the question is option 'D', as the reaction of hydrogen gas with oxygen gas to form water is an example of a combination reaction, an exothermic reaction, and a redox reaction.