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Chemical qualities of water

Hydrogen bonds and polarity

Water is an inorganic substance and it represents the most essential element for life on our planet. The water molecule (H20) is composed of three atoms – two of hydrogen (H) and one of oxygen (O). Because the two electrons involved in the bond between hydrogen and oxygen tend to spend more time in proximity of the oxygen, rather than the hydrogen, the oxygen atom ends up being partially negatively charged, while the hydrogen will be partially positively charged. This consequently causes an important characteristic of the water molecule: its polarity. A “polar” substance is a substance with two poles: + and – (like batteries).

                 
Figure 7: Polarity of water molecule (www.daviddarling.info/encyclopedia)

When another polar substance is mixed with water, the poles of the different molecules are attracted to each other and the substances blend. Thus polarity determines the solubility of a given substance. Water-soluble substances contain polar or ionic bonds. Electric polarity and hydrogen bonds are responsible for the chemical-physical properties of water. They are essential for biological processes which require the dissolving of vast varieties of ions and molecules, large or small.

All those molecules that are capable of forming hydrogen bonds will be soluble and are known as “hydrophilic”. Molecules that do not possess this ability will generally not be soluble. These are referred to as “hydrophobic”.

Water is an excellent solvent for mineral salts, but also for organic substances like saccharose. In nature water can not be found in a chemically pure condition. Even spring water in fact contains other substances dissolved in it: it is a solution. Clean rain water (non polluted), one of the purest types of water, contains small amounts of carbonic acid, produced by reaction with carbon dioxide, and nitrous oxides produced by thunderstorm phenomena. But rain can also react with substances produced through the combustion of carbon and petrol, such as sulphur and nitrogen oxides. This, in turn, is at the origin of the phenomenon of acid rain which causes severe damage to forests and aquatic ecosystems in various regions of the globe, included some areas of Europe.

pH

Water has a slight tendency to ionize, which means it forms positive H+ ions (hydrogen) and negative OH- ions (hydroxide). In pure water the number of these ions is equal and water is said to be neutral. If positive ions prevail the solution becomes acidic and if negative ions out-number the positive ones, it is referred to as an alkaline solution. pH is the term used to express this relationship between the two types of ions. The pH scale measures values usually between 0 and 14. Up to pH 7 a solution is considered acid , between 7 and 14 it is alkaline, and at pH 7 it is neutral. One should bear in mind that the chemical reactions of living systems occur within restricted pH limits which are usually around the neutral level. Water meets this requirement of neutrality through another of its characteristics, alkalinity, which permits it to neutralise acids and alkali and to prevent the water’s pH-level from varying.

Solubility of salts

As mentioned before, polar or ionic substances can be dissolved in water, but the dissolved amount varies for each substance. The maximum amount of a solute that can be dissolved in a given quantity of water (solvent), at a fixed temperature, is called solubility, and the solution obtained is called saturated solution. If we add more solute, it will precipitate and be visible at the bottom of the container in the solid form.


Figure 8:  Solid form of a salt (www.bbc.co.uk)      

Many types of substances are water-soluble and can be found in surface waters, but among them salts are the most important ones. They are classified as soluble, insoluble and slightly soluble (respectively at least 0.1 moles per litre at room temperature, less than 0.001 M at room temperature, or between these extremes). Salts in freshwater are the result of dissolving processes of rocks and soils where waters flow through, and they determine water hardness.

Salts are composed of positive and negative ions, forming strong ionic bonds in their solid form.


Figure 9:  Process of dissolving of a salt (source: wikipedia)

When salts are put into water, the ions are surrounded by water molecules and separated from each other. Hence they can move around the solution, giving it the property to conduce electricity (electrolytic solutions). In this form salts are available to roots, providing the elements essential to plant growth. Plants require three major nutrients for growth: carbon (captured from CO2 of air with photosynthesis process), nitrogen and phosphorus, available to plants as dissolved ions.

