Chemical substances: Nature and behaviour

Learn about chemical substances, their nature, behaviour and uses. Also, learn about chemical reactions and basic inorganic and organic chemistry.

Aromatic Vs. Aliphatic Compounds

Do you know the difference between aromatic and aliphatic compounds?

Compounds of carbon and hydrogen form the basis of organic chemistry. Aromatic and aliphatic compounds are two broad classes of organic compounds.

Aliphatic compounds are formed when the carbon and hydrogen interact through straight chains, branched chains or non-aromatic rings. The number of bonds may vary from one (alkanes) to three (alkynes). The bonds in aliphatic compounds can be saturated (hexane) or unsaturated (hexene). Besides hydrogen and carbon, aliphatic compounds can also contain oxygen, nitrogen, sulphur and chlorine. Most aliphatic compounds are flammable gases. Methane (CH4) is the simplest aliphatic compound.

The bond between carbon and hydrogen is weak in aliphatic compounds. Therefore, aliphatic compounds are often very reactive.

The spatial distribution of carbon and hydrogen atoms is different in aromatic compounds. The atoms arrange themselves to form a flat aromatic ring. Benzene is the prototypical aromatic compound.

The aromatic ring contains many double bonds that interact with each other to form a very stable configuration. Therefore, these rings form easily and once formed cannot be easily broken. This (the stable nature of the ring) is why aromatic compounds are not as reactive as aliphatic compounds.

The stability of an aromatic ring is more than that expected from conjugated bonds alone. Single and double bonds alternate in aromatic compounds. Therefore, electrons can flow freely in a circle through the alternating single and double bonds. Therefore, the aromatic ring can be considered as a hybrid of single and double bonds.

The first described aromatic compound is the Benzene. Benzene has the following structure.

aromatic compounds

Image from Wikipedia

Benzene has six carbon atoms. The carbon atoms arrange themselves in a perfect hexagon. The carbon atoms are held together by alpha and pi bonds. The interaction of the pi bonds forms a pi-orbital above and below the plane of the carbon atoms. As the carbon atoms are out of the plane, these orbitals can freely interact with each other and become delocalised. Therefore, the electron is shared freely with all six carbon atoms. Thus, there are not enough electrons to form double bonds at each carbon atom, but the excess electrons strengthen every bond in the aromatic ring. Therefore, the bonds between the carbon atoms are much stronger than that would be expected from simple covalent bonds alone. Therefore, these bonds do not break easily. Hence, these (aromatic) compounds are considerably less reactive than aliphatic compounds.

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Periodic classification of elements: The beginning

What is the periodic classification of elements? And why do we need to classify them?

All of us must have visited a library sometime. How are books organised in a library? Books are arranged by author name or by subject thus helping us to retrieve the books quickly. Similarly, the aim of the periodic classification of elements is to simplify the study of elements.

There are 118 elements in nature. Of these, 98 are naturally occurring. Elements are being discovered with each passing day. Therefore, scientists have been trying to classify the elements based on shared properties shared by the elements. One of the earliest methods was to classify the elements as metal or non-metal.

In 1817, Dobereiner a German chemist made the first attempt at periodic classification of elements. He grouped elements based on some properties, and he identified some groups that contained three elements each. He called these groups the Dobereiner?s Triad. He noted that when the elements are arranged in the ascending order of their atomic weight, the element in the middle had an atomic weight that was the average of the other two elements.

Dobereiner identified only three triads from the elements known at that time. Therefore, this system of periodic classification of elements did not become famous.

Dobereiner’s Triad

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Newlands, an English chemist, arranged the elements according to their atomic weight. The lightest element was hydrogen, and the heaviest element was thorium. He noted that every eight elements in the table had similar properties. He called this as the law of octaves (after the octaves in music).

law of octaves

Newlands’ Law of Octaves

However, there were problems associated with the law of octaves. The law worked fine for lighter elements up to calcium. Beyond calcium, every 8th element did not possess the same properties. Also, with the discovery of new elements, the law of octaves was found to be inadequate for the periodic classification of elements.

