Water is the most abundant chemical substance in the universe. It is vital for all living things and the main constituent of the earth’s hydrosphere. It is an inorganic, colorless, odorless, and tasteless substance. There are two types of properties of water i.e. physical and chemical properties.
Physical properties of water
The physical properties of water are directly concerned with the appearance and interaction of water. Some physical properties are listed below.
- Polarity
- Hydrogen bonding
- Specific heat capacity
- Heat of vaporization
- Density
- Vapor pressure
- Melting point
- Boiling point
- Freezing point anomaly
- Cohesion
- Adhesion
- Surface tension
- Capillary action
An introduction and a slight note on all these physical properties of water are explained below.
Polarity in water
Water is a neutral molecule. It has an asymmetric internal distribution of charge that leads to positive and negative ends known as the polarity of water. The positive charge is on the hydrogen atoms while the negative charge is on the oxygen atom. These charges are generated due to the differences in the electronegativities of hydrogen and oxygen atoms. So, with being a neutral molecular compound, it also is polar.
Hydrogen bonding
Hydrogen bonding in water molecules is the reason behind most of its physical and chemical properties. It is the electrostatic force of attraction between highly electronegative oxygen (E.N∼3.4) and partially positive hydrogen (E.N∼2.1). Thus, the electronegativity difference is 1.30.
Due to such differences in electronegativities, the oxygen atom pulls the shared pair of electrons towards itself. As result, the oxygen atom acquires a partial negative charge and the hydrogen atom acquires a partial positive charge. This polarity leads to the formation of a strong intermolecular force i.e. hydrogen bonding between water molecules and also gives it the possibility of forming such bonds with other similar molecules.
Water can dissolve many compounds like ethyl alcohol, carboxylic acid, etc by forming hydrogen bonds. This is what makes water a universal solvent.
Specific heat capacity
The amount of heat required to raise the temperature of 1 gram of a substance by 1 degree Celsius is known as its specific heat capacity. Water has a very high heat capacity of 4184 J.kg-1.K-1 at 25 °C. This means that water does not heat up quickly, it keeps resisting this change as long as it can.
Advantages of High specific heat capacity of water
Among all other physical properties of water, its specific heat capacity is very helpful because;
- It makes water a usable cooling reservoir for cars.
- It can act as a cooling agent in nuclear reactors and also in machines and everywhere else.
- In the daytime, sand gets heated more quickly than water because of its low specific heat capacity. The air around the sand becomes hot and rises. The cooled air from seawater moves towards land and forms the sea breeze.
- At night, due to the low heat capacity of sand, it releases heat more quickly than water and gets cooled. The air above the sea now gets hotter than the air above the sand. The sea air now moves towards land and forms the land breeze.
These are the reasons coastal areas have moderate temperatures.
- Along with this property, water is a cheap chemical as well, which makes its applications uncountable.
The heat of vaporization
The amount of heat energy required to change the liquid substance into a gas at a specified pressure is known as the heat of vaporization. Water has a high heat of vaporization that is 40.65 kJ.mol-1.
Density
The ratio of the mass of an object to the volume of that object is known as its density. Water has a maximum density of 1.0 gcm-3 at 4 °C. It means that water gets denser at low temperatures but this rule is only valid till 4 °C. Below this temperature, water starts to expand as an anomalous property. Such a physical property of water is known as the anomalous property of water.
The table below shows what has been explained above.
At temperatures below 4 °C, the density of ice is less than water. This is because ice has an open cage-like crystalline structure. In ice, each water molecule is surrounded by the other four molecules of water in a tetrahedral array. This structure is held together by hydrogen bonding. As this structure creates spaces between molecules due to which it has to expand, the ice gets less dense.
Thus ice floats on the water and makes an insulating layer that prevents the liquid water from freezing completely. In this way, the aquatic organisms can survive below the ice along with many other applications.
Vapor pressure
The pressure exerted by the vapors of water molecules in gaseous form is known as the vapor pressure of water. The vapor pressure of water is increasing with increasing temperature, just like any other typical liquid.
Vapor pressure is actually being opposed by the atmospheric pressure such that the liquid stays put. Now, as this atmospheric pressure gets lower at altitudes, water will boil at less temperature on hills. For example, it takes 70 °C to boil water on the top of Mount Everest.
Melting point
It is the temperature at which water melts from solid ice into liquid water. The melting point of water is 0 °C.
The microscopic study of ice shows that molecules of ice are closely packed together in a regular pattern. When the temperature of ice increases, the motion of molecules also increases. At 0 °C the motion of molecules becomes enough to start moving apart and change into liquid water.
Due to anomalous expansion of water upon freezing, it will also shrink upon melting.
Boiling point
It is the temperature at which water changes from liquid to the gaseous state. The boiling point of water depends upon the atmospheric pressure. At sea level, the boiling point of water is 100 °C, while at the top of Mount Everest where atmospheric pressure is significantly low, it boils at 70 °C.
