June 29, 2007: Curious about Water? Read on....
IN its very deceptive ORDINARINESS, water is exceptionally extraordinary. It is almost everywhere - in air; clouds; oceans; lakes; rivers; springs or glaciers. In the five-kilometer layer below the sea level on Earth, water is nearly six times as abundant as all other substances put together. Not only has it been the cause of rise and fall of great civilizations, it has also been one of the agents responsible for shaping and reshaping the face of Earth. Falling as rain or flowing in rivers, it levels mighty mountains, creates broad valleys or steep canyons by weathering the hardest rocks. Availability of water determines the economy of an agrarian society like India. As steam or hydroelectric power, it drives the wheels of modern technology, it's an indispensable ingredient in nearly all manufacturing processes, from the baking of breads to cutting steel for automobiles. Last but not the least it is the elixir of life. Life has evolved in water, and is designed around it, many organisms can live without air, but none without water. Obviously water arouses very many questions in a fertile young mind. Here are the answers to a few such questions.
How did water originate on Earth?
- Why does water around toothpaste dropped on a wet floor drain away?
- Why does a drop of oil expand over surface of water while a drop of water over the surface of oil contracts to become round shaped?
- Why water cannot be used for frying like oil?
- Why are organic compounds not soluble in water?
- Why don't oil and water mix together?
- Why is it that we can blow bubbles only with a soap solution?
- Why do impurities settle down on addition of alum to dirty water?
- Why does water get cold on addition of glucose to it?
The origin of water on Earth is linked to the formation of Earth. According to some currently accepted theories Earth began as a waterless mass of rock surrounded by cloud of gas. Radioactive materials in the rock and increasing pressure in the Earth's interior gradually produced enough heat to melt the interior of the Earth. The heavy materials, such as iron ores, then sank. The light silicates (rocks made up of silicon and oxygen) rose to the Earth's surface and formed the earliest crust. Many silicate rocks have water molecules integrated into their atomic arrangement --water can be driven out of such rocks by the action of heat. Thus the heating of the Earth's interior caused release of water contained in such rocks to the surface. Over millions of years, water thus released collected slowly in low places of the crust and formed the oceans. Whatsoever might have been the origin of water, Earth's original supply of water is still in use and very little, if any, has been added during the past billion years or so. The same water has been pumped time and again from the oceans into air, dropped upon the land and transferred back to sea. A single drop of water spends 8 to 10 days passing through air, 2 to 3 weeks in a river, as long as 100 years in a Himalayan glacier, or from 100 to 40,000 years underground.
As a chemical substance water is unique and rather odd. All its oddities can be traced to its molecular structure. It is a rather sturdy molecule. Until some 180 years ago water was believed to be an indivisible element rather than a chemical compound. Today any student of science knows that each of its molecule is made up of two atoms of hydrogen and one atom of oxygen. The bond between the oxygen and the hydrogen atom in water is polar, that is, it has positive and negative charged ends because of an unequal distribution of electrons. The oxygen atom has a denser distribution of electrons around it and hence a net negative charge. The hydrogen atoms in a water molecule, on the other hand, are positively charged. This leads to a lopsided molecule, with electrical charges concentrated on opposite sides. Water molecules, are therefore, attracted to each other as well as to other molecules having a similar charge distribution. And many of the characteristic features of water can be traced to the so-called hydrogen bond between its molecules.
When a substance dissolves in another substance, the resulting distribution of the molecules of the two substances has lesser number of molecules of either substance surrounded by its own kind. This necessitates disrupting prevailing intermolecular forces in each of them. The molecules of most organic compounds (e.g. oily substances) are non polar. As a consequence the intermolecular forces between organic molecules are much weaker than in water. If such a substance is to mix with water -the resulting distribution of molecules must lead to lowering of energy content. If more energy is required to separate water molecules from each other (by breaking hydrogen bonds) than is gained when water molecules get closer to the organic molecules, the two substances will not mix together. It is for this reason that water and oil do not mix and many organic compounds do not dissolve in water. Oil spreads on the surface of water for a similar reason, because in doing so the interaction of water molecules with the oil molecules is minimized. However, not all organic compounds are insoluble in water. Glucose is a common example. It is an organic compound, which dissolves in water. This is because the molecule of glucose has several hydrogen--oxygen bonds, which can indeed interact with the hydrogen-oxygen bonds of water favorably. In the process of mixing glucose molecules with water some energy is consumed. This energy is acquired from the heat energy of the water, hence water becomes cooler when glucose is dissolved in it.