NITRATES (NO3-). Nitrogen is present in water in many forms, but nitrate is usually the most important inorganic form, because in this form it is captured by roots and used for growth and reproduction in many algae and aquatic plants. Nitrates are soluble salts. Therefore, human activities can greatly affect their amounts in water bodies. Pollutants such as sewage or manure, wastewater treatment plants, runoff from fertilised lawns, cropland, animal manure storage areas and industrial discharges are all sources of nitrates and responsible for the increase of NO3- concentrations in water bodies. Scientists often call nitrogen a “limiting nutrient” because in low amounts, plants use up all the available nitrogen in the water and cannot grow or reproduce any further. However, an excess of these salts is responsible for eutrophication. Normally, levels of nitrate range from 0 to 10 mg/L. Nitrate usually is found in nature in very small amounts because of the ongoing growth and decay process. When plants and animals die and decompose, ammonium (NH4+), is produced. Bacteria usually turn the ammonia into nitrite (NO2-) and nitrate (NO3-). 

PHOSPHATES (PO43-). Phosphorus is present in waters in various forms, soluble or often trapped in sediments. The latter must be converted into its soluble form (PO43-) to be measured. As for nitrates, scientists call phosphorus a “limiting nutrient” because in low amounts, plants use up all the available phosphate in the water and cannot grow or reproduce anymore. The number of aquatic plants growing in a particular area is dependent on the amount of phosphorous available. Most natural waters have phosphate levels below 0,2 mg/L phosphate, but concentrations over 1 mg/L phosphate are found in some areas. Phosphates can be found in water in three different forms: orthophosphate, metaphosphate (or polyphosphate) and organically bound phosphate. Each compound contains phosphorous in a different chemical formula. Phosphates can be trapped in sediments, as its salts have often a low solubility. Organic phosphates, which are part of living plants and animals, are introduced into the environment naturally and from human activities such as: human and animal waste, fertilizers, industrial waste and human disturbance of the land and its vegetation. Phosphates are produced by natural processes of decomposition of organic matter and are found in sewage. Poly-forms are used for treating boiler waters and in detergents: they are important in nature and they can change into the soluble form.

Eutrophication

Nitrates and phosphates in water can result in the rapid growth of algae and other plants, called eutrophication. A massive growth of aquatic plants can change the water significantly. Although plants and algae add valuable oxygen to the water, overgrowth can potentially lead to reduced light levels in the water body. As plants and algae die and decay, bacteria multiply and use the dissolved oxygen in the water. The amount of available dissolved oxygen in the water may become very low and harm fish and other aquatic animals. The resulting excess plant growth can cause taste and odour problems in lakes used for drinking water or can cause nuisance and problems for users of the water body. Water becomes murky, and the water temperature rises. Therefore, excess nitrates and phosphates can cause hypoxia (low levels of dissolved oxygen) and can become toxic to warm-blooded animals at higher concentrations (10 mg/L or higher) under certain conditions. If the oxygen level drops, many types of fish and insects can no longer survive in the water. 

Dissolved oxygen

Gaseous substances can also dissolve into water. The amount of dissolved gas depends on temperature and pressure. Dissolved oxygen analysis measures the amount of gaseous oxygen (O2) dissolved in an aqueous solution. Only a small amount of atmospheric oxygen is normally dissolved in water. Dissolved oxygen is added to water through aeration (water running or splashing), diffusion, and by photosynthesis of aquatic plants. Oxygen is important to all life. Aquatic life needs oxygen to live and uses oxygen that is dissolved in the water even if is in much smaller quantities than in the air. The maximum amount of dissolved oxygen in water (saturated solution) depends on elevation (atmospheric pressure), water temperature, and salinity. Distilled water at 0°C has a O2 solubility of 14.6 mg/L at sea level. Dissolved oxygen in natural waters may vary from 0.0 mg/L to around 16.0 mg/L. Warm, still waters might have dissolved oxygen levels of about 4 or 5 mg/L. Cold, running waters might have oxygen levels at 13 or 14 mg/L. Higher levels are possible due to photosynthesis (plants, algae), lower levels are possible due to oxygen consumption by respiration of biota (fish, bacteria, etc).


Figure 10: Oxygen dynamics in coastal waters (www.ozcoasts.org.au)

If more oxygen is consumed than is produced, dissolved oxygen levels decline. Dissolved oxygen levels below 3 mg/L are stressful to most aquatic organisms. Some sensitive organisms will not live in oxygen levels less than 7.5 mg/L. Dissolved oxygen levels that drop to low levels (i.e. below 5 mg/L) are a reason for concern. The amount of dissolved oxygen in a water body also affects whether a water body can provide optimal habitats for fish and other aquatic life. If compared to the maximum allowed by the temperature of the water (saturation percentage), the measurements indicate a water body’s capacity of self-purification and/or a state of eutrophication.