At 5 °C, molecules of water slide over each other. When the temperature increases molecules move faster. At 100 70 °C, the motion of molecules is maximum and water is converted into vapors.
Freezing point
The temperature at which water changes from a liquid state into a solid is known as the freezing point of water. The freezing point of freshwater is 0 °C whereas the freezing point of seawater is -1.8 °C. This is due to a colligative property of solutions, in this case, salt + water.
When decreasing the temperature of water the motion of molecules also decreases. At 0 °C the motion of molecules of water is too slow that they prefer getting arranged in a regular pattern (a crystal shape). At this point, they can only vibrate at their mean positions just like other typical solids. Hence, all the water molecules change from liquid to solid state.
Cohesion
The force of attraction by which one molecule of water is attracted to another water molecule is called cohesion. It is caused by hydrogen bonding and Van der Waal forces.
It is important for the transport of water in plants against gravity because transpiration pull is actively driven by this cohesive force among water molecules.
Adhesion
The force of attraction by which molecules of water are attracted to other substances (containers) is known as adhesion. It is caused by electrostatic and mechanical forces. This is the property of water that makes it a wetting liquid.
Examples
- The adhesion of water molecules to the window after the rainfall.
- Adhesion of wet sheets to the table also shows attractiveness.
Surface tension
Surface tension is the measure of the force necessary to stretch or break the surface of a liquid. It is due to the intermolecular forces on surface molecules of water and the atmospheric pressure pushing against them. Its unit is N/m. The water has high surface tension than most other liquids.
The property of surface tension is responsible for the following:
- Formation of water drops, spherical shape.
- the stay of insects on the water surface without getting dipped or wet.
Capillary action
The tendency of water to rise in the thin tube is known as its capillary action. The presence of capillary action of water is due to the presence of both adhesion and cohesion forces.
Examples
- When we dip a paper in the water, the water molecules move up by capillary action. This property is used in ascending chromatography.
- In plants, water moves from roots to the leaves by the capillary action.
- Capillary action helps in the drainage of the tears from the eyes, etc.
Chemical properties of water
The chemical properties of water are listed below:
- Self ionization
- Water as an acid and a base
- Electrolysis, etc
Self-ionization of water
It is a process in which two molecules of water react with one another to form a hydroxide ion and the hydronium ion.
The equilibrium constant for the reaction is the following:
Water is present in very large excess and its very little amount is ionized. So that its concentration remains constant.
Kw is an ionic product of water.
The value of Kw is increased with increasing temperature as shown in the table below:
Water as Bronsted-Lowry acid and base
Water act as both a Bronsted-Lowry acid and a base. It depends upon the strength of other species present in the solution, which property of water will be used here. For example, when hydrochloric acid reacts with water it donates a proton to the water. This is because HCl is a strong acid than water. In this reaction, water acts as a Bronsted base.
- Hydronium ions are conjugate acids of water.
- Chloride ions are the conjugate base of hydrochloric acid.
In an aqueous solution of NH3, water acts as a Bronsted acid. This is because it donates protons to the NH3.
- Ammonium ions are conjugate acids of NH3.
- Hydroxyl ions are the conjugate base of H2O.
Electrolysis of water
Electrolysis of water is the decomposition of water into oxygen and hydrogen gas in the presence of electricity.
In the electrolysis process, the two electrodes are placed in the water and connected to the source of electricity. When an electric current is passed through water it decomposes into H+ and OH– ions. These opposite charges are attracted by oppositely charged electrodes.
- H+ ions move towards the cathode where reduction takes place.
- OH– ions move towards the anode where oxidation takes place.
In this way, hydrogen gas is produced at the cathode and oxygen gas at the anode. This process is called the electrolytic breakdown of water which produces valuable fuels like hydrogen and oxygen gases from a cheap reactant like water.
Concepts Berg
What is the unique property a water molecule has?
The water molecule has unique property because it can form four hydrogen bonds per molecule with its neighbors.
What are the 10 properties of water?
- The polarity of water
- Formation of hydrogen bond
- Cohesion and adhesion
- Melting and boiling point
- Capillary action
- Self ionization of water
- Electrolysis of water
- High specific heat capacity
- High surface tension
- Amphoteric properties
What are the properties of pure water?
Pure water has the following properties:
- Colorless
- Odorless
- Tasteless
- Clear
- Exist in all three forms solid, liquid, and gas.
What properties or factors make water a good solvent?
There are the following properties that make water a good solvent:
- Polarity
- Hydrogen bonding
- High surface tension
- Colligative properties, etc
Which property of water is responsible for water pollution?
Water is a universal solvent that’s why it can also dissolve polluting substances.
What is responsible for water’s cohesive properties?
Water has cohesive properties due to hydrogen bonding and Vaan der Waal forces.
Does ice have less or greater density than water?
Ice has less density than water because of the presence of the open cage-like crystalline structure in ice.
References
- The Properties of Water and Their Role in Colloidal and Biological Systems By Carel J. Van Oss (State University of New York, Buffalo, USA)
- Solvent properties of water (khanacademy.org)
