Chemistry – Water and Its Properties

Topic : Water and Its Properties

Content:

• Water
• Water Cycle
• Types of Natural Water
• Purification of Water
• Physical Properties of Pure Water
• Chemical Properties of Pure water
• Benefit/Uses of Water

A. Water

Water is the most abundant chemical substance in the world. It covers 70 per cent of the surface of the earth. It occurs in form of water vapour in the atmosphere and this may collect as cloud and later come down to the earth in form of rain. It is also present below the earth.

Water is a transparent and nearly colorless chemical substance that is the main constituent of Earth’s streams, lakes, and oceans, and the fluids of most living organisms. Its chemical formula is H₂O, meaning that its molecule contains one oxygen and two hydrogen atoms, that are connected by covalent bonds. Water strictly refers to the liquid state of that substance, that prevails at standard ambient temperature and pressure; but it often refers also to its solid state (ice) or its gaseous state (steam or water vapor). It also occurs in nature as snow, glaciers, ice packs and icebergs, clouds, fog, dew, aquifers, and atmospheric humidity.

Water is very important to all kinds of plants and animals even to humans. Not only is water used all over the world in vast quantities for drinking purposes, but it is used in  even greater quantities for washing, bleaching, dyeing, cooking, raising steam to drive engines and turbines to generate electricity and as a solvent in industrial processes. Water plays an important role in the world economy. Approximately 70% of the freshwater used by humans goes to agriculture. Fishing in salt and fresh water bodies is a major source of food for many parts of the world. Much of long-distance trade of commodities (such as oil and natural gas) and manufactured products is transported by boats through seas, rivers, lakes, and canals. Large quantities of water, ice, and steam are used for cooling and heating, in industry and homes. Water is a good solvent for a wide variety of chemical substances; as such it is widely used in industrial processes, and in cooking and washing. Water is also central to many sports and other forms of entertainment, such as swimming, pleasure boating, boat racing, surfing, sport fishing, and diving.

B. Water Cycle

The water of the great oceans, seas, rivers and lakes evaporates into the atmosphere as water vapour. This water vapour condenses on reaching the cooler part of the atmosphere to form clouds and eventually falls to the earth as rain. A lot of the rain goes into the rivers, seas, oceans as surface water while the remaining part sinks into the ground. The underground water eventually finds it way into the rivers which inturn flow into the seas and oceans from where it again evaporates into the atmosphere.

Water on Earth moves continually through the water cycle of evaporation and transpiration (evapo-transpiration), condensation, precipitation, and runoff, usually reaching the sea. Evaporation and transpiration contribute to the precipitation over land. Large amounts of water are also chemically combined or adsorbed in hydrated minerals.

The water cycle, also known as the hydrological cycle or the H₂O cycle, describes the continuous movement of water on, above and below the surface of the Earth. The mass of water on Earth remains fairly constant over time but the partitioning of the water into the major reservoirs of ice, fresh water, saline water and atmospheric water is variable depending on a wide range of climatic variables. The water moves from one reservoir to another, such as from river to ocean, or from the ocean to the atmosphere, by the physical processes of evaporation, condensation, precipitation, infiltration, surface runoff, and subsurface flow. In doing so, the water goes through different phases: liquid, solid (ice) and vapor.

The water cycle involves the exchange of energy, which leads to temperature changes. For instance, when water evaporates, it takes up energy from its surroundings and cools the environment. When it condenses, it releases energy and warms the environment. These heat exchanges influence climate.

The evaporative phase of the cycle purifies water which then replenishes the land with freshwater. The flow of liquid water and ice transports minerals across the globe. It is also involved in reshaping the geological features of the Earth, through processes including erosion and sedimentation. The water cycle is also essential for the maintenance of most life and ecosystems on the planet.

Water Cycle Processes

Precipitation

Condensed water vapor that falls to the Earth’s surface . Most precipitation occurs as rain, but also includes snow, hail, fog drip, graupel, and sleet. Approximately 505,000 km³ (121,000 cu mi) of water falls as precipitation each year, 398,000 km³ (95,000 cu mi) of it over the oceans.

Canopy interception

The precipitation that is intercepted by plant foliage, eventually evaporates back to the atmosphere rather than falling to the ground.

Snowmelt

The runoff produced by melting snow.

Runoff

The variety of ways by which water moves across the land. This includes both surface runoff and channel runoff. As it flows, the water may seep into the ground, evaporate into the air, become stored in lakes or reservoirs, or be extracted for agricultural or other human uses.

Infiltration

The flow of water from the ground surface into the ground. Once infiltrated, the water becomes soil moisture or groundwater.

Subsurface flow

The flow of water underground, in the vadose zone and aquifers. Subsurface water may return to the surface (e.g. as a spring or by being pumped) or eventually seep into the oceans. Water returns to the land surface at lower elevation than where it infiltrated, under the force of gravity or gravity induced pressures.

