What Is Hydrolysis?

Hydrolysis is a type of chemical reaction that occurs between water and another compound. During the reaction, chemical bonds are broken in both molecules, causing them to break apart. The water molecule splits to form positively charged hydrogen (H⁺) and negatively charged hydroxide (OH^-) ions, and the other molecule splits into two simpler sections, also with positive and negative charges. H⁺ and OH^- ions attach to each of these sections. These reactions take place when some ionic compounds, for example, certain acids, bases, and salts, dissolve in water; they are involved in processes that are essential to life; they are used in some important industrial processes, such as the manufacture of soap; and they play an important part in the weathering of rocks.

Ionic compounds can be acids, bases or salts, which are compounds that result from the reaction of an acid and a base. They are made up of positively charged cations and negatively charged anions. When they are dissolved in water, they will split into their cations and anions. The anions of weak acids, and the cations of weak bases, will react with water to some extent, resulting in hydrolysis.

Where a salt is the product of a strong acid and a weak base, the cation of the base will hydrolyze in water. For example, ammonium chloride (NH₄Cl) is the salt of a weak base — ammonia (NH₃) — and a strong acid — hydrochloric (HCl). When dissolved in water, it splits into cations and anions — NH₄⁺ and Cl^-, respectively. The cation, however, will react with water to some extent by losing a hydrogen ion:

Since this reaction produces hydronium ions (H₃O⁺), the resulting solution is acidic. Where a salt is the product of a strong base and a weak acid, the anions of the acid will react with water by accepting hydrogen (H⁺) ions, leaving hydroxide (OH^-) ions, which gives an alkaline solution. The salt of a strong acid and a strong base will not hydrolyze because the anion of the acid and the cation of the base do not react with water.

Many processes that are essential to life involve hydrolysis. An example is the release of energy by the molecule adenosine triphosphate (ATP). Cells use this compound to store energy, which can then be released when it is needed. The molecule has three phosphate (PO₄^-) groups, but it can lose one of these groups by reacting with water. This reaction actually uses up a small amount of energy, but much more is released by the subsequent reactions of the free phosphate group.

Hydrolysis is also plays a vital role in the breakdown of food into easily absorbed nutrients. Most of the organic compounds in food do not react readily with water, and usually a catalyst is required to allow these processes to take place. Organic catalysts that help with reactions in living organisms are known as enzymes. In the body, enzymes such as lipases, carbohydrases and proteases catalyze the reactions with fats, carbohydrates and proteins with water.

One example of hydrolysis is the breakdown of starch, which is catalyzed by the enzyme amylase. Starch is broken down into smaller molecules, which consist of the sugar known as maltose. Maltose may then be further broken down into glucose molecules, under the influence of the enzyme maltase. In each case, water takes part in the process, itself splitting and adding a hydroxyl group and a hydrogen ion to the new molecules formed on each side of the broken bond.

Many industrial procedures require various substances to be hydrolyzed to create useful products. Often, however, the raw materials for these processes do not react easily with water molecules, so the reactions are helped by a variety of means, such as high pressure, high temperatures and catalysts. Laboratory hydrolysis usually requires the use of a catalyst, which is typically a strong acid or alkali.

Hydrolysis has been used for a long time in the production of soap. During this process, known as saponification, fat is hydrolyzed in a reaction with water and the strong alkali, sodium hydroxide. The reaction produces fatty acid salts, commonly known as soap. Saponification sometimes occurs in old oil paintings when fatty acids in oil paint react with the metals in paint pigments. This can cause white deposits and lumps to develop on the surface of paintings, although it is not known why it only occurs on some artworks and not others.

Hydrolysis is an important process in the weathering of rocks. Various silicate minerals, such as feldspar, undergo slow hydrolysis reactions with water, forming clay and silt, along with soluble compounds. This process is important in the formation of soils, and in making essential minerals available to plants.