Chapter 6.1 - Basic details about Acids

In the previous section, we saw the relation between non-metallic oxides and acids. In this section we will see metallic oxides.
Let us do an experiment:
For this experiment, we need 'quick lime'. Quick lime is the common name for calcium oxide (CaO). It is obtained by heating naturally occuring substances like limestone or sea shells. Limestone and sea shells are calcium carbonates (CaCO₃). When it is heated, carbon dioxide escapes, and CaO is left behind. Let us write the equation:

Reactants:
    ♦ Calcium carbonate. One molecule is CaCO₃

Products: 
    ♦ Calcium oxide. One molecule is CaO

    ♦ Carbon dioxide. One molecule is CO₂

• So the skeletal equation is: CaCO₃ → CaO + CO₂ . This is a balanced equation.

1. So we have an oxide CaO. An oxide of a metal calcium. So it is a metallic oxide. We are going to do the experiment with it. 

2. Take water in a beaker. Add some quick lime to it and stir well. A reaction takes place. It is between CaO and water. If we touch the sides of the beaker, we can feel heat. This is because the reaction releases heat. In other words, it is an exothermic reaction.  Let us write the equation:

Reactants:
    ♦ Calcium oxide. One molecule is CaO

    ♦ Water. One molecule is H₂O

Product: 
    ♦ Calcium hydroxide. One molecule is Ca(OH)₂

• So the skeletal equation is: CaO + H₂O → Ca(OH)₂. This is a balanced equation.

3. How are we going to use this calcium hydroxide?

• We added quick lime to water. The quick lime was converted into Ca(OH)₂ (calcium hydroxide).
• Some water will be used up for the reaction. But there will still be some excess water. Because all the water will not be used up. 
• So the product, which is Ca(OH)₂, will dissolve in the excess water. And we get a solution of Ca(OH)₂ in water. 
• But all the Ca(OH)₂ will not dissolve in water. Some of it will precipitate to the bottom of the beaker. The clear portion above the precipitate is called 'supernate'.

■ We see a separation as a precipitate and a supernate because, there is not enough water for all the Ca(OH)₂ to dissolve. That portion of Ca(OH)₂, which is unable to dissolve, will form the precipitate. That means, the supernate is a solution of Ca(OH)₂ in water. 

Take some of this supernate in a test tube. Add a drop of red litmus to it. We can see that, a blue colour appears in the solution. So we can infer that Ca(OH)₂ is alkaline in nature. Let us analyse what we have done:

• We took quick lime. It is an oxide of calcium, which is a metal. So quick lime is a metallic oxide.
• This metallic oxide, when added to water, gave an alkali. We can say this: Metallic oxides are generally alkaline in nature.

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■ So, if we are given a list of oxides, we can classify them as acidic or alkaline. Given below is a list:

SO₃, NO₂, CaO, K₂O, P₂O₅, Na₂O, CO₂, MgO

Let us classify them into acidic and alkaline:

Acidic oxides	Alkaline oxides
SO3	CaO
P2O5	K2O
CO2	Na2O
NO2	MgO

The above classification is based on the following two facts:
■ Non-metallic oxides are acidic in nature
■ Metallic oxides are alkaline in nature

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Following are some of the properties of acids and alkalies:

■ Acids:

• Have sour taste

• Changes blue litmus to red

• Produces carbon dioxide gas on reacting with carbonates

• Produces hydrogen gas on reacting with metals like Fe, Mg etc.,

■ Alkalies:

• Have bitter taste

• Changes red litmus to blue

• Slippery to touch

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Let us explore some more about acids:

Acids can be classified as Mineral acids and Organic acids

■ Mineral acids are also known as inorganic acids. They are prepared from one or more inorganic compounds, and are stronger than organic acids. Most common mineral acids are Sulphuric acid, Nitric acid and Hydrochloric acid. Mineral acids should not be tasted.
■ Organic acids are those acids found in naturally occuring organic compounds. They are generally weak acids. The sour taste of some of the food items is due to the presence of organic acids. Some examples are given below:

• Lime – Citric acid

• Tamarind – Tartaric acid

• Tomato – Oxalic acid

• Sour milk – Lactic acid

• Vinegar – Acetic acid

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The common factor in acids

Let us write the chemical formula of some commonly known acids:

Name of acid	Chemical formula
Hydrochloric acid	HCl
Nitric acid	HNO3
Acetic acid	CH3COOH
Carbonic acid	H2CO3
Sulphuric acid	H2SO4

• From the above list, we find that hydrogen is present in all the acids. So hydrogen is a common factor in acids.