In 1872, Mendeleev, a Russian chemist published his periodic classification of elements. He classified the 63 known elements into groups and periods. Elements were organised based on their atomic mass. On the basis of his work he postulated the periodic law- ?the properties of the elements are the periodic function of their atomic mass?.

There were some gaps in Mendeleev?s periodic classification of elements. He predicted that these holes corresponded to elements that were undiscovered at that time.

Mendeleev

Mendeleev’s Periodic Table

Although Mendeleev?s periodic classification of elements was an important step in chemistry, there were a few limitations to his periodic classification of elements. The position of Hydrogen in Mendeleev?s table was ambiguous. Secondly, isotopes were a challenge to Mendeleev?s periodic classification of elements. Thus, there was a need to refine Mendeleev?s periodic classification of elements to incorporate the newly discovered elements and also to plug the loopholes in Mendeleev?s table. ?We will learn about the modern periodic table in another article.

 

 

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The PH scale: Its importance in everyday life

The PH Scale is a measure of the strength of an acid or base. The PH of a solution is measured using a paper impregnated with a universal indicator.

In the previous articles, we have learned that acids produce hydrogen or hydronium ions in a solution. The strength of an acid is proportional to the concentration of the hydronium ions. Therefore, if we have a way to measure the concentration of hydronium ions, then we would be able to quantify the strength of an acid. Similarly, the strength of a base is directly proportional to the concentration of the hydroxide (OH) ions.

The PH Scale is the log of the hydronium ion concentration. An acid is any substance whose hydronium ion concentration is more that 10-7 moles/litre. Therefore, an acid will always have a PH of less than 7. As the strength of the acid increases, the PH decreases. For example, lemon juice has a PH for 2.3. The hydrochloric acid in our stomachs has a PH of 1.5-2, and the PH of concentrated hydrochloric acid is nearly 0.

The PH scale is also used to measure the strength of bases. In general, bases have a PH greater than 7. Stronger the base, more is its PH. The maximum value possible on the PH scale is 14. A concentrated solution of sodium hydroxide would have a PH value of 14.

So, what is the importance of the PH scale in the everyday file?

Most life processes can occur within a narrow range of PH. For example, the PH of blood is tightly regulated in the range of 7.2 to 7.4. Similarly, the PH value of the soil is of great importance in agriculture. Most food crops grow best at a PH of 7-7.8. The growth of plants suffers when the PH value is outside this range. The PH of soil is usually tightly regulated by the microflora present in the ground. However, excessive use of ammonia-based fertilisers can make the soil acidic.

Have you heard of acid rain? The atmosphere contains gasses like nitrous oxide and sulphur oxide. When these dissolve in rain, they produce nitric and sulphuric acid. If the concentration of these acids in high in rainwater, it can lower the PH value. Acid rain has a value of less than 5.6 on the PH scale. ?Acid rain is another cause of the increase in soil acidity.

The PH scale plays a critical role in the human body. Different parts of the human body have different PH. The mouth has a slightly alkaline PH while the stomach has a very acidic PH. The skin has a slightly acidic PH, and the PH in the Vagina is again very acidic. Why is this so?

The saliva in the oral cavity makes the PH of the mouth alkaline. The alkaline PH facilitates the action of salivary enzymes. It also prevents tooth decay. Why do we wash our mouth after eating anything? The sugars present in the food can ferment to produce acid. This acid can damage the enamel of the teeth, thus causing tooth decay.

The PH of the stomach is between 1.3 -2. The glands in the stomach secrete a hydrochloric acid that makes the PH of the stomach acidic. The acidic PH serves two functions- it kills any microorganisms, and it facilitates the action of enzymes like trypsin.

Similarly, the acidic PH of the skin and vagina prevent colonisation by bacteria, thus serving as a natural defence against infection.

Thus, the PH scale plays a crucial role in everyday life. Students are frequently asked to calculate the PH in the board exams. Therefore, the PH scale is an important topic that the students of class 10 science should master.

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Acids: Properties and behaviour

 

What are the adjectives one would associate with acids? The first word would be corrosive. What about sour? Are acids sour in taste? Only a fool would taste a concentrated solution of acid. But what about a dilute solution of acids, like vinegar? You will note that acids are sour in taste.