Evaporation

The transformation of water from liquid to gas phases as it moves from the ground or bodies of water into the overlying atmosphere.  The source of energy for evaporation is primarily solar radiation. Evaporation often implicitly includes transpiration from plants, though together they are specifically referred to as evapotranspiration.

Sublimation

The state change directly from solid water (snow or ice) to water vapor.

Deposition

This refers to changing of water vapor directly to ice.

Advection

The movement of water — in solid, liquid, or vapor states — through the atmosphere. Without advection, water that evaporated over the oceans could not precipitate over land.

Condensation

The transformation of water vapor to liquid water droplets in the air, creating clouds and fog.

Transpiration

The release of water vapor from plants and soil into the air. Water vapor is a gas that cannot be seen.

Percolation

Water flows vertically through the soil and rocks under the influence of gravity

Plate tectonics

Water enters the mantle via subduction of oceanic crust. Water returns to the surface via volcanism.

Water cycle thus involves many of the intermediate processes.

C. Types of Natural Water

There are four major types of natural water

1. Rain Water – This is the purest form of natural water because it is formed as a result of the condensation of water vapour in the atmosphere. Rain water contains small impurities like gases such as CO₂ from the air and also dust particles present in the air. Dissolved oxygen and nitrogen will always be present in rain water. After electric storms during which oxygen and nitrogen react to form oxides of nitrogen, rain water may actually be an extremely dilute solution of trioxonitrate(V) acid.
2. River Water – Rivers are used extensively for sewage disposal. River water contains a lot of dissolved air, mineral salts, bacteria and organic remains. It is dirty and needs to be purified especially before drinking.
3. Spring Water – Spring water is purer than River water. It contains a considerable amount of mineral salts but very little suspended impurities such a s dust and bacteria. It is good for drinking.
4. Sea Water – This is the most impure form of natural water because impure water is consistently flowing into it and at the same time, purer water is constantly leaving it through evaporation. The main inorganic metallic salt present in sea water is Sodium chloride, but Magnesium, Calcium and Potassium salts are also present. Of all the solid impurities in sea water, the most important ones are Calcium tetraoxosulphate (vi) and Calcium hydorgen trioxocarbonate (iv).

D. Purification of Water

Purification is the act of removing pollutants from water so that it is fit for drinking. This can be achieved by a number of methods including filtration, sedimentation, chlorination and other chemical treatments.

1.  Filtration – It is one of the methods to purify water but the water might not be safe enough for drinking, as it can still contain germs. Filtration would make the water clearer and can be done in the laboratory in small quantity.
   Experiment:
   a. Using a Filter Paper – Get some water from a nearby river. Put the filter paper like a cone inside the funnel and place the funnel on the conical flask.
   b. Using a Sand Filter – Instead of the filter paper, carefully arrange some large pieces of gravel in the bottom of the funnel. Arrange the layer of smaller pieces of gravel on top of these and then a layer of fine sand.
2. Sedimentation – For drinking purposes the clear water obtained by filtration is not pure enough and particles which are not retained by the filter beds have to be removed. This is done by adding powdered potash alum. This will coagulate all the particles to settle at the bottom of the container.
3. Chlorination – The clear water from the above method may still contain harmful bacteria and other microbes. These germs can be killed by the boiling water if we need small quantity of water. For a large scale water supply, chlorine is added to the water to kill harmful germs.

E. Physical Properties of Pure Water

1. It freezes at 0^oC
2. It is clear and colourless
3. It has no smell and has insipid taste
4. It boils at 100^oC, when the barometer stands at 760mm and pure will boil away completely with no change in temperature
5. It is neutral to litmus paper
6. Its maximum  density is 1gcm^-3 at 4^oC.
7. It turns anhydrous copper(II)tetraoxosulphate(VI) to blue. – This test denotes merely the presence of water

