Water chemistry FAQ Frequently Asked Questions
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?
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.
The molair weight of separate atoms can be found in the periodic table of Mendelejef.
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 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) |
Advanced Water Chemistry FAQ Frequently Asked Questions
The question library on water related issues
How are water molecules bond together?
Hydrogen atoms are attracted to other atoms such as oxygen atoms, because the electrons are pulled closer to the oxygen atom, due to its greater attraction for electrons. As a result the oxygen atom has a slightly negative partial charge and the hydrogen atoms have a slightly positive partial charge. The slightly positively charged hydrogen atoms are then attracted to the slightly negatively charged oxygen atoms of other water molecules. These forces of attraction are called hydrogen bonds. The forces holding two molecules together in a hydrogen bond are much stronger than those between molecules and hydrocarbons. The attraction between hydrogen and oxygen atoms results in a much higher boiling point of water than anticipated for a different molecule with the same mass.
Hydrogen bonds can also be formed between atoms of hydrogen and sulphur or nitrogen, typically SH- and NH2- groups.
In this picture you can see what hydrogen bonds in water look like:
 Hydrogen bonds How does water evaporate?
To make water evaporate, energy has to be added. The water molecules in the water absorb that energy individually. Due to this absorption of energy the hydrogen bonds connecting water molecules to one another will break. The molecules are now in the gaseous state; this is called water vapour. The phase change from liquid to vapour is called evaporation. During evaporation a molecule of water absorbs latent heat.
What are thermal properties of water?
Thermal properties of a liquid are properties that have everything to do with heat transfer through this liquid. Thermal properties can be divided up into specific heat and latent heat.
The specific heat is the amount of heat per unit mass that is required to raise the temperature of a liquid by one degree Celsius. The relationship between heat and temperature change is usually expressed by the upcoming relationship, where c is the specific heat. Q = c*m*dT Q = heat added c = specific heat m = mass dT = change in temperature The specific heat of a liquid is expressed in kilo Joules per kilogram, per degree Celsius. The specific heat of water is 4.18 kJ/ kg * oC at 0oC. Latent heat means energy that is absorbed by water molecules, in order to evaporate. It is heat that is hidden in the water molecule and that is used, when water is heated. When the water cools down the energy is hidden inside the molecule again. Latent heat is expressed in kilo Joules per kilogram (kJ/kg). The latent heat of water is 2250 kJ/ kg at normal pressure and at a temperature of 100oC. How do we determine the solubility of gasses in water?
Henry's Law determines the solubility of gasses in water. This law links the partial pressure of a gas to its mole-fraction in the liquid phase, and thus declares how much of the gas is dissolved. According to calculations from Henry's Law oxygen is more water-soluble than nitrogen.
Henry's Law describes as follows: P = H * x In which p is the partial pressure of a gas, H is a constant that differs for each gas and x is the mole-fraction of the gas in the liquid phase. Some gasses have a specific way of acting when they are dissolved. Carbon dioxide, sulphuric acid and various volatile agents, such as hydrogen chlorine, dissolve and then combine. Their solubility coefficient is much higher than that of other gasses. How do we determine the solubility of liquids in water?
Water molecules are polar. That is why the solubility of a liquid is determined by its polarity. Polair substances often contain OH-, SH- and NH2- groups. The more of these groups a liquid contains, the more water-soluble it is. This is because the groups in question can form hydrogen bonds with water, which are very strong. Because these bonds are very strong a molecule that contains more OH-, SH- and NH2- groups is more water-soluble.
What is oxidation and reduction?
Water can take part in oxidation and reduction reactions. Water can be a donor of electrons; this is called a reducing agent. The type of reaction where a compound takes up electrons is called an oxidation reaction. The acceptor of electrons is called the oxidant. Usually oxygen originates during such reactions. Water can also act as an acceptor of electrons, an oxidant. The type of reaction where a compound accepts electrons is called a reduction reaction.
An example of a redox reaction of copper (Cu) and zinc (Zn): Cu2+(aq) + Zn(s) -> Zn2+(aq) + Cu(s) When we spread this reaction into two parts we can see the oxidation and reduction of electrons (e-) separately: Zn(s) -> Zn2+ + 2e- (oxidation) Cu2+(aq) + 2e- -> Cu(s) (reduction) In this reaction zinc is a donor of electrons; zinc is the reducing agent. Copper is the oxidizing agent, because copper is the acceptor of electrons. |
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