So how does one identify an acid? Let us assume that you are given two test tubes with a colourless solution. One of them is an acid and the other is not. How do you know which one is the acid?

One method is to use a litmus paper. The Litmus paper contains a dye produced by a class of plants called ferns. This dye changes colour when exposed to acids. There are two types of Litmus papers- red and blue. When you dip the blue litmus paper in an acid, it will change colour to red while the red litmus paper will remain red on exposure to acids. The Litmus paper can also be used to identify a base. If you dip a red litmus paper in a basic solution, it turns blue in colour. Thus, by using a combination of these two Litmus papers, we can identify acids and bases. Apart from Litmus paper, we could also use substances like Turmeric, methyl orange and phenolphthalein to identify acids and bases. These pigments are also known as indicators.

These were the physical properties of acids. Let is now discuss a little about the chemical properties of acids

In the previous article, we learned about the definition of an acid. We learned that acids were in general proton donors. Therefore, acids can react with any substance that accepts a proton. These materials include bases and metals. Acids can also be defined on the basis of the transfer of electrons. An acid is a substance that accepts a pair of electrons from another species.

The following equation summarises the reaction between acids and metals:-

Metal + Acid = Metal Salt + Hydrogen gas

Consider this example:-

Zn + 2HCl = ZnCl2 + H2

In the above equation, one atom of Zinc reacts with two molecules of hydrochloric acid to form one molecule of hydrogen and zinc chloride. Here, hydrochloric acid is behaving as a Lewis acid as it accepts a pair of electrons from the zinc atom.

So, how do acids and bases react with each other?

Consider this equation:-

2HCl +2NaOH = 2H2O +2 NaCl

We can devise a simple experiment using the above equations. Take a test tube with 5 ml of NaOH. Add a few drops of phenolphthalein to this solution. What do you observe? You will note that the colour of this solution turns pink. Now, start adding hydrochloric acid to this solution drop by drop. What do you observe? You will note that the pink colour gradually fades and at some point, the solution will become entirely colourless. When you add hydrochloric acid to a solution of NaOH, the acid neutralises the base to produce salt and water. The basic solution will become transparent at the point where all the base is neutralised. ?In fact, you will be using this neutralisation reaction in Class 12 to quantify the exact concentration of an acid or base in a solution.

To summarise, acids are corrosive substances with a sour taste that change the colour of blue Litmus to red. Acids are capable of reacting with a multitude of materials. We will learn more about acids in the next article.

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Acid: Definition

Can you think of a few uses of an acid in everyday life? We use vinegar to preserve and flavour our food. Acid was and is still used to clean toilets and bathrooms, and car batteries also use acids. Apart from these uses, acid is also used in many industrial processes. In fact, acids are ubiquitous in our life. So, what exactly is an acid?

There are three definitions of an acid; the Arrhenius, the Br?nsted-Lowry & the Lewis definition.

Arrhenius definition

A Swedish chemist, Arrhenius, defined an acid is any substance that increases the concentration of hydrogen, or more accurately, hydronium ions in a solution. Study the following equation:-

H2O + H2O = H3O+ + OH

Most of the molecules of water exist as H2O. But, some molecules exist as hydronium, and some exist as hydroxide ions. As the number of hydronium and hydroxide ions is the same, water in neutral. However, if any substance were to increase the concentration of hydronium ions, then the solution will turn acidic. Conversely, any substance that decreases the concentration of hydroxide ions will also be an acid, as a reduction in hydroxide ions will turn a solution acidic.

Have you ever wondered why a PH < 7 is considered acidic? An acid is any solution that contains hydronium ions in excess of 10-7 moles per litre. Since PH is the negative log of hydrogen or hydronium ions, a PH< 7 is considered acidic.

Br?nsted- Lowry definition

Although the Arrhenius definition explains many reactions involving acids, there are some limitations to its use. Consider this equation:-

CH3COOH + NH3 = CH3COO + NH4+

In the above reaction, there is no hydronium ion, yet we know that acetic acid (CH3COOH) is an acid. This flaw was rectified by Br?nsted and Lowry, two chemists, who independently postulated that an acid is any species that donates a proton. Thus acetic acid, in the above example, donates a proton (H+) ion to ammonia. Thus, it behaves as a Br?nsted acid.