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Hydrogen is the component present in all acids and is responsible for the common properties of acids. 

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Let us see a reaction in which an acid is a reactant:

Hydrochloric acid reacts with iron to give ferric chloride and hydrogen. Let us write the equation:

Reactants:

    ♦ Iron. One molecule is Fe

    ♦ Dilute Hydrochloric acid. One molecule is HCl.

Products:

    ♦ Ferric chloride. One molecule is FeCl₂.

    ♦ Carbon dioxide. One molecule is CO₂.

• So skeletal equation is:

Fe + HCl → FeCl₂ + H₂. This is not a balanced equation. The steps for writing the balanced equation are shown below:
Step 1: Fe + HCl → FeCl₂ + H₂
Step 2: Fe + 2HCl → FeCl₂ + H₂

	Reactants			Products
	Fe	H	Cl	Fe	H	Cl
Step 1	1	1	1	1	2	2
Step 2	1	2	2	1	2	2

■ So the balanced equation is: Fe + 2HCl → FeCl₂ + H₂

We can say this:

The acid HCl, reacts with the metal iron to liberate hydrogen. In fact, it is a common property of all acids:

■ All acids react with metals to liberate hydrogen

We saw another example in the preparation of hydrogen in the lab. Details here.

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We will now discuss about acids in detail:
We know that in HCl:

• There is a covalent bonding between H and Cl.
• Cl is more electronegative, and so attracts the shared pair of electrons towards itself
• So the hydrogen has a partial positive charge, and chlorine has a partial negative charge. 
• We have seen the above details here. 
• When HCl is mixed with water, the bond between H and Cl breaks. The hydrogen will have to leave with out taking it's electron.
• This is because, Cl is more electronegative, and so, will keep both the shared electrons
• The H atom thus becomes H⁺ ion and Cl becomes Cl^-
• The H⁺ ions will go towards the water molecules. They will attach with the water molecules. 
• The water molecules will then become ions because of the positive charge brought by the H⁺ ions. 
• The new ion is written as H₃O⁺. It is called the hydronium ion. 
• These hydronium ions are responsible for the characteristics of acids.

Consider the following scenario:

We have a situation in which we want the effects of an acid. We have a bottle of HCl. If we keep the HCl in the bottle itself, or pour it into another suitable container, there is no effect. To see the effects of acid, the acid must come in contact with other materials.

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Note that it is very important to handle acids carefully. All safety measures should be taken. Some of the compulsory safety measures are:

• Wearing lab coat or lab apron

• Wearing safety goggles for protection of eyes

• Using acid resistant gloves

Experiments should be performed only under the guidance of authorised professionals

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• We have seen the effect when HCl comes into contact with water: The hydronium ions (H₃O⁺)are formed, and that gives the acidic effect.

• If more H+ ions are liberated, more hydronium ions will be formed, and there will be greater acidic effect

■ If one molecule of an acid gives one H⁺ ion, It is a monobasic acid

Eg: HCl → H⁺ + Cl^-.

HNO₃ → H⁺ + NO₃^-

■ If one molecule of an acid gives two H⁺ ions, It is a dibasic acid

Eg: H₂SO₄ → 2H⁺ + SO₄^2-

It may be noted that, the two H⁺ ions are obtained as a result of two steps as shown below:

Step 1: H₂SO₄ dissociates into H⁺ and HSO₄^-

Step 2: HSO₄^- dissociates into H⁺ and SO₄^2-

Step 1 gives one H⁺ and one HSO₄^-. But after step 2, the HSO₄^- is no longer present. It has dissociated into H⁺ and SO₄^2-

So we get a total of two H⁺ ions

Another example: 

H₂CO₃ → 2H⁺ + CO₃^2-.

Here also, the two H⁺ ions are obtained as a result of two steps as shown below:

Step 1: H₂CO₃ dissociates into H⁺ and HCO₃^-

Step 2: HCO₃^- dissociates into H⁺ and CO₃^2-.

So we get a total of two H⁺ ions

■ If one molecule of an acid gives three H⁺ ions, It is a tribasic acid

Eg: H₃PO₄ → 3H⁺ + PO₄^3-

Here, the three H⁺ ions are obtained as a result of three steps as shown below:

Step 1: H₃PO₄ dissociates into H⁺ and H₂PO₄^-

Step 2: H₂PO₄^- dissociates into H⁺ and  HPO₄^2-.

Step 3: HPO₄^2- dissociates into H⁺ and  PO₄^3-.

So we get a total of three H⁺ ions

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We have seen some of the important details about acids. In the next section, we will discuss about alkalies. 

