Category: Chemistry Notes PPT

To dissociate a molecule of hydrogen into two constituent atoms, one needs to apply energy that is greater or equal to the dissociation energy. It is a type of energy that holds the bond together, and similar or greater energy needs to be supplied to separate the atoms. For hydrogen, the dissociation energy is 104 kcal/mole.

Reactivity towards hydrogen in its atomic form is highly reactive. It reacts with several elements to form its hybrids. For example, potassium hydride, sodium hydride, etc. It also reacts to reduce metal oxides, which produces metal in this element stage. Metallic surfaces that do not have real behaviour with hydrogen to produce stable hydrides catalyze the recombination reaction of the hydrogen atoms to form molecules of hydrogen. They are thereby heated to illumination by the energy that is released during this reaction.

The reactivity of hydrogen changes with temperature. Hydrogen molecules react with numerous compounds and elements. At room temperature, the rate of reactivity is slow and negligible, but at high temperatures, the rate of reaction is extremely elevated. This is due to the heightened dissociation energy present between the atoms. 

Hydrogen Reaction With Metals

Most of the metals don’t react with molecular hydrogen as both metals and hydrogen form compounds by giving up electrons. Therefore, atomic hydrogen rejects electrons offered by metallic atoms to form a compound. However, a few active metals like calcium, sodium, potassium, and magnesium react with hydrogen atoms by forcing them to accept the electrons and form ionic salt-like compounds known as metal hydrides. An example of reaction of metal with hydrogen is given below.

Gaseous hydrogen is passed over heated sodium to form sodium hydride.

2Na (s) + H2(g) → 2NaH (s). 

The Reaction of Potassium Metal With Water

Potassium metal reacts with water rapidly, very rapidly to give a basic colourless solution of potassium hydroxide or KOH along with hydrogen in a gaseous state. The reaction goes on even after the solution is turned basic. The end product is a basic solution of the dissolved hydroxide in the solution. The reaction of potassium metal with water is exothermic, which means it produces heat during or after the reaction. As potassium metal reacts with water, it gets so heated that it catches fire and burns in a characteristic shade of pale lilac colour. Hydrogen gas is evolved as one of the products during the reaction.  

2K (s) + 2H2O → 2KOH (aq) + H2(g)

This reaction is slower than the reaction of rubidium with water. However, it is faster than the reaction of sodium with water. 

Reaction of Sodium Metal With Water

Sodium metal reacts with water instantly to form a basic colourless solution of sodium hydroxide or NaOH and hydrogen gas or  H2(g). The reaction doesn’t stop even after the solution turns basic. The dissolved hydroxide is responsible for turning the resulting solution basic. The reaction of potassium metal with water is exothermic, which means it produces heat during or after the reaction. As sodium metal reacts with water, it gets so heated that it catches fire and burns in a characteristic shade of orange colour. 

2Na (s) + 2H2O → 2NaOH (aw) + H2 (g)

Reactivity of Hydrogen Halides

Hydrogen halides undergo an electrophilic reaction with alkenes to produce alkyl halides. This reaction is called electrophilic because the reaction is initiated due to the accumulation of an electrophilic proton. Furthermore, the addition of hydrogen halides is very regiospecific. This means the hydrogen halide only particularly reacts with the pi bond, and it can only give one of the two specific products. The result of this phenomenon is summarized by Markovinkov’s rule. All the hydrogen halides, including HBr, HCl, HI, HF, can take part in the reaction. All the halides have different rates of reactions because the H-X bond becomes weak as the size of X increases. The rate of reactivity of the halides in increasing order is: HI > HBr > HCl > HF

Reactivity of Hydrogen Atoms Attached to Carbon

The reactivity of hydrogen atoms attached to atoms of carbon present in alkanes differs according to the number of carbon atoms attached to one carbon atom. 

The rate of reactivity of hydrogen atoms attached to carbon in the increasing order is tertiary carbon > secondary carbon > primary carbon. The differences in the reactivity degree are due to the differences in the C-H bond dissociation. Naturally, the weaker bonds can be more easily broken than the stronger bonds. 

