Why is water so special?
- Water is an essential part of the earth system. Water is special not only because it covers over 70% of the earth’s surface, but also because it is the only known substance that can exist in gaseous, liquid, and solid phases within the relatively narrow range of temperatures and pressures found on earth
- Water’s special qualities come from the unique shape of the water molecule.
- Each molecule contains two atoms of hydrogen and one atom of oxygen, arranged such that one side of the molecule (nearest the hydrogens) is positively charged while the other side (nearest the oxygen) is negatively charged.
- If two water molecules come together, the positive side of one is attracted to the negative side of the other, making the molecules cling together.
- This simple fact accounts for the high heat capacity, surface tension, cohesion, adhesion, and other characteristics that make water so important to the earth’s biosphere.
Effects of cohesion and adhesion
- Surface tension is the result of cohesive forces between adjacent molecules. The molecules in the bulk of a liquid are pulled equally in all directions by the neighbouring molecules. But the surface molecules do not have molecules on all sides. Hence they are pulled inwards causing the liquid to shrink to form a surface with minimum area, a sphere. Hence water droplets are spherical.
- Water molecules bead together on a wax paper because surface tension is greater than the adhesive forces between the paper and water molecules.
- The surface tension of water allows objects heavier than it to float across it. When water molecules do not stick to the object (non-wetting) and the weight of the object is less than the forces due to surface tension.
Concave and Convex Meniscus: The meniscus is concave when adhesive forces are stronger than cohesive force, e.g. water. It is convex when cohesion is stronger, e.g. mercury
The curved surface of a liquid inside a container is the meniscus.
· When the cohesive forces between the liquid molecules are greater than the adhesive forces between the liquid and the walls of the container, the surface of the liquid is convex. For example, Mercury in a container.
· When the cohesive forces between the liquid are less than the adhesive forces between the liquid and the container, the surface curves up. For example, water in a glass container.
· When both adhesive and cohesive forces are equal, the surface is horizontal. For example, distilled water in a silver vessel.
Capillary action is the result of cohesive and adhesive forces. When a liquid flows through a narrow space the cohesive and adhesive forces act together to lift it against the natural force of gravity. Wetting of a paper towel, water flowing in xylem cells up from the roots to the tip of a plant are a few examples of capillary action.
Capillary action in action
- Adhesion is used for the proper functioning of glue, paint, tar, cement, ink etc.
- Adhesive and cohesive forces together induce capillary action which is the principle used in lamp wicks. Synthetic fibres use wicking to remove sweat from the skin.
In general, when considering the states of matter, solids are denser than liquids and liquids are denser than gases. Water is a bit different to this rule.
- When water is in its solid state (ice), the water molecules are packed close together preventing it from changing shape. Ice has a very regular pattern with the molecules rigidly apart from one another connected by the hydrogen bonds that form a crystalline lattice.
- These crystals have a number of open regions and pockets making ice less dense than liquid water.
- This is why ice floats on water. Ice forms when the temperature is below freezing (0°Celsius or 32°Fahrenheit).
- When ice is warmed above freezing, it melts and becomes liquid water.
- As a liquid, the attractive forces between molecules weaken and individual molecules can begin to move around each other.
- Because the molecules can slip and slide around one another, water takes the shape of any container it is in.
- The third state of water is the gaseous state (water vapour). In this state, water molecules move very rapidly and are not bound together.
- Although we cannot see water in its gaseous state, we can feel it in the air on a hot, humid day.
- Commonly, water boils at a temperature of 100°C or 212°F, forming water vapour.
Water cycles endlessly throughout the atmosphere, oceans, land, and life of planet earth, occurs in each of the physical states at one time or another.
Approximate amounts of water available on earth:
- 70% of the earth’s surface is covered with water but of this, approximately 97% is salt water, with the remaining 3% being fresh water. Of this 3%, less than 1% is available for life on earth, whilst the rest is in the form of ice at the poles.
- So, what are we doing with the 1%?