When a foodstuff is fried some water is removed from it since the cooking medium is at a temperature higher than the boiling point of water. Oils have a boiling point much higher than that of water, hence frying is defined as a process of cooking in excess of fat or oil, thus cooking in water cannot be called frying.
Molecules of some of the compounds used to make soap and tooth paste have a long non polar chain like molecules, but on one end of these chains there is a polar bond. Dissolved in water, they form a structure in which all the non polar chains are stacked together in a layer, so that they have a minimal contact with water molecules. When some toothpaste is dropped on a wet floor some of its molecules spread out on the nearby area, and since they are hydrophobic (they repel water) water layer recedes from this area. When we blow bubbles using a solution of soap in water soap molecules rearrange from a micellar (spherical globules) to a lamellar (like a thin film) arrangement such that air can be trapped in between. Molecules of water themselves cannot arrange themselves into such stable arrangements, hence we cannot blow bubbles using pure water because while a soap solution can form a stretchable film plain water cannot.
When alum (aluminum sulfate) is added to dirty water, a gelatinous precipitate of aluminum hydroxide (known as flocs) is formed. When this precipitate settles down it carries with it bacteria, mud and other impurities as they stick to it, thus clearing the turbidity.
Rivers, Ponds, Wells and Oceans
- Why is the seawater salty?
- From where do the rivers like Godavari and Kaveri get water since they are not formed from glaciers like the river Ganga and Yamuna?
- Why does sea water makes soil infertile whereas river water doesn't?
- Why are there no tides in a pond, like in the oceans?
- Why does the water from a well or tubewell feel warm in winter but cold in summer?
- On islands in the sea, from where do the people get drinking water?
- When bubbles are at the bottom of a pond or river they are small but they get bigger as they come to the surface?
- Although water in ponds, etc. remains fresh for years, kept in a vessel why does it often get contaminated after a few days?
- How can we perform a test at home to check if the water from a handpump is suitable for drinking?
- How does the seawater move?
As rivers flow over land they dissolve many minerals on the way to the oceans. Oceans and seas therefore contain the washings of the entire land area of Earth of several million years. Many undersea volcanic activities as well as underground springs also add salt to the sea. Most of these salts remain entrapped in sea water, and thus sea water is salty. Rivers originate either on mountain glaciers or underground springs at high altitudes. River water therefore has very low salt content and does not taste salty.
Most plants cannot withstand high salinity of the soil because the water uptake by a plant through its roots takes place due to osmosis. If the salt content of the water in the soil is higher than in the plant cells water will tend to flow out of plant tissue rather into it-resulting in its drying up. Soils irrigated with sea water are therefore infertile for common plants, however, there are plants like mangroves which can survive in soils having high salinity.
Rivers like Ganga originate from the glaciers on Himalayas hence they are perennial. Peninsular rivers like Godavri, Krishna or Narmada, on the other hand, are largely rain fed. They have their origins in western ghats (or Vindhyachal). Rain water accumulated underground in such mountainous ranges comes out in the form of numerous springs which merge together to form these peninsular rivers.
The periodic rise and fall of the oceans, or other large bodies of water, relative to the surrounding land is commonly known as tides. They are produced by a combination of number of external forces, with the principal force being the gravitational attraction of the Moon. If the Moon attracted every point within the Earth with equal force, there would be no tide. It is the small difference in direction and magnitude of the lunar attractive force, on one point of the Earth's mass relative to another, which give rise to the so-called tidal stresses. Oceans and seas are spread over a much larger area than any pond or lake. The maximum possible difference in gravitational attraction due to Moon within a pond (or lake) is therefore much smaller than in oceans and seas. Thus although a pond or a lake is subject to the same forces the oceans, the observed changes in water level in them is much smaller. For example, Lake Superior, a very large lake between the United States and Canada, has tides that rise and fall about 5 centimeters compared to the rise and fall of a few meters in some oceans.