Reaction of Alkali Metals With Hydrogen

Alkali metals are the name given to the elements present in the Group 1 of the modern-day periodic table. Following metals are included under the alkali metal: Sodium, Francium, Lithium, Potassium, Cesium, Rubidium. Hydrogen is often associated with Group 1 metals, but it rarely exhibits any similar behaviour as the rest of the metals. Alkali metals are highly reactive, and they react with dry hydrogen at extremely high temperatures to form their corresponding hydrides. The hydrogen is reduced to give the hydride H- ion. 

What are Resonance Structures?

Resonance structures are the Lewis structure sets that describe the electron’s delocalization in a molecule or a polyatomic ion.

In most of the cases, a single Lewis structure has always failed to explain the bonding in either a polyatomic or molecule ion because of the presence of fractional bonds and partial charges in it. In such cases, the structures of resonance can be used to describe chemical bonding.

Define Resonance Structures?

To define resonance in chemistry, the bonding in specific ions or molecules by merging several contributory forms or structures, jointly known as resonance structures or canonical structures within the valence bonding theory, into a hybrid resonance (or the hybrid structure).

The different structures of resonance of the carbonate ion (CO32-) can be illustrated above. Also, electrons‘ delocalization can be described through the fractional bonds (denoted by dotted lines) and the fractional charges present in a resonance hybrid.

At times, the resonance structures are not equivalent, and it is essential to determine which one (s) describes the best actual bonding. A formal charge is used to predict which resonance structures are most favoured.

Resonance Structures of NO2– Ion

In the case of nitrite ion, the bond lengths present in both nitrogen-oxygen bonds are equal. Whereas the Lewis dot structures of the NO2– ion highlight a difference in the 2 N-O bonds’ bond order. Moreover, the resonance hybrid of this polyatomic ion, which is obtained from its various resonance structures, is used to explain the equal bond lengths, same as illustrated below.

The resonance hybrid of the NO2– ion suggests that every oxygen atom holds a partial charge of magnitude with -½. The bond length of the N-O bonds can be given as 125 pm.

Resonance Structures of NO3– Ion

Nitrogen is considered to be the central atom in the nitrate ion. It is singly bonded to 2 oxygen atoms and doubly bonded to 1 oxygen atom. The oxygen atoms which are singly bonded to the nitrogen hold a charge of -1 (to satisfy the octet configuration). The central nitrogen atom also holds a charge of +1, and the overall charge on the nitrate ion is given as -1. The 3 possible resonance structures of NO3– can be illustrated as follows.

If a resonance hybrid of the polyatomic molecule is drawn from the set of Lewis structures that is provided above, the partial charge present on every oxygen atom will be equal to -(⅔). And, the net charge on the central atom remains as +1. This resonance hybrid can be illustrated as follows.

The resonance hybrid of the ozone molecule holds a +1 charge associated with the oxygen at the centre and also a partial charge of -(½), which is associated with the other oxygen atoms.

Resonance Structures of the Carbonate (CO2−3) Ion

Carbonate ion is considered as a common polyatomic ion that is found in baking powder, baking soda, and limestone. The addition of acid to the carbonate ion causes the carbonic acid formation, which decomposes rapidly into carbon dioxide and water. In baking, the carbon dioxide, which is released, will cause bread to rise and makes its texture lighter.

Our first attempt in drawing carbonate ion’s lewis dot structure results in the structure as represented below. Carbonate ion holds 24 electrons, where 2 of them are responsible for a charge of -2. This is probably from the electrons of sodium, calcium, or whatever salt resulted in a cation that the donated electrons to the carbonate anion. Still, the carbon atom present in the pure structure needs to share the electrons to satisfy the octet rule at which the atom is most likely involved in electron sharing.

Using the formal charges present on the atoms, we could reconfigure our electrons to participate in the double bond with the carbon molecule. At this point, the positive charge present on the carbon atom is gone, and all the valence is filled. Then, the octet rule is satisfied. The formal charge’s sum is equivalent to the charge present on the carbonate ion. This is considered to be a good Lewis dot structure for carbonate.

Resonance Structure of Nitrobenzene

The electron density present in the aromatic ring of nitrobenzene molecule is less than the benzene owing to the presence of an electron-withdrawing group, which contains a double bond adjacent to the nitrobenzene’s phenyl ring as illustrated by the nitrobenzene’s resonance structures.