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Chapter 6 - Acids, Alkalies and Salts

In the previous section, we completed the discussion on Hydrogen and Chlorine. In this chapter we will see Acids and Alkalies.

We know that, when metals or non-metals react with oxygen, various oxides are formed.

• When metals react with oxygen, we get metallic oxides.

• When non-metals react with oxygen, we get non-metallic oxides.

We will first consider non-metallic oxides.

We know that carbon is a non-metal. Nitrogen and sulphur are also non-metals. When these non-metals react (the reaction can be combustion or ordinary reaction) with oxygen, their oxides are produced. And these oxides are ‘non-metallic oxides’.

Here we have to learn about 3 non-metallic oxides. CO₂, NO₂ and SO₂. We will first consider CO₂. 

Let us do an experiment:

1. Take some calcium carbonate (CaCO₃) in a boiling tube. Add 5 ml dilute hydrochloric acid (dil.HCl) to it using a thistle funnel. See fig.6.1 below:

Fig.6.1

2. The calcium carbonate reacts with HCl. We get calcium chloride (CaCl₂) and carbon dioxide (CO₂) gas as products. Let us write the equation:

Reactants:

    ♦ Calcium carbonate. One molecule is CaCO₃. 

    ♦ Dilute Hydrochloric acid. One molecule is HCl.

Products:

    ♦ Calcium chloride. One molecule is CaCl₂.

    ♦ Carbon dioxide. One molecule is CO₂.
    ♦ Water. One molecule is H₂O

• So skeletal equation is:

CaCO₃ + HCl → CaCl₂ + CO₂ + H₂O. This is not a balanced equation. The steps for writing the balanced equation are shown below:
Step 1: CaCO₃ + HCl → CaCl₂ + CO₂ + H₂O 
Step 2: CaCO₃ + 2HCl → CaCl₂ + CO₂ + H₂O

	REACTANTS								PRODUCTS
	Ca	C	O	H	Cl				Ca	C	O	H	Cl
Step 1	1	1	3	1	1				1	1	3	2	2
Step 2	1	1	3	2	2				1	1	3	2	2

■ So the balanced equation is: CaCO₃ + 2HCl → CaCl₂ + CO₂ + H₂O

Thus we successfully produced a non-metallic oxide CO₂.  There is more to do:

3. Pass this CO₂ through water in a test tube. When we do that, we get a solution. The solution formed by dissolving CO₂ in water. 
4. Now dip a blue litmus paper in the solution. The blue litmus turns into red. 
5. So we can conclude that, the solution is acidic in nature.

■ That means, an acid is formed when CO₂ dissolves in water. 
• In fact, when the CO₂ dissolves in water, a reaction takes place. The product formed is carbonic acid (H₂CO₃). Let us write the equation:

Reactants:
    ♦ Carbon dioxide. One molecule is CO₂

    ♦ Water. One molecule is H₂O

Product: 
    ♦ Carbonic acid. One molecule is H₂CO₃

• So the skeletal equation is: H₂O + CO₂ → H₂CO₃. This is a balanced equation.

■ From the above experiment, we get the following information:

The non-metallic oxide CO₂ dissolves in water to give an acid

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Generally, CO₂ will not dissolve in water easily. If we want to dissolve large quantities of CO₂ in water, we will have to apply pressure. Soda water is produced in this way.

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Now we consider another non-metallic oxide. That of nitrogen:

Nitrogen does not take part in reactions easily. This is because of the triple bond in the nitrogen molecule. But under the action of thunder and lightning, nitrogen in the atmosphere combines with oxygen. Thus oxides of nitrogen are produced. We saw the details in the previous chapter when we saw Nitrogen cycle. We will write the steps again:
1. First, nitrogen combines with oxygen to form nitric oxide. The balanced equation is:
N₂ + O₂ → 2NO

2. The NO thus formed will combine with more oxygen to become Nitrogen dioxide NO₂. The balanced equation is:
2NO + O₂ → 2NO₂
3. Thus we reach the stage of a non-metallic oxide NO₂. We want to know the further actions of this NO₂.

4. The NO₂ dissolves in rain water. It may be noted that, oxygen is required for this dissolution to take place. The balanced equation is: 
4NO₂ + 2H₂O + O₂ → 4HNO₃.

5. So, as a result of the 'dissolving of nitric oxide in water', we get HNO₃, which is nitric acid. 
■ Thus so far, we have seen two cases:

• The non-metallic oxide CO₂ dissolves in water to give carbonic acid

• The non-metallic oxide NO₂ dissolves in water to give nitric acid.