There are currents in the oceans which make water move in them. Water in an ocean moves because of drifts of water due to temperature differences or the prevailing winds above. Thus ocean currents are fast flowing currents of seawater generated by the wind or by the variations in water density between two areas.
The specific heat of water, that is, the heat energy required to raise the temperature of 1 gram of water by 1 degree Celsius is relatively high. Thus temperature of a large mass of water on the Earth changes rather sluggishly. Water in a well is largely insulated from the temperature variations in the atmosphere above as it has only a very limited contact with it. The temperature of groundwater, therefore, does not vary much through various seasons. During winters it feels warmer because the atmospheric temperature is relatively lower and during summers it feels cold because the atmospheric temperature is relatively higher.
Drinking water on oceanic islands is usually obtained from rain-fed lakes or groundwater which is sweet.. Normally groundwater (which can be pumped up) is fit for human consumption, because as the water percolates into an underground reservoir most of the microbes get filtered off. However, if the soil contains substances from which harmful ions can leach out, the water which passes through it may become contaminated. There have been reports of arsenic leaching into underground water in certain areas in West Bengal. Testing such water often requires chemicals and apparatus not readily available at home.
The quantity of water in a pond is much more than in any domestic vessel. It is ultraviolet radiation from the Sun and the flora and fauna inside a lake or a pond, which keeps it clean. Water from such a pond, if kept in a domestic container, may contain spores of some bacteria, which may grow in it, thus contaminating it. However chlorinated water kept even for several months in a domestic pot does not get infected because the dissolved chlorine prevents any microbial growth.
The external pressure of water on air contained in a bubble deep down in a pond is much more than the atmospheric pressure. Thus as an air bubble rises it is subject to lesser and lesser pressure. It therefore grows bigger. In addition small bubbles coalescing together as they rise also give rise to larger bubbles.
Ice and Snow
- Why do pipes carrying water often burst in cold countries during winter?
- A shower of hail often accompanies the first rain after summer season, why?
- Why are the hailstones hard while the snowballs soft?
- Whereas common salt is often added to the frozen snow in Kashmir during the winters -- to melt it, during summers in Delhi ice-cream hawkers add common salt to ice-jackets surrounding ice-cream container so that ice will not melt. Why this contradiction?
- Why is snow softer than ice?
- Why does ice melt but waxes harden when subjected to pressure?
- Why does water occupy less volume than the ice formed by same amount of water?
- Why does the level of water in a container remain unchanged when an ice cube floating in it melts?
- Why do small pieces of ice stick together to form a block when pressed?
- Can water be converted to ice by application of pressure?
- Why are ice cubes preferred to water at zero degree Celsius to cool a soft drink?
- Why does ice melt on addition of salt?
- Why is there no snowfall in deserts although hailstone showers as precipitation are common?
When water is cooled to 0 degree Celsius it freezes, that is, it crystallizes into ice, it expands. This is so because the structure of ice is such that the water molecules are spaced farther apart than in liquid water. Water molecules therefore occupy less space as liquid water than as ice and the density of liquid water is more than that of ice. Ice therefore floats on water and when floating ice melts it does not result in any increase in volume. It is for this expansion of water that water pipes if kept closed at the ends often burst in winter if the ambient temperature drops below zero.
Hail consists of rounded particles of ice which fall out of cumulonimbus clouds. This class of clouds are characterized by a tall mountain like appearance and they span from about 1000m. to 7000m. in the atmosphere above the sea level. Such clouds are formed by a strong vertical movement of hot humid air, which is possible mainly in deserts or plains near them where there has been a spell of hot summer. No wonder thunderstorms accompanied by hail are common only in such regions. Larger hailstones commonly exhibit an onion like structure of alternating shells of clear and opaque ice. The clear ice layer consists of large crystals of water that has cooled rather slowly. The thinner opaque layer contains air bubbles and is often an accumulation of snow crystals.