Lewis definition

In the same year, another chemist, Gilbert N. Lewis, defined an acid in terms of electron transfer. He defined an acid as any species that accepts a pair of electrons from another substance. All Br?nsted acids are Lewis acids. However, all Lewis acids need not be Br?nsted acids. Consider this equation:-

BF3 + F = BF4

Here, the fluoride ion donates a pair of electrons to boron trifluoride. This reaction cannot be explained by the Br?nsted theory. However, by the Lewis definition, boron trifluoride is an acid as it accepts a pair of an electron from fluoride ion.

The Arrhenius and the Br?nsted definitions are the most relevant definitions for an acid, and the Br?nsted definition is the most commonly used method to define an acid.

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Redox reactions

Oxidation and reduction reactions (redox) are asked frequently in class 10 science. We shall discuss the definitions of redox reactions in this article.

Oxidation and reduction (redox) reactions form the basis of many life processes and life on earth would not be possible without these reactions. Oxidation and reduction reactions happen together and hence there are referred as redox reactions.

Oxidation can be defined in a number of ways. This can be defined on the basis of the transfer of oxygen, hydrogen or electrons between the reactants.

Redox reactions in terms of oxygen transfer.

In a reaction, when one reactant gains oxygen, it is said to be oxidised. Therefore, oxidation can be defined as a gain of oxygen. For this reactant to gain an oxygen atom, another reactant has to donate an oxygen atom. The donor is called an oxidising agent. And the donor reactant is in turn reduced. Therefore, loss of oxygen defines a reduction reaction.

Consider this example:

Fe2O3 + 3CO = 2Fe +3CO2

In this equation, iron loses its oxygen to carbon monoxide to produce carbon dioxide. Here, iron oxide is the oxidising agent. And since carbon monoxide removes an oxygen from iron oxide, it is called a reducing agent.

Redox reactions in terms of transfer of hydrogen atoms.

This definition is useful in organic chemistry. Let us consider this equation:

In organic chemistry, one will come across two terms- alcohol and aldehyde. Alcohols and aldehydes are inter-convertible. When alcohol loses hydrogen, it converts to an aldehyde and this process is called oxidation. Similarly when an aldehyde gains hydrogen, it forms alcohol. This process is called reduction. Equations in organic chemistry are complex, therefore we have simplified this reaction using ethanol and its corresponding aldehyde ethanaldehyde.

CH2CH3OH — ———–> CH3CHO

Loss of hydrogen

 

Redox reactions in terms of transfer of electrons.

This is the most useful and comprehensive definition of oxidation and reduction reactions. Based on the transfer of electrons, oxidation is defined as loss of electrons while reduction is defined as a gain of electrons. Consider this basic equation:

CuO + Mg + Cu + MgO

In the above equation, copper oxide loses oxygen to magnesium, therefore magnesium is getting oxidised and copper oxide is the oxidising agent. Now let us rewrite this equation as an ionic equation (copper oxide and magnesium oxide are both ionic, the metals aren?t).

Cu2+ + Mg = Cu + Mg2+

In this equation, the oxygen atom is simply a spectator. Thus if we look at this equation in terms of electron transfer, the copper gains 2 electrons from magnesium, hence it is being reduced and magnesium loses 2 electrons to copper, hence it is getting oxidised.

This can be more easily remembered as:

OIL(oss) RIG(ain)

Oxidation= loss of electrons

Reduction= gain or electrons

Therefore, to summarise:

  1. Oxidation is loss of electrons.
  2. An oxidising agent oxidises another agent.
  3. Thus, an oxidising agent must gain electrons.

Similarly, a reducing agent reduces another substance, thus it must lose electrons (as the reduction is a gain of electrons).

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Balanced Chemical Equations

Chemical equations form a very important component of CBSE class 10 science. Students are often asked about balancing a chemical equation. This article will discuss some of the methods used in balancing equations.

A chemical equation is a symbolic, written representation of a chemical reaction. In an equation, the reactants are on the left and the products are on the right. There are a few rules to be followed when writing a chemical reaction.

