Types of Chemical Bonding

Molecules are made of atomic building blocks, and these atoms are bonded into compounds by two main methods, covalent and ionic. In covalent bonding, the atoms share an electron, while in ionic bonding, the compounds stay together in solid form due to their atomic constituents having an electrical charge.

An example of a covalent bond compound is sucrose (table sugar). Even when dissolved in water, the sugar molecule maintains its shape. Conversely, when ionically bonded compounds are dissolved in water, they typically (depending upon the strength of the bonds) separate into their constituents. For example, when common table salt (Na+Cl-) is dissolved in relatively pure drinking water, it separates into Na+ and Cl- ions.

Note that, in order to remain in solution, ionic substances must be in balance, meaning they must have the same number of negative charges and positive charges.

Acidity, Alkalinity and pH

Water (i.e., H20) is covalently bonded, but it can also dissociate ionically into H+ or OH-. If there is a higher concentration of H+ ions, the solution is acidic, and if there is a higher concentration of OH- ions, the solution is alkaline. Alternatively, if the solution is balanced (all H+ and OH- ions are in balance), it has a pH of 7.0. Solutions that are acidic have a pH below 7.0, and solutions with a pH above 7.0 are alkaline.

The strength of an acid is its tendency to dissociate (i.e., to divide into an H+ ion with an associated negative ion) in a solution. A strong acid found in coffee is phosphoric acid (which contributes to the perceived high acidity in Kenya coffees), while a weak acid is chlorogenic acid, which tastes more bitter than acidic due to its tendency not to dissociate.

Solubility

How quickly, or whether, a compound will dissolve in water is referred to as solubility. Many substances, like the stainless steel metal in a spoon, will not dissolve easily, but can do so, given enough time under the right conditions. There are few substances known that are completely insoluble.

When discussing coffee, solubility is the ability of substances, including the soluble solids, liquids and gases contained in ground coffee, to dissolve into a medium – in this case, water. Water is a particularly good solvent because it has a high di-electric constant, which is a complicated way of saying it dissolves other electrically charged compounds. For example, salt (Na+ and Cl- ions) dissolves easily in water, but not as easily in other solvents, like gasoline or alcohol. The larger component (in this case, water) is called the solvent, and the remaining components are called solutes. The solubility of a substance can either increase or decrease with higher temperature, which is why particular temperatures are recommended for coffee brewing.

In order to accurately describe a substance’s solubility, one must consider 1) the amount of solvent, 2) the amount of the solute, and 3) the temperature of the solution. Some solutes will not dissolve unless they reach high temperatures, while some will partially dissolve at that same high temperature. Still, others will dissolve completely and immediately at a low temperature. The rate of solubility refers to how quickly the dissolving process occurs. Some actions, such as pulverizing (i.e., grinding), stirring and heating, can speed up the process. The art of brewing coffee is properly managing this solvating process, and the quality of the water is a major factor in this control.

Additional Properties of Water

In the world of solvents, water has unique properties. It’s boiling point and melting point are relatively high. Water’s density increases upon heating, and its volume shrinks upon melting. These distinct properties are a result of the attraction between water molecules, a processed referred to as hydrogen bonding.

Water is composed of two hydrogen atoms covalently joined with one oxygen atom. Besides being joined on an atomic level, water molecules can also be joined on a molecular level. The oxygen atom is highly electronegative, resulting in the two hydrogen atoms clustering to one side of the molecule. The result is a charged molecule, with a negative charge on the oxygen side and a positive charge on the hydrogen side. A polar bond forms between water molecules as the positive hydrogen attracts the negative side of the oxygen. This is called a hydrogen bond.

The hydrogen bonds between water molecules cause the melting and boiling points of water to become relatively high compared to other solvents. The interaction between the water molecules due to hydrogen bonds is similar to how a magnet is attracted to some metals, and is attracted, or repelled, by other magnets. This tendency of attraction or repulsion is referred to as polarity.

Water is a polar solvent, having negative and positive sides. Conversely, a non-polar solvent is one in which the molecule is symmetric in structure, and where there is an equal sharing of electrons between atoms, so no attraction or repulsion occurs.

The Structure of Water

At the molecular level, water molecules form clusters as the result of hydrogen bonding. These mini-structures are always moving and changing in form, and they are especially active when water is heated. They are further influenced by the presence of other charged compounds and the pH of the liquid.

Water can behave as if it is a large aggregate of multiple sets of hydrogen and oxygen atoms (this is due to hydrogen bonds). The shape of these aggregates determines the polarity of their contact with the solutes they touch, the speed at which they are hydrolyzed (i.e., dissolved), and thus, the final flavor of the brew. These aggregates can change according to temperature, pH and the presence of various substances.

Pure water molecules can come together in variety of structures, from 4-molecule structures to complex 280-molecule structures, all with various shapes and combinations. When water is heated, the molecules gain energy and break some of its hydrogen bonds. This enables the water to hydrate other chemicals. The size of the structures decrease as the number of structures increase, which allows more charged sites to be exposed and more material to be hydrolyzed. This is another reason temperature is important in brewing. When other substances other than water are present, these well-ordered structures change considerably and affect the extraction process.