F. Chemical Properties of Pure Water

1. Water is known as a universal solvent because most substances dissolve in it to some extent forming aqueous solution.
2. Oxides of non metals dissolve in water to form acid solutions while oxides of metals which are soluble in water form alkaline solution
3. Water reacts with many metals to liberate hydrogen. Some metals like Potassium, Sodium and Calcium react with cold water while metals like Magnesium, Aluminium, Zinc and Iron react with Steam.
4. Electrical conductivity and electrolysis – Pure water has a low electrical conductivity, which increases with the dissolution of a small amount of ionic material such as common salt. Liquid water can be split into the elements hydrogen and oxygen by passing an electric current through it — a process called electrolysis. The decomposition requires more energy input than the heat released by the inverse process (285.8 kJ/mol, or 15.9 MJ/kg).
5. Polarity and Hydrogen Bonding – Since the water molecule is not linear and the oxygen atom has a higher electronegativity than hydrogen atoms, it is a polar molecule, with an electrical dipole moment: the oxygen atom carries a slight negative charge, whereas the hydrogen atoms are slightly positive. Water is a good polar solvent, that dissoves many salts and hydrophilic organic molecules such as sugars and simple alcohols such as ethanol. Most acids dissolve in water to yield the corresponding anions. Many substances in living organisms, such as proteins, DNA and polysaccharides, are dissolved in water. Water also dissolves many gases, such as oxygen and carbon dioxide — the latter giving the fizz of carbonated beverages, sparkling wines and beers. On the other hand, many organic substances (such as fats and oils and alkanes) are hydrophobic, that is, insoluble in water. Many inorganic substances are insoluble too, including most metal oxides, sulfides, and silicates. Because of its polarity, a molecule of water in the liquid or solid state can form up to four hydrogen bonds with neighboring molecules. These bonds are the cause of water’s high surface tension and capillary forces. The capillary action refers to the tendency of water to move up a narrow tube against the force of gravity. This property is relied upon by all vascular plants, such as trees.

Action of Potassium on Water – Pour cold water into a large trough. Use tongs to take a piece of potassium out of the bottle, cut  a small piece of about 2mm with a knife and drop it into the water and watch the reaction.

Observation : The potassium melts into a silvery ball and darts about in the water. A gas (hydrogen) is given off. The metal becomes smaller as it reacts with water to form potassium hydroxide.

2K_(s) + 2H₂O → 2KOH_(aq) + H_2(g)
                          potassium hydroxide

The solution turns red litmus paper to blue

Action of Sodium on Water – Sodium reacts with water in a similar way as potassium except that it is less reactive and the hydrogen does not usually burn. The solution remaining in the trough is Sodium hydroxide

2Na_(s) + H₂O_(l) → 2NaOH_(aq) + H_2(g)

Action of Calcium on Water – Calcium is a denser and harder metal than either Sodium or Potassium. When a piece of Calcium is dropped into a dish of water, it sinks because it is denser than water. Effervescence occurs and the gas evolved is collected The gas gives a positive test for hydrogen when it is tested with a lighted sprint. The solution remaining turns red litmus paper to blue. The solution is cloudy but on filtering the filtrate becomes clear. When carbon(iv)oxide is bubbled through this clear solution, it turns milky due ti the formation of insoluble calcium trioxocarbonate(iv).

Ca_(s) + 2H2O_(l) → Ca(OH)_{2(aq) {Calcium Hydroxide}} + H_2(g)

Ca(OH)_2(aq) + CO_2(g) → CaCO_3(s) + H2O_(l)

Action of Magnesium on Water – Magnesium reacts with steam to produce hydrogen and forms oxides. This gas can be detected by applying a flame to the tube of gas collected. A characteristic ‘pop’ sound will be heard. The white ash remaining pis magnesium oxide.

Mg_(s) + H2O_(g) → MgO_{(s) {magnesium oxide}} + H2_(g)

Action of Water on Iron – Iron does not attack ordinary water but readily attacks excess steam at red heat What will be remaining in the tube is black residue which is an iron oxide called tri-iron tetraoxide.

3Fe_(s) + 4H₂O_(g) → Fe₃O_{4(s) {tri-iron tetraoxide}} + 4H_(g)

Action of Water on Non metals – Non-metals like chlorine, oxygen and silicon also react with water. Chlorine acts on water to form a mixture of two acids: oxochlorate(i) acid and hydrochloric acid.

Cl_(aq) + H2O_(l) ↔ HOCl_{(aq) {Hypochloric acid}}+ HCl_{(aq) {Hydrochloric acid}}

G. Benefits/Uses of Water

1. Water is used as a universal solvent in most chemical processes
2. Water is an essential component for the effective functioning of our body. Approximately 50 to 70% of our body mass is made up of water, including skin, tissues, cells and the organs.
3. Water staves off dehydration. Dehydration is a condition in which the body doesn’t have enough water to support the vital functions.
4. Water removes toxins and waste products from the body thereby cleansing it.
5. Water is required to process all the nutrients from the food we eat and carry the assimilated nutrients to the cells by circulating through the lymphatic system.
6. Lack of water in the body can cause constipation, asthma, allergy, hypertension, migraine and many other health problems.
7. Water is used in dams to supply electricity (hydroelectric power)
8. Water is used for agricultural processes and farming activities e.g. Irrigation, Watering of Ornamental plants, Gardening and Fishery and Livestock activities.
9. Water is life and it is essential for all domestic activities such as drinking, washing, cooking, bathing, cleaning etc
10. Animals and plants also need water to survive
11. Water is used for transportation of both humans and cargoes in riverine areas
12. Water is used for sporting and recreational activities. e.g Boating, Swimming, Beach {Elegushi, Bar Beach}
13. Water is used in industrial processes e.g Mining, Manufacturing of Products, Petroleum Refineries.