Category: Chemistry Notes PPT

Before discussing what is variable valency, the concept of valency has to be understood. So, what is valency? Elements generally do not exist independently in nature and combine with the other elements. Valency is the combining capacity of one element with another element. Elements combine with one another to attain a stable state. It determines the number of electrons an element can donate or accept to form a stable electronic configuration.

Types of Valency

1. Valency in ionic compounds

2. Valency in covalent compounds

3. Variable valency

Valency in Ionic Compounds or Electrovalent Compounds

Ionic compounds or electrovalent compounds are compounds that form by the combination of metal ions and non-metal ions. For example, sodium chloride (NaCl). In this compound, sodium (Na) is the metal, and chloride (Cl) is the non-metal. It is formed by the transfer of electrons. What is electrovalency? This type of chemical bonding between metal and non-metal ions is called electro valency, and these compounds are called electrovalent compounds. Electro valency can be defined as the number of electrons lost or gained by the atoms in an ionic compound.

For Example: In NaCl

Na → Na⁺ + 1 e⁻

Cl + 1 e- → Cl⁻

Na⁺ + Cl⁻ → NaCl

In the above example of sodium chloride (NaCl), the electro valency of sodium (Na) and chloride (Cl) is 1, as 1 electron is lost by sodium and gained by chloride ion. 

Valency in Covalent Compounds

Covalent compounds are compounds formed by the chemical bonding of non-metals. These compounds are formed by the sharing of electrons between atoms. 

Non-metal + Non-metal → covalent compounds.

The valency of these compounds can be defined as the number of bonds by which the atom is directly attached to the other atom. These compounds exhibit co-valency.

For Example

Methane (CH₄)

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The carbon is attached with the four hydrogens with the help of four single covalent bonds. Therefore, the co-valency of carbon in methane is four. One hydrogen is attached with the one-carbon by the single covalent bond and hence, the co-valency of hydrogen is one. 

What is Variable Valency?

Some elements show more than one type of valency; these types of valency are called variable valency. These types of compounds show valency in one compound and another valency in other compounds. Variable valency is shown by elements like Iron, mercury, and copper. Transition elements show variable valency. For example: in some cases, iron shows a valency of 2 like ferrous sulfate (FeSO₄), and in some, it shows valency of 3 like ferric chloride (FeCl₃). Copper shows two types of valencies 1 and 2. Mercury shows two types of valency 1 and 2.

Why do some Elements show Variable Valency?

Let’s see an example of iron:

Iron (Fe)

The atomic number of iron = 26

Electronic configuration of iron = 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁶

4s² 3d⁶ shown electronic configuration makes iron an unstable molecule. The two electrons will be first removed from the 4s orbital. Then the electronic configuration of iron becomes 4S⁰ 3d⁶. 3d⁶ is not a stable electronic configuration. After losing one electron from the d subshell it will become a half-filled subshell. This 3d⁵ is a stable electronic configuration. Hence the element of iron shows 2 and 3 valencies. These valencies are called iron variable valencies. The elements show variable valency to acquire stability. The half-filled subshell shows more exchange energy which further lowers the energy of the compound. Therefore, the compound becomes more stable.

Elements with Variable Valency

Copper (Cu) = Cuprous (Cu²⁺) and cupric (Cu³⁺)

Iron (Fe) = Ferrous (Fe²⁺) and Ferric (Fe³⁺)

Mercury = Mercurous (Hg ⁺¹) and mercuric (Hg ⁺²)

Silver (Ag) = Argentous (Ag⁺¹) and argentic (Ag⁺².)

Stannum (Sn) =  Stannous (Sn⁺²) and Stannic (Sn⁺³)

The element that exhibits lower valency will be suffixed with “ous”. While the element that exhibits higher valency will be suffixed with “ic”.

 

Did you Know? 

The value of valency can never be zero.

The primary valency is the fixed property of elements.

Valency shows the bonding potential of the elements.

As we know that around three-fourths of the earth’s surface is covered with water and around 96.5% of the global water resources come from the oceans and seas only. But the total volume of usable freshwater is around 2.5%  and stored groundwater is 30% only. Many research has shown that by 2026, India, along with many other countries will face a serious scarcity of water. Many regions in our country are already under ‘water stress’. (‘water stress’ happens when the available water falls below 1000 cubic metres per person per day). Let’s discuss some water conservation methods and management.