Therefore, the phenyl ring of the nitrobenzene is less nucleophilic compared to the benzene. From the resonance structures, the para and ortho positions are positive. Hence, for the electrophilic aromatic substitution reaction, the electrophile will not be going to react at these positions, whereas, instead, at the meta position. Therefore, it appears that if a double bond falls in conjugation with the phenyl ring, which is the electrophilic aromatic substitution product will be said as the meta substituted product.

Resonance Structures of Benzene

Benzene is a much essential aromatic hydrocarbon of organic chemistry. It has the chemical formula C6H6. The benzene molecules contain a cyclic structure consisting of alternating single bonds and double bonds between the adjacent carbon atoms. Every carbon atom is also bonded to 1 hydrogen atom. The 2 possible resonance structures of the benzene molecule can be illustrated as follows.

The benzene molecule can be stabilized using resonance, where the pi electrons are delocalized around the ring structure. This particular delocalization causes every carbon-carbon bond to hold a bond order of 1.5 by implying that they are stronger compared to the regular C-C sigma bonds. In the benzene’s resonance hybrid, the delocalization of pi electrons can be described using a circle inside of the hexagonal ring.

In benzene molecule, Kekule’s first sug
gested 2 cyclohexatriene Kekule structures that have been taken together, and they constitute the general structure as the contributing structures. The hexagon replaces 3 double bonds in the hybrid structure on the right side and also represents 6 electrons in a collection of 3 molecular orbitals with a nodal plane in the molecule plane.

Therefore, in this manner, we can also discuss the resonance structures of a few polyatomic ions and molecules.

Ruthenium is described as a chemical element having the symbol Ru and with an atomic number 44. This metal is a rare transition one that belongs to the platinum group of the periodic table. Similar to other metals of the platinum group, Ruthenium is said to be inert to most other chemicals. In 1844, Karl Ernst Claus, who is a Russian-born scientist of Baltic-German ancestry had discovered this element at Kazan State University and was named Ruthenium in honour of Russia. In 2017, the annual production of Ruthenium has risen from about 19 tonnes in 2009 up to 35.5 tonnes.

Properties of Ruthenium

• In the periodic table, Ruthenium is a member of the platinum group.

• Whereas in the environment, it can be found as a free metal, or it can be found occasionally as a chemical combination with osmium, iridium, and platinum ores. At times, it is also associated with the deposits of nickel.

Let us look at some important properties of Ruthenium

Important Properties of Ruthenium

Chemical Properties of Ruthenium

Characteristics of Ruthenium

Ruthenium contains four crystal modifications, and it does not tarnish at ambient conditions; it also oxidizes upon heating to 800 °C (1,070 K). This metal dissolves in fused alkalis to produce ruthenates (RuO₂−4), which is not attacked by the acids (even te aqua regia), but it is attacked by halogens at high temperatures. Indeed, the ruthenium metal is more readily attacked by oxidizing agents. The small amounts of ruthenium increase the palladium and platinum hardness. The titanium’s corrosion resistance is markedly increased due to the addition of a less quantity of Ruthenium.

This metal is plated by thermal decomposition and electroplating. A ruthenium-molybdenum alloy is referred to be superconductive at temperatures below 10.6 K. Ruthenium is one and only 4d transition metal that can assume the group oxidation state of +8, and even it is less stable there compared to the heavier congener osmium. This is said as the first group from the left side of the table, where the second and third-row of the transition metals represent notable differences in their chemical behaviour. Similar to iron but dissimilar to osmium, Ruthenium can produce aqueous cations in its lower oxidation states of +2 and +3.

Occurrence

Being the 74th most abundant element in the crust of Earth, ruthenium metal is relatively rare, which is found in about 100 parts per trillion. Generally, this element can be found in ores with the other platinum group metals present in North and South America and in the Ural Mountains. Less, but commercially important amounts are also present in pentlandite mined from Ontario, Sudbury, Canada, and pyroxenite deposits in South Africa. Ruthenium’s native form is a very rare mineral (where Ir replaces part of Ru in its structure).

Production

Around 30 tonnes of Ruthenium are mined every year, with world reserves at an estimation of  5,000 tonnes. The composition of the mined Platinum Group Metal (PGM) mixtures widely changes, based on the geochemical formation. For suppose, the PGMs which are mined in South Africa contain on average of 11 percent of Ruthenium while the PGMs which are mined in the former USSR contain only 2 percent (as of 1992). Osmium, iridium, and Ruthenium are considered to be the minor platinum group metals.

