Anhydrous vs Monohydrate
There are substances in solid, liquid and gaseous phases. They differ due to their constituents. The same chemical has different properties and characteristics according to the state they are in. Since water is found everywhere, the probability of containing water in chemicals is very high. Water vapors are in the atmosphere. Though we keep chemicals in a place where there is no water, atmospheric water can be absorbed by some chemicals. Therefore, if we want to have a chemical without water, it is necessary to store it in a water free, dry place. Sometimes we can use another substance like silica gel to absorb the atmospheric water in a container, so that chemicals encounter minimum amount of atmospheric water.
The way substances absorb water differs from chemical to chemical. Some chemicals are completely non polar. These do not like to interact with water; therefore, they repel water molecules. For example, ether, benzene, acetone are substances without any water. Anhydrous is the term used in chemistry to describe such compounds. Some chemicals absorb and contain water. The molecules with water are termed as hydrated molecules. The chemicals that have the capability to absorb moisture from the air are said to be hygroscopic. The rate of water absorption can differ from chemical to chemical. A piece of sodium in the air can absorb moisture very fast whereas sugar absorbs water slowly. Not only the water absorption rate, but the amount of water a substance can absorb differ from one substance to another. For example, some substances like sodium absorb water till it dissolves. Some substances contain only one water molecule per one of that substance molecule. Likewise, some have 2, 3, 4, 5, 10, water molecules etc. As mentioned before, amount of water of a substance can change its properties. For example, we know that a salt (sodium chloride) crystal dissolves when it absorbs water. But there are some compounds in the solid state. They may incorporate one or few water molecules. But the forms with water and forms without water may have differences in color, texture, reactivity, etc.
A chemical is said to be anhydrous, when it doesn’t contain any water. For some reactions, it is specially specified to carry out the reaction under anhydrous conditions. At that condition, we should take chemicals without water and carry out the reaction also in water free containers. Grignard reaction is one such reaction where the reaction should be carried out in an anhydrous condition. Copper sulfate can be found in anhydrous form where it is white in color (otherwise it exists in pentahydrate form and has a blue color). We can obtain anhydrous solutions by boiling. Boiling evaporates water and gives the anhydrous liquid. Or else, we can use a substance which absorbs all the water and makes the substance dry. Or else we can use molecular sieves or add alkali bases like potassium hydroxide.
Monohydrate contains one water molecule per formula unit. Usually, the number of water molecules a substance molecule has is written as “chemical formula. n H2O”. n gives the number of water molecules and, if the compound is monohydrated, n is one.
What is the difference between Anhydrous and Monohydrate?
• Anhydrous means without water and monohydrate means containing one water molecule.
• Anhydrous form and the monohydrate form of chemicals may differ from their reactivity, color and phase.