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What is Water Management?

The activity to control water resources in order to minimise the damage of property and life and also to maximise the efficient use is termed as water management or in simple words It can be termed as the process of planning, developing, and distributing for the optimum use of water resources under defined water policies and regulations.  With the rapid increase in the population of the world to over 6 billion people in the past few years, the use of water has also increased up to 500%. Water is an essential resource for life on earth not only for humans but for plants and animals also, and therefore it must be conserved. In fact, historically also, humans had learned some techniques to conserve the available water resources by building dams, Using Drip irrigation, doing Water harvesting, etc.

What is Water Conservation?

The most common misconception people believe is, water is replenishable and will be around us forever. The reality is, many of them are uneducated about the conservation of water resources. If we do not do something now to conserve water, Our future generations will not be able to have access to pure water. By doing proper planning, water can be supplied to many places regularly in town or city. But many times some amount of water is wasted through leakage of pipe and many other reasons. As we know that proper water management is necessary for water conservation methods. Thus, it is important for CWA authorities to take proper care of these problems while distributing water to our homes.

Most of the rainwater gets wasted even though it is one of the most precious natural resources. Farmers can play an important role in water conservation methods by using suitable techniques like rainwater harvesting and drip irrigation.

 

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Water Conservation and Management

The different methods of water conservation are:

It is the process of collection and storage of rainwater, rather than allowing it to run off. Rainwater is collected from the roof and is redirected to a tank, reservoir, cistern, or natural tanks, etc.

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It is a method for saving water placed under the ground to control the groundwater flow in an aquifer and to raise the water table.

It is a type of irrigation that saves water and fertiliser by dripping water slowly to the roots of various crops, either on the soil surface or directly to the root zone, through a network of pipes, tubing, and valves. This process saves more water compared to the traditional watering method.

Dams are simple hydraulic structures that act as a barrier between the source and destination of flowing water. Earlier times, these dams were very small and hand-made while in modern times, new engineering techniques and methods are used to construct large dams.

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There are various good habits to conserve water for a long time. Some of them are Fixing leaky taps, Keeping the tap closed while brushing, taking a shower of 5 mins instead of long baths are a few examples of saving water.

The Indian practice in old times of cleaning water using brass vessels is well known and still continues. Even today water filter systems made from brass are very common. Older people in India use brass pots in the evening to store water and drink it during the daytime.

As time passes many technological devices are being developed to minimise water wastage, the impact will be greater if each and every individual starts contributing to water conservation by minimising or optimising the use of groundwater for daily work. Today, water management is becoming extremely important. Water management often involves modifying policies, such as drainage levels of groundwater, or allocating water for different purposes.

Ways to Water Management

Water is the most important natural resource. Many factors over the years have resulted in the degradation of natural resources including water bodies. Let us discuss the steps that can be taken for the conservation of water and what can be done on our behalf for the same. The activity of developing, planning, managing and distributing the optimum use of water resources is defined as water resource management.

Through precipitation and evaporation the water cycle maintains hydrological systems which support a variety of Aquatic ecosystems and forms lakes and rivers . Intermediate forms between aquatic and terrestrial ecosystems are wetlands that contain species of animals and plants that are highly moisture dependent Both security and economic development are placed at risk by poor water management and  water is increasingly becoming a Priority policy issue at the national level.

Rainwater harvesting:

The method of storage and collection of rainwater into reservoirs or natural tanks is known as rainwater harvesting.

Groundwater harvesting: 

A method to save water placed under the ground is groundwater harvesting.

Drip irrigation:

When the irrigation is done through dripping water slowly with the roots of various crops either directly onto the root out onto the soil surface in the method of drip irrigation.

Rainwater harvesting:

The rainwater is stored in big ponds or other things in the method of rainwater harvesting. This stored water can be reused in the future.

Water-saving habits: 

There are various wise habits to conserve water. Light taking a quick shower instead of long baths, lesser use of water during washing the clothes and fixing leaky taps.