  1. The chemical formulas of the reactants and products must be mentioned correctly.
  2. As mentioned above, the reactants are placed on the left side and the products are on the right.
  3. Law of conservation of matter states that matter can neither be created nor destroyed. Therefore, the total number of atoms should be the same on both sides of the equation.

Now consider this equation:

H2 +?O2?= H2O

There are 4 atoms on the left while there are only 3 atoms on the right. This is against the law of conservation, therefore this equation will need to be modified. This process of modifying the equation to make it conformant with the above law is called balancing an equation and the equation itself is called a balanced chemical equation. So let us proceed to balance the above equation.

H2?+ O2?= H2O

H=2????????? H=2

O=2????????? O=1

As there is only 1 oxygen atom on the right, we will add a coefficient to the oxygen atom on the right. The coefficient, in this case, is 2 (the number of oxygen atoms on left).

H2?+ O2?= 2H2O

H=2????????? H=4

O=2 ?????????O=2

Now you note that the number of oxygen atoms is same, but the number of hydrogen atoms is more on the right. So we now need to add a coefficient to the hydrogen on the left.

2H2?+ O2?= 2H2O

H=4 ???????????H=4

O=2???????? ???O=2

Now there are 6 atoms on either side and the equation is balanced.

This was a fairly simple equation to balance. Let us now consider a more complex equation. This equation represents the events that happen when propane burns in the presence of oxygen to produce water and carbon dioxide.

C3H8?+ O2?= H2O + CO2

There are three elements in this equation. Carbon, oxygen and hydrogen. We will balance the carbon first, the balance the oxygen and hydrogen molecules. We will follow the same process we did in the previous equation.

C3H8?+ O2?= H2O + CO2

C=3??????????????? C=1

H=8??????????????? H=2

O=2??????????????? O=3

There are 3 carbon atoms on the left but only one on the right, so we add a coefficient, which, in this case, is 3 (the number of carbon atoms on left).

C3H8?+ O2?= H2O + 3CO2

C=3??????????????? C=3

H=8??????????????? H=2

O=2??????????????? O=7

We now note that the carbon atoms are balanced, but hydrogen and oxygen are not. We will now balance the hydrogen atoms in the equation (we do hydrogen before oxygen because hydrogen is present in only a single molecule on both the left and right side). There are 8 hydrogen atoms on left and 2 on right, so we add a coefficient to hydrogen on the right, i.e. 4 (8/2=4).

C3H8?+ O2?= 4H2O + 3CO2

C=3??????????????? C=3

H=8??????????? ????H=8

O=2??????????????? O=10

Now both carbon and hydrogen are balanced. We only need to balance the oxygen atoms and we are done. This we do by adding a coefficient to the oxygen on the left (there are 10 atoms on the right but only 2 on left). The coefficient would be 5 (10/2=5).

C3H8?+ 5O2?= 4H2O + 3CO2

C=3??????????????? C=3

H=8??????????????? H=8

O=10????????????? O=10

Using this method one can balance any type of chemical equation, ranging from simple ones like above to more complex equations. However, as equations get complex, this method will become tedious, hence there are other methods which we shall discuss in another article.

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Types of chemical reactions

When wood burns, it is a chemical reaction. When food gets bad, it is a chemical reaction. Similarly, rusting of iron is also a chemical reaction. However, in each of the above examples, the nature of the chemical change undergone by the reactants is different. There are many different chemical reactions, therefore it is often useful to classify them into different types. The common types of chemical reactions include:-

Combination or synthesis reaction: A combination reaction is a reaction where two or more reactants combine under suitable conditions to form a new substance. As a new substance is being formed, this type of reaction is also called as a synthesis reaction. Take for example the rusting of iron. During the process of rusting, iron combines with oxygen present in the air to form iron oxide. It is this iron oxide which is visible as rust. As iron is reacting with oxygen, this type of reaction is also called as an oxygenation reaction.