Now we will see yet another non-metallic oxide. The oxide of sulphur. It is called sulphur dioxide (SO₂) We have seen it's formation when we studied about the reactions of oxygen with non-metals. Details here. The balanced equation for the formation of SO₂ is:

S + O₂ → SO₂.

• This oxide also dissolves in water to form an acid. The acid formed is sulphurous acid (H₂SO₃). Let us write the equation:

Reactants:

    ♦ Sulphur dioxide. One molecule is SO₂. 

    ♦ Water. One molecule is H₂O.

Products:

    ♦ Sulphurous acid. One molecule is H₂SO₃.

• So skeletal equation is:

SO₂ + H₂O → H₂SO₃. This is a balanced equation.

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So we have one more case to add to our list. The modified list is given below:

■ Three cases seen so far:

• The non-metallic oxide CO₂ dissolves in water to give carbonic acid

• The non-metallic oxide NO₂ dissolves in water to give nitric acid.

• The non-metallic oxide SO₂ dissolves in water to give Sulphurous acid.

We can say this: Non-metallic oxides generally give acids when they dissolve in water

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Let us now see the ill-effects of such acid formation:

■ Factories, thermal power plants, automobiles etc., send out large quantities of non-metallic oxides like NO₂ and SO₂. These acids dissolve in rain water, and form acids. When large quantities of acids come down with rain, we call it an acid rain. Following are the ill-effects of acid rains:

• Acid rains destroy the leaves of plants. The plants then will no longer be able to produce food by photosynthesis.

• Acid rains destroys the greenery of a region. This will increase air pollution

• Acid rains can cause deaths of fish and other marine organisms

• Acid rains cause destruction of substances containing calcium carbonate (CaCO₃). Because, CaCO₃ reacts with acids

    ♦Corals contain CaCO₃. When acids reach them, reaction takes place, and the corals will be destroyed. The effects can be seen here.

     ♦ Marble contain CaCO₃. When acids reach them, reaction takes place, and the beauty of marble is lost.
     ♦ Limestone is an important naturally occuring veriety of stone. It is essential for many industries. They are obtained from limestone quarries. The quarries as a whole will be destroyed if the acid rains reach them. The destruction takes place due to the reaction of CaCO₃ with the acids. The effects can be seen here.

We have to take steps for the prevention of acid rains. Following are some of the effective steps:

• Reduce the use of fossil fuels like coal and petroleum

• Use those fossil fuels from which sulphur compounds have been removed. In this way, oxides of sulphur will not be formed during the combustion of the fuel.

• Regularly check that, the gases emitted from automobiles does not cause pollution above the accepted level.

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We have completed our present discussion on the formation of acids from non-metallic oxides. In the next section, we will discuss about metallic oxides. 

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Chapter 5.3 - Details about Hydrogen and Chlorine

In the previous section, we completed the discussion on Nitrogen. In this section we will see Hydrogen and Chlorine.

We have seen that nitrogen is the most abundant element in the atmosphere. But  when we consider the universe as a whole, hydrogen is the most abundant element. Hydrogen is the main component of Sun and other stars.

■ Hydrogen combines with other elements to form various compounds. Some of it's important compounds are:

• Water (H₂O), • Sulphuric acid (H₂SO₄), • Hydrochloric acid (HCl), • Methane (CH₄) etc.,

Preparation of Hydrogen

■ Hydrogen is produced when metals react with dilute acids. This property can be used to prepare hydrogen in the lab. The following experiment will demonstrate the procedure:

Take 5 ml of dilute hydrochloric acid in a test tube. Add some zinc granules to it. The zinc will react with hydrochloric acid to produce hydrogen. Let us write the equation:

• It is a reaction between zinc and hydrochloric acid. One molecule of zinc is Zn. One molecule of hydrochloric acid is HCl. So we will write it on the left side.

• The products are zinc chloride and hydrogen. One molecule of zinc chloride is ZnCl₂. One molecule of hydrogen is H₂. We will write them on the right side. 

• So the skeletal equation is:  Zn + HCl → ZnCl₂ + H₂ .

This is not a balanced equation. The steps for balancing the equation are given below:

So the balanced equation is: Zn + 2HCl → ZnCl₂ + H₂ .

According to the above equation, hydrogen will be produced in the test tube. But we have to detect it's presence in the test tube. For that, bring a glowingspliter, or an incense stick to the mouth of the test tube. The flame is put out with a pop sound. But the gas burns with the formation of water droplets on the side of the test tube. A video showing a similar procedure can be seen here.