Snow is a conglomerate of many small ice crystals. A snowfall can take place only in such places where the ambient temperature falls below zero degree Celsius for a rather long spell of time. As the packing of such crystals can change with the application of pressure snow is softer than ice or hailstones. When a block of ice is subject to external pressure it melts because the crystal structure of water molecules in ice breaks down under the pressure and water molecules cannot be crystallized in an alternate crystal structure unless the pressure is drastically raised above 2000 atm. However, as soon as the pressure is released the water at zero degree Celsius freezes back into ice. Thus when we press together two different pieces of ice, it melts at the boundaries due to increased pressure and then freezes back on release of pressure. The ice thus formed at the periphery is common to both pieces hence the two pieces join together into one piece. Water can be converted into ice only by application of phenomenally very high pressures. At a pressure of about 20,000 atm, liquid water freezes to ice VII at about 1000C.
Ice is preferred over water at zero degrees Celsius to cool a drink because ice ( at 00C) can absorb much more heat (than water at 00C) before it is converted to water at say 10 degrees Celsius. This is so because water at 00C loses some more heat (known as latent heat of fusion ) before it is converted to ice at 00C.
The transition temperature of pure water into ice is zero degree Celsius at normal atmospheric pressure. But this transition point shifts to a lower temperature ( about -150C) if sufficient salt is added to water. A mixture of ice and salt at a temperature above its fusion temperature will therefore be converted to a liquid salt-water solution by absorbing heat from the surroundings. Thus when salt is added to snow on roads covered with snow in Srinagar, the snow melts. Ice cream vendors in Delhi add a lot of salt to ice in the ice cream trolleys (almost in the ratio 1:1). In the process the temperature of the mixture falls down because the process of dissolution of salt into water is endothermic, that is, it entails some absorption of heat from the surroundings. Therefore salt added to water lowers the temperature of the mixture. Some ice does melt but the undissolved salt at subzero temperature helps in keeping the ice-cream frozen for a longer period.
Vapours, Steam and Clouds
- Why does the water in a covered pot boil faster?
- We know that clouds contain +ve and -ve charges but when clouds become rain where do these charges go ?
- During natural evaporation of water, neither boiling point of water is reached nor latent heat is given. So how does water change its state?
- Clouds contain positive charges on the upper side and negative charges on the lower side(towards ground). Why the negative charge is on the lower side?
- When we pour chilled water in a tumbler, how do water droplets form on its outer surface?
- Can we save fuel by boiling water in a pressure cooker?
- Why does a direct contact with hot steam cause more severe burns than contact with boiling water?
- How is it that water can be boiled in a paper cup without burning the cup?
Water can exist in the three states -- solid, liquid and gas, simultaneously. A piece of ice at zero degree Celsius can has around it liquid water and water vapours at the same time, even though ice is the most stable state of water at this temperature. This is so because there are always some molecules in ice or liquid water which have enough kinetic energy to escape from the bulk. Thus water vapours co-exit with liquid water or ice at all temperatures. Their relative quantities depend on how far off the conditions are from the freezing or boiling point of water. Water in a pond is therefore constantly evaporating from it and condensing into it what ever be its temperature. Water on the outside surface of a cold tumbler is largely due to condensation of water in the atmosphere. Liquid water is not converted into water vapours only at 100 degrees Celsius alone. In fact if we decrease the pressure on its surface it can boil at lower temperatures.
