In the solid state, all molecules are arranged with a higher degree of order and, in most cases, also more densely than in a liquid. However, water behaves anomalously. Its density in the solid state is lower than in the liquid state. This is due to the large empty space resulting from the arrangement of water molecules in six-membered rings (Fig. 1.4). That is the reason why ice floats on liquid water. Solids are rigid and have a definite shape and volume. However, even in this state their molecules are not completely motionless. Depending on the temperature, they vibrate to a greater or lesser extent while keeping their average positions.

The interaction of cellulose and water is governed by competing electrical attraction forces that determine the cohesion of water molecules and their adhesion to cellulose. Water consists of very polar molecules and therefore its molecular attraction forces strongly influence its properties as they are experienced on the macroscopic scale. The polarity of water determines its high density in the liquid state, its relatively high boiling point, its high viscosity and its wetting behaviour when it comes into contact with materials of different molecular composition (see Chapter 2). Cellulose, which forms the structure of the papermaking fibre, is also a highly polar compound. Water interacts with cellulose because it is also polar. It therefore wets the cellulose surface and penetrates into its matrix, causing it to swell. In the gas phase, water is also attracted by cellulose, which absorbs and desorbs water vapour from the air in equilibrium with the environment (see Chapters 4 and 10). Conservators work with water and paper mostly in the physical, not the chemical, sense. When they humidify paper, they initiate the migration of water into the cellulosic fibre substrate, a process that is driven forward by the physical attraction between water molecules and cellulose. When they flush out acids and discolouration from paper during a washing treatment, they make use of the fact that water molecules are able to surround mobile polar molecules so that they can be removed. When conservators dry paper, they have to overcome the cohesion between the water molecules and the cellulose.

Almost every chemical principle is related to the concept of energy. By definition energy is the capacity of doing work, where work is the motion against an opposing force. Energy moves atoms, molecules and electrons. Energy could be regarded as the fuel of the universe. It is the driving force for both the formation of chemical bonds and the breaking of chemical bonds. Processes by which chemical bonds are formed or separated are chemical reactions. They cause the transformation of the involved reactants into products. Energy determines how and in what direction chemical reactions proceed. One