Another experiment:

Fix a balloon to the mouth of the test tube in which hydrogen is produced. The hydrogen gas begins to fill up the balloon. So it swells. Tie the mouth of the balloon with a thread. Now release the balloon. It will rise high up in the air. This is because hydrogen is lighter than air.

Water and Heavy water

We know that water is a combination of oxygen and hydrogen. Now, hydrogen has two isotopes: Deuterium and Tritium. We have seen the details here. Out of these two isotopes, Deuterium combines with oxygen to form heavy water. 
• We know that hydrogen has no neutron. But deuterium has one neutron. 
• So deuterium is heavier than hydrogen. We can write schematic equations:

    ♦ Hydrogen + Oxygen → Ordinary water

    ♦ Heavy hydrogen (deuterium) + oxygen → Heavy water

■ The above equations gives us an idea about why 'Heavy' water is called so. Heavy water is indeed heavy. It is 10 percent heavier than ordinary water. That is., if a certain volume of ordinary water weighs 100 gm, the same volume of heavy water will weigh 110 gm.

The chemical formula of heavy water is D₂O. It is present in nature. But only in very small quantities. If we divide a given quantity of water into 6000 equal parts, one part will be heavy water.

Uses of heavy water:

• As a moderator in nuclear reactors  • For the preparation of deuterium   • As a tracer in chemical reactions

Uses of hydrogen:

• As a fuel   • To fill balloons   • To prepare a fuel called 'water gas'   • In the manufacture of fuel cells.

Hydrogen as a fuel

We know that, there are many types of fuels. Some examples are: Methane, Petrol, Ethanol etc., Hydrogen is also a fuel. Of all the fuels available to us, hydrogen has the highest calorific value. That means, if we take the same quantity of various fuels, hydrogen will give the highest heat energy value. Hydrogen is present in large quantities. It does not cause any pollution while burning. Because, the only product of it's combustion is water.

Though there are many merits, we can not use hydrogen as a fuel very easily. This is because, it is not easy to make hydrogen into usable form. It burns explosively in air. It's storage and distribution is difficult.

Chlorine

Chlorine is an element that is usually seen in combined form with other elements. This is because, chlorine is highly reactive. It does not occur free in nature.

Some important compounds of chlorine are: NaCl, KCl, HCl, CaCl₂ etc.,

Preparation of chlorine

In the laboratory, chlorine is prepared by heating potassium permanganate (KMnO₄) with concentrated hydrochloric acid (HCl). The balanced equation is:

2KMnO₄ + 16HCl → 2KCl + 2MnCl₂ + 8H₂O + 5Cl₂.

The arrangement for preparation of chlorine in the lab is shown in fig.5.4 below:

Fig.5.4

• The required quantity of HCl is added to KMnO₄ using a thistle funnel. 
• This HCl can vapourise into gaseous state when the heat is applied. 
• Also, from the equation, we can see that water is also a product. This will also be in the gaseos state as water vapour. 
• So, the gas which comes out of the boiling flask will be a mixture of HCl, water vapour and chlorine gas. We have to separate the chlorine gas from the other two. 
• For that, the delivery tube that comes out of the boiling flask is made to go into a conical flask containing water. This water will absorb the HCl. 
• The resulting gas that comes out of the conical flask will be a mixture of chlorine gas and water vapour. So it is passed into another conical flask containing concentrated sulphuric acid. 
• This sulphuric acid will absorb the water vapour. So the delivery tube that comes out of the second conical flask, will give pure chlorine gas.

Bleaching action of chlorine

Chlorine has the ability to bleach materials. That is., chlorine can remove colour of materials. For the bleaching action, presence of moisture is essential. This is because, water is a reactant in the 'chemical reaction that takes place during bleaching'.

Place pieces of moistened colour papers, petals, coloured cloth pieces etc., in a jar containing chlorine gas. After some time, we will find that, the colours have been removed from the materials. A video showing the bleaching action can be seen here.

Let us now see how the bleaching action occurs. A product known as 'bleaching powder' is available in the market. The main constituent of this powder is chlorine. In a wet atmosphere, this powder will release chlorine. This chlorine reacts with water to form hypochlorous acid. This acid is unstable, and so will decompose to form ClO^- ions. These ClO^- ions have the ability to bleach colours.

Uses of Chlorine:

• As a bleaching agent • To produce bleaching powder • To produce insecticides • To remove stains from fabrics • In the purification of water.

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We have completed our present discussion on hydrogen and chlorine. In the next chapter, we will discuss about Acids, Alkalies and Salts. 

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