The most common solvent in everyday life is water
Water [: Its chemical formula is H2O, meaning that its molecule contains one oxygen and two hydrogen atoms, that are connected by covalent bonds. Water strictly refers to the liquid state of that substance, that prevails at standard ambient temperature and pressure; but it often refers also to its solid state (ice) or its gaseous state (steam or water vapor)]
Solvent [: a solvent (from the Latin solvō, "loosen, untie, solve") is a substance that dissolves a solute (a chemically distinct liquid, solid or gas), resulting in a solution]
Water is the liquid solvent most commonly used for cleaning, has a property called surface tension. In the body of the water, each molecule is surrounded and attracted by other water molecules. However, at the surface, other water molecules only on the waterside surround those molecules. A tension is created as the water molecules at the surface are pulled into the body of the water. This tension causes water to bead up on surfaces (glass, fabric), which slows wetting of the surface and inhibits the cleaning process.
You can see surface tension at work by placing a drop of water onto a counter top. The drop will hold its shape and will not spread. In the cleaning process, surface tension must be reduced so water can spread and wet surfaces. Chemicals that are able to do this effectively are called surface-active agents, or surfactants. They are said to make water "wetter."
Reactivity – [: the tendency of a substance to undergo a chemical reaction and to release energy].
Warm or hot water melts fats and oils so that it is easier for the soap or detergent to dissolve the soil and pull it away into the rinse water. Thermal energy (hot water) gets things cleaner along with mechanical energy (abrasion) and chemicals (surfactants and etc.); if you reduce the thermal energy you need to increase the other two to compensate. However, there are some newer surfactants and enzymes that work better in cold water. The temperature at which a surfactant becomes insoluble in water; this becomes important when designing detergents for use in hot water work at 85 O.F (30°C) or below.
How Water Hardness Affects Cleaning Action
pH Value [: In chemistry, pH - potential of hydrogen- is a numeric scale used to specify the acidity or basicity of an aqueous solution]
Solutions with a pH less than 7 are acidic and solutions with a pH greater than 7 are basic. Pure water is neutral, at pH 7, being neither an acid nor a base.
Although soap is a good cleaning agent, its effectiveness is reduced when used in hard water. Hardness in water is caused by the presence of mineral salts - mostly those of calcium (Ca) and magnesium (Mg), but sometimes also iron (Fe) and manganese (Mn). The mineral salts react with soap to form an insoluble precipitate known as soap film or scum.
Soap film does not rinse away easily. It tends to remain behind and produces visible deposits on clothing and makes fabrics feel stiff. It also attaches to the insides of bathtubs, sinks and washing machines.
Some soap is used up by reacting with hard water minerals to form the film. This reduces the amount of soap available for cleaning. Even when clothes are washed in soft water, some hardness minerals are introduced by the soil on clothes. Soap molecules are not very versatile and cannot be adapted to today's variety of fibres, washing temperatures and water conditions.