Conclusion

Out of 70% of the Earth’s surface water only 3% is freshwater. Of which  only 1%is usable water in lakes, subsoiler aquifers and rivers and 2% is in polar ice caps . Fractions of this can only be utilised at a global level, 70% of water is used for agriculture, about 25% for industry and only 6% for domestic use. This Article’s primary focus is on ways of Water Management. 

India continues to be an agricultural economy predominantly because much of our population resides in villages. As a civilization, we have always been in touch with the earth and its abundance. With the invention of modern-day machines, farmer-friendly technology has made its way into the agricultural space. 

If you have had the privilege of listening to these stories from your parents or grandparents or even better, visiting your native village – have you seen the women of the family sit out on the porch with grains of rice filled in a basket-like structure? They keep throwing the husk aside so that they are left with rice grains alone. This is an example of how winnowing used to be performed in our own kitchens!

What is Winnowing? 

Winnowing refers to the separation of quality rice grains from husk or chaff through the blowing of strong winds. It is crucial in the process of paddy cultivation. In older times, this used to be a tedious process as farmers would have to wait for hours for winds to blow to separate their rice grains from chaff after making these dried grains fall from a height using shovels or sieves. 

Soon enough, the winnowing machine was introduced as industrialization spread globally. Husk is lighter than rice grains, hence it easily gets separated due to the force of the wind. The wind winnowing machine was first invented in Scotland by Andrew Rodger as early as 1737 when he wanted to separate corn. 

Today’s wind winnowing machines have a fan that is rotated through pedaling. The velocity of the wind produced can be controlled through this process of pedaling. Farmers no longer have to wait for the natural winds to blow. They can winnow hundreds of kilograms of rice or paddy in a short span of time. Since they are machines, they can be operated inside a large closed space too which means farmers can benefit even during the rainy season.

Separating grains from the chaff is called winnowing. It is an agricultural technique being used since ancient times. Now a days many machines are available for winnowing.

Winnowing means is the separation of grains from straw by the use of the current of air. The word winnow is originated from the old English word ‘windwian’ which means separation of the mixture through the wind. 

Principle of Winnowing

Winnowing is based on the fact that if a mixture is containing two components and one is lighter than another one, then both the components can be separated by a blow of air. The mixture is allowed to fall from a height. The lighter component gets separated from the heavier component due to wind.

In the above image, it is shown that as empty seeds and debris are lighter than filled seeds, so they get separated by the current of air. 

Wind winnowing machines- It is said that the first winnowing machine was developed by Andrew Rodger in 1737, in Scotland. He was a farmer and developed that machine for separating corn. He named the machine ‘fanner’. Then during the industrial revolution, many types of winnowing machines were developed.

Ancient winnowing machine developed in China 

Modern Winnowing Machine 

Now a days, a single machine is available for winnowing and threshing. Threshing is a process of separating stalks from grain. Farmers used to beat the grains manually to separate their edible part from non-edible parts. Now a days, machines are available for threshing also. Even now a days farmers use only one machine for threshing and winnowing. It makes their work easy and they can complete two tasks at a time. This saves their time and energy. 

Single machine for winnowing and threshing 

Example of Winnowing 

1. Wheat is separated from husk by using winnowing method. Wheat and husk mixture is allowed to fall from a height. Husk being lighter than wheat grains get separated from wheat because of wind or air blow. 

2. Dirt particles can be removed from grains by winnowing. 

3. Corns are separated from straw by winnowing.

Advantages of Winnowing

1. Winnowing is very useful in agriculture. 

2. We get grains and husk separate by winnowing, so husk can be used as food for animals.

3. We can separate grains such as wheat, corn, millet etc. from lighter impurities such as husk, straw, stones etc. by using winnowing. 

Disadvantages of Winnowing

1. This process of separating the components of a mixture does not work for heavier particles such as stones. 

2. Since in winnowing the components of a mixture are separated with the help of wind, it is practically not possible to separate the components that are heavier than grains. Therefore, this process of separation of mixtures has a limited scope.

3. Also, winnowing machines are more efficient than manual winnowing.

Difference Between Winnowing and Threshing 

Conclusion

The study notes on Winnowing provide a complete overview of the process of winnowing, including an overview of its origin, description, and the advances made in the process with the progress of industrialization.

