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khadidja
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PostSubject: Water chemistry FAQ   Sat Feb 27, 2010 5:16 am



What is water?


Water is a very important substance, as it makes up the larger part of an organism's body. But what exactly is water? Inside the body of a human being there is a skeleton, which makes your body solid and makes sure you can stand up without falling apart. Water is also a kind of
skeleton. It consists of tiny particles, the atoms, just like every other substance on earth. One of these atoms is called hydrogen and the other is called oxygen. As you probably know the air that we breathe also contains oxygen. One particle of water is called a molecule. When
lots of water molecules melt together we can see the water and drink it or use it, for instance to flush a toilet.

How is a water molecule built up?




A water molecule consists of three atoms;an oxygen atom and two hydrogen atoms, which are bond together like little magnets. The atoms consist of matter that has a nucleus in the centre.
The difference between atoms is expressed by atomic numbers. The atomic number of an atom depends on the number of protons in the nucleus of the atom. Protons are small positively charged particles. Hydrogen hasone proton in the nucleus and oxygen has eight. There are also
uncharged particles in the nucleus, called neutrons. Next to protons and neutrons, atoms also consist of negatively charged electrons, which can be found in the electron cloud around the nucleus. The number of electrons in an atom equals the number of protons in the nucleus. The attraction between the protons and electrons is what keeps an atom together.


How much does a water molecule weigh?

The weight of a molecule is determined by the atomic masses of the atoms that it is built of. The atomic mass of an atom is determined by the addition of the number of protons and neutrons in the nucleus, because the electrons hardly weigh anything. When the atomic masses of the separate atoms are known, one simply has to add them up to find the total atomic mass of a molecule, expressed in grams per mol. A mol is an expression of the molair weight of a molecule, derived from the weight of a hydrogen molecule, which is 1 mol.
Hydrogen has a relative atomic mass of 1 g/ mol and oxygen has a relative atomic mass
of 16 g/ mol. Water consists of one oxygen atom and two hydrogen atoms. This means that the mass of a water molecule is 1g + 1g + 16g = 18 g/mol. When the number of moles of water is known, one can calculate how many grams of weight this is, by using the molar weight of water.



In what states (phases) can water be found?

Water exists in three states: solid, liquid and gaseous. At a normal temperature of about 25oC it is liquid, but below 0oC it will freeze and turn to ice. Water can be found in the gaseous state above 100oC, this is called the boiling point of water, at which water starts to evaporate. The water turns to gas and is then odourless and colourless. How fast water evaporates depends on the temperature; if the temperature is high, water will evaporate sooner.



What happens if water changes phase?





The phase changes of water


The changes from a liquid to a solid or to a gas are called phase changes. When a substance such as water changes phase, its physical appearance changes, but not its chemical properties. This is because the chemical structure remains the same, but the molecules of which it
consists will float a little further apart. In the solid state the water molecules are fairly close together, but in the liquid state they are a bit further apart. The water becomes liquid as a result of parting molecules. When water changes from liquid to gas the molecules will part even further, that is why we cannot detect it.



Why does ice float on water?

When substances freeze, usually the molecules come closer together. Water has an abnormality there: it freezes below 0oC, but when temperatures goes below 4oC, water starts to expand again and as a result the density becomes lower. Density of a substance means the weight in kilograms of a cubic metre of a substance. When two substances are mixed but do not dissolve in one another, the substance with the lowest density floats on the other substance. In this case that substance is ice, due to the decreased density of water.



How come not all substances are water-soluble?

Polarity determines if a substance is water-soluble. A polair substance is a substance that has two kinds of 'poles', as in a magnet. When another substance is also polair the poles of the substances attract each other and as a result the substances mix. A substance then
dissolves in water. Substances that contain no 'poles' are called apolair substances. Oil for instance is an apolair substance, which is why oil does not dissolve in water. In fact it floats on water, just like ice, due to its smaller density.




What is hard water?

When water is referred to as 'hard' this simply means, that it contains more minerals than ordinary water. These are especially the minerals calcium and magnesium. The degree of hardness of the water exceeds, when more calcium and magnesium dissolve. Magnesium and calcium are positively charged ions. Because of their presence, other positively charged substances will dissolve less easy in hard water than in water that does not contain calcium and magnesium. This is the cause of the fact that soap doesn't really dissolve in hard water.



What are physical and chemical properties?

Physical properties of a substance are properties that have everything to do with the substance's appearance. Chemical properties are properties that are often used in chemistry, to address the state of a substance. Physical and chemical properties can tell us something about the behaviour of a substance in certain circumstances.



Which physical and chemical properties does water have?

There are several different physical and chemical properties, which are often used alternately. We can name the following:

- Density: The density of water means the weight of a certain amount of water. It is usually expressed in kilograms per cubic metre. (physical)

- Thermal properties: This refers to what happens to water when it is heated; at which temperature it becomes gaseous and that sort of thing.(physical)

- Conductivity: This means the amount of electricity that water can conduct. It is expressed in a chemical magnitude. (physical)

- Light absorption: This is the amount of light a certain amount of water can absorb over time. (chemical)

- Viscosity: This means the syrupiness of water and it determines the mobility of water. When the temperature rises, the viscosity degrades; this means that water will be more mobile at higher temperatures. (physical)

- The pH: The pH has its own scale, running up from 1 to 14. The pH shows whether a substance is acid (pH 1-6), neutral (pH 7) or basic (pH 8-14). The number of hydrogen atoms in the substance determines the pH. The more hydrogen atoms a substance contains, the lower the pH will be. A substance that contains many hydrogen atoms is acid. We can measure the pH by dipping a special colouring paper in the substance, the colours shows which pH the substance has. (chemical)

- Alkalinity: This is the capacity of water to neutralize an acid or a base, so that the pH of the water will not change. (chemical)
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PostSubject: thanks   Sat Feb 27, 2010 5:20 am

thank you Khdidja



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