Applications of Ruthenium

Let us look at some of the applications that are related to Ruthenium.

Nearly 30.9 tonnes of Ruthenium were consumed as of 2016, whereas 7.7 in catalysis, 13.8 of them in electrical applications, and 4.6 in electrochemistry.

Because it hardens the alloys of palladium and platinum, Ruthenium can be used in the electrical contacts, where a thin film is enough to achieve the wanted durability. With the same properties and lower cost compared to the rhodium, electric contacts are a primary use of Ruthenium. The ruthenium plate can be applied to the electrode base metal and electrical contact by sputtering or electroplating.

Ruthenium dioxide with bismuth ruthenates and lead are used in the thick-film chip resistors. These both electronic applications account for 50 percent of the consumption of Ruthenium.

Ruthenium is seldom alloyed with metals outside of the platinum group, where the small quantities improve a few properties. The added corrosion resistance present in the titanium alloys led to the special alloy development with 0.1 percent of Ruthenium. Ruthenium can also be used in a few advanced high-temperature single-crystal superalloys, including the applications such as the turbines in jet engines.

Uses of Ruthenium

• Ruthenium is used in the manufacturing of electronic devices and low-cost solar cells.

• It also acts as a versatile catalyst in the synthesis of Fischer Tropsch and olefin metathesis.

• It can be used as exotic materials.

What is Scarcity?

Scarcity, an economic concept, refers to the basic life fact that there is only a finite amount of human, nonhuman resources where the best technical knowledge can produce only limited optimal amounts of each economic good. If the scarcity conditions didn’t exist and an “infinite amount of each good could be produced, or the human wants fully satisfied, there exist economic goods; it means, relatively scarce goods. Scarcity is the limited availability of any commodity, which can be in demand in the market.

Why is there Scarcity Around Us?

We all look at the ocean and reassure us that water is abundant and will last us till the infinity level. Prior to dwelling on this, let us take a detour. 

Some express that money is the root of all evil. But it is a wonder, if we think about since money was invented by us, to connect humans with their needs. But our evolution as a species has made us consider the same money with a negative connotation. We have also descended down to the level where we are no longer having any regard for the food, shelter, and water provided to us.

If we consider this case, half of the world feasts producing enough wastes that could resultantly feed the hungry half of the world. This is one of the primary reasons for these grave scarcities. Also, it is an extremely easy leap to conclude that greed is the root of all evils. We also like to pretend that we have kept our own personal greed in check, which is not always true. If we look around, food scarcity, water scarcity it’s all around us.

Types of Scarcity

One should understand that anyone cannot wish the concept of scarcity to vanish. It is defined as a fundamental reality of economic life. However, we do not have a history of judicious use. But the need for both the judicious and environmentally healthy use of all the resources is the basic need of the hour. The oceans are draining, leading to water scarcity, and also, the glaciers are melting because of the global warming concept. The resources of the world are not what one would call fair or one that serves equality. Many people in developing nations are excessively facing food scarcity and water scarcity. Not only are humans suffering, but due to the greed of the human race, the animal kingdom is also becoming a victim of this scarcity.

Let us look at the prime types of scarcity.

Water Scarcity

Water scarcity is one of the primary causes of concern due to the reason that people die because of it. Most of us waste water because it is available for us at an abundant level all the time, we fail to imagine those places and countries where getting a glass of clean or pure water to drink is still a challenging task to manage. Not only this, water scarcity affects human beings, but also it affects the animals. Some of the root causes of the water scarcity are listed below:

• Water Pollution

• Overuse of water due to ignorance

• Natural calamities like Drought

• Political conflict between states

• Government access

Food Scarcity

Food shortage is another major issue which is being faced by the world for so long. The other name for food scarcity is defined as famine, which means extreme scarcity and severe food shortage. Famine is also an issue because various environmental issues lead to a shortage of food. A few reasons for the scarcity are listed below:

Scarce Goods

A scarce good has more quantity demanded compared to quantity supplied at a price of $0. Scarcity is the possible existence of conflict over the possession of any finite good. We can say that, for any scarce good, at times, control and ownership excludes the control of someone else. 

Scarcity falls into 3 distinctive categories: supply-induced, demand-induced, and structural. Demand-induced scarcity takes place when the resource demand increases and the supply stays similar. Supply-induced scarcity takes place when a supply is very low to the demand. This mostly happens due to environmental degradation, such as drought and deforestation. At the same time, structural scarcity takes place when the population part doesn’t have equal access to resources because of the location or political conflicts.