Zinc is one of the transition earth metals which has a lustrous bluish-white colour. The zinc symbol is represented as Zn.  The Zn atomic number is 30. The abundance of the zinc element is estimated at almost 65 grams for every ton of the Earth’s crust. The word ‘zinc’ has a German origin whereas its exact derivation goes beyond the Persian word ‘sing,’ which means stone.

Zn in chemistry is a very important element and has several chemical properties.

Zn + 2HCl → ZnCl2 + H2 

2Zn + O2 → 2ZnO

• Zinc consist of five different natural stable isotopes

• These isotopes include; 64Zn, 66Zn, 67Zn, 68Zn and 70Zn

• Zinc has the lowest melting point after Cadmium and Mercury among all the  transition metals

• Zinc does not react with water directly. However, metallic zinc can react with oxygen present in the air in the presence of water vapour and thus, form zinc hydroxide.

Zn + 2H2O → Zn(OH)2 + H2

Chemical Properties of Zinc

Uses of Zinc

Zinc is one of the heavily used metals within the industry. Here is a list of some of the applications of zinc:

• The majority of the zinc is used in the manufacturing of zinc oxides as well as creating roofing materials

• Zinc oxides are mainly used as an additive to rubbers for the production of tires. This helps to withstand higher temperature as well as prevent any unnecessary wear and tear

• Zinc is also used in galvanizing other metals such as iron and steel. It prevents iron from rusting. On the other hand, galvanized steel is mainly used in manufacturing vehicles

• Alloys of metals are also formed using zinc. Some of the examples include nickel silver, brass, and even aluminium solder

• Zinc is also used in the production of die-casting. It plays an integral part in electronic hardware.

• Zinc oxide has also found its way into cosmetics, inks, pharmaceuticals, and even plastics

• Zinc sulfide is used for making fluorescent lights, x-ray screens as well as luminous paints

 Biological Significance of Zinc

The zinc atomic structure plays an integral role in every other living organism. In fact, almost 20 metalloenzymes have an active site formed through zinc. About 2.5g of zinc is present in an average human body, whereas we consume almost 15 milligrams per day. Some of the most common sources of zinc in food include beef, lamb, sunflower seeds, herrings and cheese.

On the contrary, zinc has carcinogenic properties too. Inhaling zinc (II) oxide can cause ‘zinc chills’ or commonly known as ‘oxide shakes.’

Physical properties of Zinc

Some of the most astounding physical properties of zinc include:

• At normal room temperature, it remains brittle with a crystalline state

• One of the properties of zinc is that when heated between 110oC to 150oC, it becomes malleable and ductile

• Zinc is highly reactive with dilute acids causing the release of hydrogen

• Zinc also shows its reactive properties while combing with oxygen or any other non-metal

Most Common Zinc Compounds

It is imperative to understand that the majority of the zinc compounds are widely used in organic synthesis. Below is a list of some of the most widely used zinc compounds:

• Zinc halogenides (fluoride, bromide, chloride, iodide)

• Zinc carbides

• Phosphides

• Sulfite, selenide, zinc arsenide

• Thiocyanates, cyanides and thiosulfates

• Ammonia complexes

• Hydroxo-zincates: amphoteric compounds formed from hydroxide and zinc oxide

 Interesting Facts about Zn Element

Here is a list of some of the most interesting facts about zinc that are worth mentioning:

• The 24th most common element that one can find on Earth’s crust is zinc

• Zinc contributes to almost 0.0075% of the entire Earth’s crust

• Zinc is also available in seawater, although it’s about 30 parts per billion

• The first known use of zinc dates back to 1000 BC

• The majority of the zinc (95%) are mined within ore deposits of sulfide

• Zinc is the fourth most commonly used metal within the industry

• Current zinc production includes 70:30; mining and recycling ration

• In 1746, Marggraf defined the zinc atomic structure while proclaiming it has a distinct element

The pH Of Various Substances?

Chemistry has three major classifications as acids and bases. These are the essential elements of chemistry to carry out reactions and form water and salt. In our daily life, we use different compounds that are either acids or bases. Daily life examples are milk turning sour into curd because of the presence of lactic acid. It all depends upon the pH of various substances. 

How To Define Acids And Bases?

