Introduction of water-characteristics of water
1. Water characteristics
The molecular formula of water is H2O, the relative molecular mass is 18.015, it is a colorless, odorless and odorless transparent liquid at room temperature, and pure water is almost non-conductive. Water has the following characteristics:
1) The state of water Water has three states of gas, liquid and solid at normal temperature, the melting point of water is 0 ℃, and the boiling point is 100 ℃;
2) Density of water The density of water is the largest at 3.98 ℃, which is 1g / m3. When the temperature is higher or lower, the density is less than 1g / m3;
3) Specific heat capacity of water Almost all liquid and solid materials, the specific heat capacity of water is the largest, and there is a large steam heat and heat of dissolution.
4) Water dissolving ability Water has a large dielectric constant and extremely strong dissolving ability. It is a very good solvent. The substance dissolved in water can carry out many chemical reactions and can react with many metal oxides, Metal oxides and active metals produce chemical synthesis.
5) Conductivity of water Because water is a very weak electrolyte, it can ionize a small amount of hydrogen and hydroxide ions, so even ideal pure water also has a certain electrical conductivity, which is commonly expressed by electrical conductivity, 25 The specific resistance of pure water at ℃ is 18.3MΩ.
6) Chemical properties of water Water can react with metals and non-metals to release hydrogen, and can also react with many metal and non-metal oxides to form alkalis or acids.
2. Material composition of natural water
Natural water bodies are in constant contact with the atmosphere, soil, rocks, various minerals, animals and plants during natural circulation. Since water is a very strong solvent and is easily mixed with various substances, natural water is a complex containing many soluble and insoluble substances and very complex components.
2.1 Classification of impurities in water
From the needs of water purification and treatment, it is assumed that the materials in the water are spherical and divided into three categories according to their diameters: suspended matter, colloid and dissolved matter:
1) suspended matter
The largest particle is a suspended substance, the particle size is about 10-4mm or more, which is visible to the naked eye. These particles are mainly composed of sediment, clay, protozoa, algae, bacteria and organic matter;
Followed by colloidal material, the particle size is 10-4 ~ 10-6 mm. Colloid is a collection of many ions and molecules. Inorganic mineral colloids in natural water are mainly compounds of iron, aluminum and silicon, and organic colloidal substances are mainly humus;
3) Dissolved substances
The smallest particles are ions and molecules, called dissolved substances, with a particle size of [10-6mm, which are mainly various ions and gases dissolved in water in the form of low molecular dissolved salts.
2.2 Influence of impurities in natural water on water quality
1) suspended matter:
Sediment, clay: make the water turbid and produce sticky mud
Algae and protozoa: make the water color, smelly, turbid and produce slime
Bacteria: causing disease, producing slime and corrosion
Other insoluble materials: deposits
2) colloidal substance
Sol (such as silica gel): causes scaling
High molecular compound (such as humic acid colloid, etc.): make water turbid, produce adsorption and deposition
3) Dissolved substances:
HCO3-, CO32-, OH-: make water have a tendency to form carbonate scales, such as calcium carbonate and magnesium carbonate.
SO42-: The water has a tendency to form sulfate scales, such as calcium sulfate and barium sulfate.
Cl-: Metal corrosion occurs.
F-: Overdose can cause disease.
Ca2 +, Mg2 +: scaling may be formed.
Fe3 +, Mn2 +: generates odor, corrodes metals, and may form hydroxide precipitates.
CO2: lower the pH of water.
O2: Corrosive metal.
3. Commonly used water quality indicators
3.1 mg / L (ppm)
Milligrams per liter is a unit that indicates the content of a substance in water. The unit volume of water is usually expressed in liters, and the amount of substances contained in water is usually expressed in grams. Since the volume of one liter of water is usually 1000 grams, 1 mg / liter of impurities is equivalent to one part per million of impurities in water, so the unit of ppm is equivalent to the impurities in water.
3.2 Salt content, conductivity
The salinity of water (also called salinity) is the amount of salt in the water. Since dissolved salts generally exist in the form of ions in water, they can also be expressed as the sum of the amounts of various cations and anions in water, and the unit is mg / L. Because dissolved salts mostly exist in the form of anions and cations in water. When a pair of energized electrodes are inserted into the water, the charged ions will move directionally under the action of the electric field, making the water conductive. The conductivity is expressed by the conductivity, and its unit is μs / cm. The greater the salt content of water, generally speaking, the greater the electrical conductivity, but because different ionic conductivity and quality are different, there is no strict correspondence between the salt content and electrical conductivity.
3.3 Turbidity of water
The suspended and colloidal particles present in the water make the water turbid. The degree of turbidity is called turbidity. The turbidity of drinking water shall not exceed 5 degrees.
3.4 Water hardness
Calcium and magnesium ions in water are combined with some anions, and scale may form when the water is heated or concentrated. The content of calcium and magnesium in water is the hardness of water. Expressed in mmol / L. It is also usually expressed as the mass concentration of CaCO3 in equimolecular amounts, in mg / L CaCO3.
3.5 Alkalinity of water
The alkalinity of water refers to the amount of substances in water that can accept [H +] neutralization reaction with strong acid. In natural water, alkalinity is mainly composed of HCO3- salts. The unit is mmol / L. It is also usually expressed as the mass concentration of CaCO3 in equimolecular amounts, in mg / L CaCO3.
3.6 pH of water
The pH value of water is a negative logarithmic value indicating the concentration of hydrogen ions in water, expressed as
PH =-Lg [H +]
The concentration of hydrogen ions is a sign of the acidity and alkalinity of water. When the concentration of H + is 10-7mol / L, the water is neutral. The more H +, the stronger the acidity of the water, and the stronger the alkalinity of the water. The corresponding pH indicates that when the pH is 7, the water is neutral. When the pH is 0-7, the water is acidic, and when it is 7-14, the water is alkaline.