Nonscarce Goods

On the other side, there exist nonscarce goods. These goods do not require to be valueless, and a few can even be indispensable for one’s existence. As Frank Fetter, in his Economic Principles, explains: “A few things, even which are indispensable to existence, may yet, due to their abundance, fail to be desired objects and of choice. Such things are referred to as free goods.

Sewage management is basically the process by which the waste is treated for the advantage of man. There are different processes involved in sewage treatment; they are the sewage, treating and screening of the sewage. This also involves disposing of the sewage in such a way that it does not cause any hazard or harm to nature and the health of humans also. In this article, you are going to get a brief knowledge about sewage management and why it is necessary to perform sewage management.

Methods

As you know, waste generation is one of the most inescapable and natural activities of the human lifestyle. For the existence of humans, water is compulsory to perform various activities. These life processes include domestic works, industrial manufacturing, services like railways, restaurants, etc. All these usages generate a large amount of wastewater. Even though the natural water reservoir replenishes periodically, the resource is not unlimited. As the human population is increasing day by day so is the use of the water for various domestic work is increasing, so in order to protect this water for our future generations it is necessary that you reuse the water that you have wasted so that the resources may not replenish away till our future generation comes and we cannot even imagine our life without water and other natural resources as we are completely dependent upon these resources.

Thus, for protecting our environment, we use these sewage management methods by recycling the used wastewater because it is necessary for the present and next generations. If this misuse of water resources continues, adverse situations such as scarcity or drought are inevitable in the upcoming days.

To prevent this type of pollution, you need to learn about the systematic methods of sewage treatment. Here in this article, you are going to get brief information about sewage management that will help you to protect the water and environment from getting polluted.

Definition of Sewage Treatment

Sewage treatment suggests a process that can eliminate impurities from household sewage and industrial wastewater. Majorly, four methods of sewage water treatment are followed – physical, biological, chemical, and sludge water treatment. By following these methods, the wastewater is disinfected from all the sewage materials and converted into treated water that is safe for both human usage and the environment.

Talking about basic sewage treatment methods that is

Physical Method: This is basically the process of removal of small and large floating and suspended solids from sewage through two processes of sedimentation and filtration.

Firstly, the suspended particles or floating particles are removed by filtration. The filtration is then kept in large open tanks where the suspended impurities are allowed to settle down.

Biological Method: In this method, the human waste or other degrading wastes are treated by the bacteria and microbes that convert the sewage waste into byproducts such as sludge.

Chemical Method: In this method, chemicals are used in wastewater treatment. For this, chemicals are used to sterilise the wastewater so that it does not transmit any kind of infection.

Activated Sludge Method: Activated sludge method uses microorganisms to feed on the organic components of the wastewater. This process, in turn, produces a purified effluent. This solution is injected and supplied with a large amount of air to meet the oxygen demand of the microorganisms.

This entire containment removal process occurs before this impure used water reaches the natural water bodies such as lakes, rivers, oceans, and estuaries. As the availability of pure water is scarce, the difference between clean and polluted water is completely based on impurity concentration and intent of usage. 

In other words, water is considered polluted when it is not fit for any specific purpose like drinking, fishing, swimming, etc. Water contaminations primarily happen due to the drainage of impure wastewater into groundwater or surface water. And different sewage management methods can prevent water pollution significantly. And the water that is safe for drinking and other domestic uses as well as for swimming, fishing etc., is considered as non-polluted water or water that is safe for drinking.

However, before a discussion about these methods, you should know about the principal pollutants and sewage types first.

Types of Wastewater

Typically, three types of sewage are there-

Domestic wastewater includes the used water from apartments or households. This type of wastewater is also known as sanitary sewage. It can be the water that runs down from the shower or body waste as well. This wastewater is generated by our homes daily due to daily household activities.

Industrial wastewater carries contaminated water from chemical or manufacturing processes. Various pollutant chemicals can be present in this sewage. As this wastewater mainly comes from industries, it can be toxic and contaminated with heavy metals.