The two chemical compounds: acids and bases have different definitions according to different people. Arrhenius defined both of them as the ionizing compounds and differentiated acids and bases as:

Arrhenius Theory Of Acids And Bases

According to Arrhenius’s theory, when we dissolve a compound in aqueous solution, it releases some ions. Thus he defines his theory on acids and bases. 

Acids: Acids leave H+ ions when dissolved in the solution or water. Further, these ions form H3O+ or hydronium ions when combined with water molecules. 

Bases: Bases are the compounds when dissolved in water leaves OH– ions in the solution.  

Here is the reaction explaining chemical equation of formation of hydronium ions:

HCl(aq)  -> H+ (aq) + Cl–(aq)

In the above reaction, when HCl is dissolved in the aqueous solution, it forms H+ ions. Further, these H+ ions dissolve to form H3O+ ions.

HCl(aq) + H2O(l) -> H3– (aq) + Cl– (aq)

The reaction of base dissolving in water:

NaOH (aq)  -> Na+ (aq) + OH–  (aq)

Acids And Bases- pH Scale

Based on the concentration of hydrogen ions, solutions are classified as basic or acidic. Acidic solutions are the one, which has a higher H+ concentration as compared to water. However, basic solutions are the one, which has lower H+ concentration. The concentration of Hydrogen ion is expressed pH that is calculated as:

pH = -log10 [H+]

The square brackets around the hydrogen ion state the concentration. The concentration of H+ in water is 1×10-7 M, which is the value of 7.0 (neutral pH). However, the concentration of H+ moves away whenever an acid or base is added to a water-based solution. 

In terms of pH, an acid in chemistry is a compound that enhances the concentration of H+ in the solution. In contrast, a base provides OH– ion that combines with H+ and removes them from the solution. As a result, bases are the substances, which increase the pH value, and acids are the substances that decrease the pH value.

The pH scale is generally used to rank the solutions in terms of alkalinity (basicity) and acidity. It ranges from 0 to 14, and most of the solutions fall in this range. 

(Image to be added soon)

The above picture shows that anything below 7.0 in pH scale is acidic, and above 7.0 is basic. 

What pH Values Correspond To Acids And Bases?

Strong Acids:

Generally, acids are substances with a pH value of less than 7.0. The value goes on decreasing as the amount of H+ ion increases in the solution. Strong acids are the substances that release H+ ions rapidly or that are 100% ionizable in the solution. Thus, the strong acids have less pH value, nearly 0 to 1. The lower the pH value, the higher is the concentration of H+ ions in the solution, and hence, stronger is the acid. 

For example,

HCl (aq) -> H+ (aq) + Cl– (aq)

It shows that hydrogen chloride, when dissolved in solution, splits out to give hydrogen and chloride ions. Some other examples of strong acids include hydrobromic acids (HBr), sulphuric acid (H2SO4), and many more.

Strong Bases 

The pH of various substances decides its nature. However, bases are substances with pH values greater than 7.0. The value goes on increasing as the amount of H+ decreases in the solution. Strong bases are those substances that release OH– ion in the solution rapidly. These ions scoop the H+ ion present in the solution, and as a result, increase the pH value of the solution. Thus, strong bases usually have pH values, approximately 13 or 14. 

For example, sodium hydroxide (NaOH) is the strong base that splits in the aqueous solution to produce sodium ion and hydroxide ion. Some other examples of strong bases include potassium hydroxide (KOH), and hydroxides of alkali metals.

Weak Acids

A weak acid is the one that fails to ionize in the solution completely. It releases H+ ion in the low concentrations, and thus pH ranges from 5 to 7. Some of the examples include formic acid (HCOOH), acetic acid (CH3COOH), and many more.

Weak Bases

Weak bases are the substances that do not undergo complete dissociation. As a result, there is a decrease in OH– ion concentration and an increase in pH value. Some of the examples include methylamine and ammonia. 

Not every organism is capable of synthesizing food from carbon dioxide (CO₂) and water (H₂O), so the organisms gifted with this functionality are termed as ‘photoautotrophs’. Although the process of process of Photosynthesis is quite complex, we can briefly represent it in the form of following chemical equation which was first proposed by Cornelis van Neil.

 

 

This Glucose that is synthesized by plants is later consumed by humans or animals to release energy, carbon dioxide and water in a process called respiration.