  1. Decomposition reaction: Have you ever wondered what happens to the food we eat? The food we eat is broken down in our intestines by proteins called as enzymes. These enzymes break food into its elemental forms (through a series of complex reactions) and it is the elemental form which is absorbed from the intestines. For example, bread is broken down into glucose and absorbed by the body. Similarly, meat is broken down in amino acid and absorbed by the body. These are all examples of the decomposition reaction. Thus, a displacement reaction is the opposite of a combination reaction.
  2. Displacement reactions: This is a type of reaction where a more reactive element replaces a less reactive element from a compound. Displacement reactions are in turn classified as cation replacement reactions or anion replacement reactions. These reactions are also called as substitution reactions.
  3. Double displacement reactions: In this reaction, elements from each of the reactant molecules are exchanged thus forming two totally new compounds.
  4. Neutralisation reactions: When equal quantities of acids and bases react with each other, salt (not necessarily common salt) and water is produced. This type of reaction is called as neutralisation reaction.
  5. Isomerisation reaction: Butane is a four carbon compound which is used in the manufacture of gasoline. When it is heated, it undergoes a change in its structure and forms isobutane. However, the chemical composition of isobutane is the same as that of butane. This type of reaction is called as isomerisation reactions.
  6. Oxidation reactions: What happens during rusting of iron? Iron reacts with oxygen to form iron oxide. This is a form of the oxidation reaction. Oxidation reactions are one of the commonest chemical reactions in the world. Burning of coal or gasoline is an oxidation reaction. Our bodies produce energy for living by oxidation reactions.
  7. Reduction reaction: Reduction reactions are the opposite of oxidation reaction. When oxygen is removed from a compound, it is called a reduction reaction. This explanation is, however, a very simplistic view of chemistry. In reality, any reaction where an element gains electrons is called a reduction reaction.

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Chemical reactions: The Basics

Have you ever wondered how striking a matchstick produces fire, or how a burning candle produces light? The tip of the matchstick is coated with a chemical substance (commonly a combination of nitrates and phosphorus) which when struck, reacts with the oxygen present in air. This reaction leads to the production of heat and light, and the product of this event is quite different from the original chemical substance. Therefore in the above instance, a chemical reaction is said to have occurred.

Now take another example- when salt is mixed in water, it dissolves. However salt can be brought back to its original state by boiling off the water. In this example, even when dissolved in water,?the chemical composition of salt remains?unchanged. There is only a change in the physical?state?of the substance?(salt).,?On the other hand, it is not possible to return the matchstick to its original state. The later is an example of a physical change while the former is a case of a chemical change. Whenever a chemical change occurs, a chemical reaction is said to have occurred.

Thus a chemical reaction can be inferred to have taken place when the following events occur:-

  1. Physical effects, such as the emission of heat and light
  2. The formation of a precipitate
  3. The evolution of gas, or
  4. A color change.

The substance or substances initially involved in a chemical reaction are called reactants. Reactants in turn are composed of tiny particles called atoms. This concept was first alluded to by John Dalton in the early 19th century. In his atomic theory, he postulated that matter is composed of small, indivisible particles called atoms. The atoms of each element are unique, and when these atoms re-arrange in a chemical reaction, new substances are formed. All chemical reactions are based on this fact.

There are two significant rules one must be aware of while studying chemical reactions:-

  • ?Law of conservation of matter
  • Law of constant composition

What is the law?of conservation of matter?

The matter is neither created nor destroyed. The ash left behind after burning of wood weighs less than the original piece of timber. Has that piece of wood has lost some mass? No, if we put this piece of wood inside a closed container and weigh the container before and after burning, we will note that the weight of the container remains the same. Thus, wood has merely transformed.

What is?the?law?of constant composition?

Now consider this example- when coal burns in air, the carbon in coal reacts with oxygen in the air to form carbon dioxide. Gasoline (petrol) is another carbon-containing compound. When gasoline burns, the carbon and oxygen react to form carbon dioxide. The carbon dioxide produced by both these processes contains one atom of carbon and two atoms of oxygen, and this ratio remains constant. Thus, we could say that one atom of carbon will always combine with two atoms of oxygen.

To summarise, A chemical reaction is a process that results in a chemical change in the reactants. In this process the product or products, are, in general, different from the reactants.

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