Stormwater implies the runoff sewage that comes from the atmosphere in the form of water particles collected in open channels. This can include rain, drizzle, snow, etc. Above 99.9% of the entire wastewater comes from domestic sewage. Even though the principal contaminants are described as plant nutrients and organic materials, domestic wastewater also contains harmful microbes. This wastewater contains the nutrients that can be collected after sewage treatment.

 Major Pollutants

The quantity of perishable or biodegradable organic materials in wastewater is calculated by BOD or biochemical oxygen demand. BOD is the oxygen amount required by microbes to decompose these organic substances in wastewater. It is one of the most vital criteria for the operation and design of sewage management methods. 

Suspended solids are another crucial element of sewage. The amount of sludge generated in a water treatment plant depends on the entire suspended solids included in the wastewater. Storm and industrial sewage carry a larger volume of suspended solids than domestic wastewater. 

Primarily, domestic wastewater carries chemical elements like phosphorus and nitrogen, which are the fundamental nutrients for plants’ growth. If these elements are excessively present in surrounding water bodies like lakes, it can boost the growth of algae. This may accelerate the natural ageing of these water bodies.

Moreover, domestic wastewater also carries various microbes that come from the intestinal tract of humans. Coliform bacteria are found in high concentrations. In present days, an excessive coliform count has been observed in sewage pollution.

By sewage treatment methods, these pollutants are removed from the sewage water so that it can be reused.

Sewage Treatment Methods

By employing various wastewater treatment methods, contaminants like chemicals and sewage can be eliminated from wastewater and recycled for further usages. Mostly, there are four methods of sewage treatment that are listed below in tabular format with important points-

Disposal Methods of Sewage

Sewage disposal methods are the basic components of a sewage management system. It disposes the used water from households, factories, etc. and helps to keep the environment clean. Following are some well-known sewage disposal methods practised around the globe.

In the municipal wastewater system, there is a treatment plant connected to the source of wastewater. By treating the used water, this plant efficiently removes near about 95% of impurities. Then, by an anaerobic process, the sludge is again treated to ensure the safety of using this water.

Due to the increasing urbanisation, the off-site sewage system has resulted. In urban areas, the houses are built on plots, and each plot is connected to a wastewater line. These wastewater lines collect waste from various households and flow towards a community sewage treatment plant. Here, excess water is directed towards a nearby river or irrigation area once the treatment is done. On-Site Sewage SystemAnother name of the on-site sewage system is a septic system. This disposal system is composed of a septic tank where the sewage can be settled, treated and similarly slurred on the same place of waste generation. In this process, the wastewater is disposed of and treated in natural ways. Usually, an on-site system comprises a septic tank and disposal field that is the soil absorption place. The wastewater slurry is carried out to this leach field, where microorganisms can decompose it over a period. These sewage disposal methods are reliable, hygienic, economical, and efficient.

In a full wastewater disposal system, the sewage water is collected from households and then directed to various sewer pipes. During this process, all solid wastes go through multiple sharp blades or macerators. It helps to decrease the dimension of solid waste before further processing.

Lagoons signify large open ponds that can collect wastewater from households. These water bodies contain a large number of microorganisms that decompose the waste. Sunlight and wind act as catalysts in this process by accelerating the decomposition process. Moreover, algae also assist in the breeding process of bacteria in waste. Due to the presence of algae, these lagoons appear greenish.

Pit latrines are the century-old sewage disposal method found in human civilisation. In present days, these are found in rare places with restricted water supply. This disposal system includes a borehole, trench latrines, and ventilated improved pit. Shallow trench latrines are used by large gatherings for a shorter time and once they are nearly full, closed with soil.

Methods of Sewage Collection

A sewage system is typically designed by various underground channels to carry wastewater discharged by localities. The collection system is configured with pipes, manhole drains, holding basins, catch basins, inlets, and pimp stations that can move sewage from the collection point to discharge. The pipe systems and other appurtenances are primary methods of sewage collection in urban places.

A combined collection system carries both storm and domestic wastewater. This collection system is typically constructed with pipelines or tunnels of wide diameter. However, in rainy seasons, this system sometimes fails as water treatment plants cannot treat a heavy volume of storm sewage.

In relatively newer cities, separate wastewater systems that carry domestic and storm wastewater separately are found. The surface runoff sewage or stormwater is disposed of in open water streams like a river. Smallholding basins or catch basins can be installed for heavy water flow during wet seasons. However, domestic wastewater is directed to a treatment plant. From the in-detailed discussion about methods of sewage treatment, you have understood the treatment process, collection process, and disposal methods. 