When water boils the liquid and the vapour states are nearly equally probable. Hence if its temperature is maintained at the boiling point more and more vapours will be formed. If the vapours are allowed to escape the pressure does not rise but if they are not allowed to escape the pressure may rise and lesser heat is required to keep it boiling. In a pressure cooker water boils at a temperature higher than 1000C. because the pressure due to vapours inside it can rise much above the atmospheric pressure. It saves fuel required for cooking because heat used to vaporize water inside it does not escape with the vapours, as in an open vessel during the time taken for cooking. If we boil water in a pressure cooker, more heat will be required to raise its temperature to the boiling temperature (which will be higher than 1000C) but lesser heat may escape along with the steam as the temperature rises to the boiling point. One cannot therefore give an offhand answer because the relative masses of the container/pressure cooker also need to be considered.
One calorie of heat is enough to raise the temperature of 1 g of water at 990Ct to 1000C, the boiling point of water. But after we supply this much heat 1 g of water will not be converted to 1 g of steam. Some additional heat is required to be put in to convert water at 1000C to steam at 1000C. This additional quantity of heat is known as latent heat of vaporization. Water has a rather high latent heat of vaporization, 540 cal./g. Thus steam at 100 degrees Celsius has more heat in it than liquid water at the same temperature. No wonder steam can cause more severe burns than boiling water. Just like freezing, addition of salts to water affects its boiling also - it raises the boiling point. Hard water therefore boils at a temperature higher than 100 degrees Celsius.
When we boil water in a paper cup almost all the heat supplied from the heater is conducted off to raise the temperature of water or convert it into steam. The temperature of the paper cup remains around 100 degrees, which is not sufficient to burn it.
Cosmic rays ionize the water and air molecules in the upper strata of the clouds by knocking off electrons leading to positively charged molecules. Some of the electrons thus liberated combine with other molecules in the lower strata to give rise to negatively charged molecules. Since the Earth carries a net positive charge, these ions distribute themselves so that negatively charges ions are concentrated in the lower segment of the clouds. When the clouds precipitate as rain the negative and positive ions can interact to neutralize the charges, or the excess negative charge may flow into the Earth.
Floating and Swimming
- Why does a ship travel faster in warm water than in cold water?
- How do huge and heavy ships made of steel float while a steel spoon sinks in water?
- Why does a poorie or pakoda tends to float up to the top when it is being fried in boiling oil?
- A needle can float on the surface of water but not on the surface of a soap solution, why?
- Why is it easier to swim in a sea than in a river?
- Why do tea leaves float on top when soaked in a tea pot but sink down when stirred?
- Is there any place in the sea where the density of water is so much that a drowning ship is suspended freely instead of going to the sea bottom?
- Some people claim that certain sadhus can walk on water, is it plausible?
- Why does ice float in water but sink in alcohol?
A solid body as it sinks displaces some of the water. The water exerts an upward thrust on it. This upward thrust is equal to the weight of water the solid displaces. A ship is a hollow structure of steel since it displaces much more weight of water than its weight it floats while a spoon does not because as it sinks the weight of water displaced by it is less than its weight. The buoyancy due to different types of water depends on its density also(Archimedes principle. Seawater is denser than river water because of the dissolved salts, therefore the buoyancy in seawater is greater and lesser effort is required to swim in it. Similarly the much lower density of alcohol is responsible for insufficient thrust to keep an ice cube floating in it. However a ship travels faster in warm water than in cold water because warm water offers lesser frictional resistance to the motion of ship than cold water because of a looser packing of molecules.
The molecules on the surface of liquid water are asymmetrically attracted towards other molecules in it because there are very few molecules above the surface. This leads the surface to behave almost like a stretched membrane. A force, known as surface tension, characterizes the membrane like feature. The magnitude of surface tension is proportional to the intermolecular attraction between the water molecules on the surface. A needle can be floated on the surface of water because of the surface tension of water. When soap is dissolved in water the surface tension of water is decreased and is not sufficient to support the weight of a normal needle.
Tea leaves float in water because they have air trapped in them, when they are stirred the air escapes or gets dissolved in water. Poories and pakoras float in oil while being fried because the water vapours released (or entrapped between layers) during the process of frying make them less dense. A human being cannot walk on water because the weight of an erect human being is spread over a very small area. Thus the surface tension of water cannot hold it. Thus according to the principles of physics it is impossible that a human being can walk on the surface of water.