You can also check our online classes on the website for other chemistry chapters. For a further note on sewage management methods, you can now download our app too. will provide you with detailed information of all the chapters that will not only increase your knowledge but also will help you to score well in your examination.

History of silk

The history of production of silk fibre is very interesting. According to one legend, the story of silk began in 2640 BC. Si-Ling-Chi, a Chinese Empress, was walking around her garden while sipping a cup of tea when the cocoon of a silkworm fell into her cup. The cocoon soon began to unravel revealing a long silken fibre. When she looked up, she saw a Mulberry tree with several other cocoons hanging from it and a number of silkworms crawling around. This led her to conclude that the cocoon had come from the silkworm caterpillars. 

For 2500 years, the Chinese kept the art of making silk to themselves. They sold silk fabric but refused to reveal the secret of how the fabric was made. In spite of their secrecy, the knowledge of how to make silk reached Korea and India in 200 BC and 140 BC respectively. 

Silk production started in Europe when two monks who were visiting China learned the secret of silk-making. They returned to Constantinople and shared this knowledge with Emperor Justinian. The Emperor then sent the monks back to China to source silkworm eggs and mulberry shoots, which they smuggled out of China in their hollowed-out walking sticks. 

 

The life cycle of silkworm

The life cycle of silkworm ranges from 6 to 8 weeks. This is largely dependent on the ambient temperature. The warmer the weather, the sooner the silkworm will complete its life cycle. Other factors that influence the life cycle of the silkworms include exposure to sunlight and humidity. In optimum conditions the eggs can hatch in a week or ten days. The entire lifespan of a silkworm can be broken down into the following stages-

 

Hatching from Eggs

Female silkworm lays eggs on the leaf of mulberry tree. Eggs hatch about 10 days after they are laid. 

 

Silkworm Larva

As the eggs hatch, they form worm like larvae. This stage of the silkworm’s lifecycle lasts for about 24-33 days. The silkworm larva will moult 4 times before it goes into the pupa stage. Periods between moults are known as silkworm instars. Before the first moult, the silkworm’s head will turn darker than the rest of the body. The silkworm’s growth is fastest during the first instar. During each moult, the larva will shed its skin and grow into a larger, new skin. After the first moult, the silkworm emerges as a white, soft, smooth caterpillar. 

 

Pupa and Cocoon

At an age of between 20 and 33 days, the appearance of the silkworm will change and it will turn yellowish and translucent. This indicates that they are ready to build a net of silk around them which is actually a liquid protein secreted from the head of silk moth or caterpillar. This silk is used as an anchor from which the worm swings back and forth to draw a long continuous fibre and build the cocoon. The fibre can be as long as 1 kilometre. Silkworms can take up to 48 hours to build a complete cocoon. Inside the cocoon, the silkworm moults for the last time and begins the process of becoming a moth. 

Sometimes, two silkworms may build a cocoon together. In this case, the cocoon is noticeably larger. 

 

Moth

The moth develops within the cocoon for about 10-14 days. The moth emerges as a hairy male or female moth. These moths have a wingspan of about 50mm but cannot fly due to centuries of domestication. Moths have an average lifespan of 5-10 days with females living shorter lives than males. The female moth is bigger than the male and begins searching for a mate almost immediately after emerging from the cocoon. For this, it releases pheromones to attract the males. After mating the female moth lays 300-500 eggs on mulberry leaves. Most female moths die soon after laying the eggs. 

 

Processing of silk 

Sericulture refers to the production of silk by raising silkworms. The silkworm must be cared for from the time it hatches from an egg to the completion of the cocoon stage. Silk is harvested by killing the silkworm pupa using heat. The cocoon filament is then wound on a reel, softened and unravelled. The silk produced at this stage is called raw silk. Extracting raw silk from the cocoon of silk moth is known as processing of silk. 

Raw silk contains sericin, a gummy substance that protects the strands. This is retained until the yarn is boiled in soap and water. Later, several silk strands may be twisted together to make stronger yarn. It is then treated with chemical substances to add density, increase weight and improve the quality of its draping. Silk is valued for its softness, lustre, resistance to breakage, strength and heat resistance. It may lose its strength and decompose when extensively exposed to sunlight but is usually resistant to mildew.