Inorganic chemistry, another important branch of Chemistry Topics, deals with the properties and behavior of inorganic compounds and elements.
Importance of Chemical Change and Differentiate Between Physical and Chemical Changes
Those changes in which new substances are formed, are called chemical changes. The properties of new substances formed in chemical changes are entirely different from those of the original substances. During chemical change, a substance undergoes a change in its chemical composition (or change in chemical properties). Chemical changes are also called chemical reactions.
The chemical changes are permanent changes which can usually not be reversed to form the original substance. A lot of energy (in the form of heat, light, sound, etc.) is either absorbed or given out in chemical changes. Thus, the various characteristics of a chemical change are as follows :
- One or more new substances are formed in a chemical change.
- A chemical change is a permanent change. A chemical change usually cannot be reversed.
- A lot of energy (in the form of heat, light, etc.) is either absorbed or given out in a chemical change.
- Sound may be produced in a chemical change.
- A change in smell may take place or a new smell may be given off in a chemical change.
- A permanent change in colour may take place in a chemical change.
- A gas may be formed in a chemical change.
Let us take an example to understand a chemical change clearly. If we burn a piece of paper with a lighted match stick, then entirely new substances such as carbon dioxide, water vapour, smoke and ash are produced. So, the burning of paper is a chemical change (see Figure).
Heat and light are also given out during the burning of paper. The burning of paper is a permanent change which cannot be reversed. For example, we cannot combine the products of burning of paper to form the original paper again.
In fact, the burning of any substance is a chemical change. Burning is always accompanied by the production of heat. Some light is also produced during burning.
Some more examples of chemical changes are: Souring of milk ; Formation of curd from milk; Cooking of food (like rice and chapatis); Spoilage of food ; Change in colour of cut apple (cut brinjal or cut potato) on keeping in air ; Photosynthesis ; Digestion of food ; Neutralisation reaction ; Explosion of a firework (or cracker) ; Burning of magnesium ribbon; Burning of fuels (like burning of wood, coal, kerosene, LPG and biogas) ; Burning of dry leaves ; Burning of candle wax ; Burning of incense stick (agarbatti) ; Rusting of iron ; Ripening of fruits ; Reaction between vinegar and baking soda (which produces carbon dioxide gas) ; Passing carbon dioxide gas through lime water (which produces calcium carbonate precipitate); Reaction between copper sulphate solution and iron (to form iron sulphate and copper); Formation of manure (or compost) from leaves ; Formation of biogas from animal wastes (like cow-dung).
All these changes are chemical changes because new substances are formed in them. Moreover, these changes cannot be reversed. We will discuss some of these changes in a little more detail.
When food gets spoiled, it produces a foul smell. This shows that new substances have been formed in the spoiled food which have foul smell. So, the spoilage of food is a chemical change. If we cut an apple into slices and keep it aside for some time, we will find that the cut surface of apple acquires a brown colour.
The cut surface of an apple slice acquires a brown colour due to the formation of new substances by the action of oxygen (of air). So, the change in colour of a cut apple slice on keeping in air is due to a chemical change. Similarly, the cut surface of a brinjal (or potato) turns black on keeping in air for sometime due to chemical change. This is because new substances are formed which impart black colour to their cut surfaces.
During photosynthesis, the plants combine carbon dioxide and water in the presence of chlorophyll and sunlight to form two new substances : glucose (food) and oxygen gas. So, photosynthesis is a chemical change. In the process of digestion, the various food materials break down to form new substances which can be absorbed by the body. So, the process of digestion is a chemical change.
When an acid reacts with a base, then a neutralisation reaction takes place in which two new substances, salt and water, are formed. So, neutralisation is a chemical change. The explosion of a firework (like a cracker) is a chemical change because many new susbstances are formed in this process. The explosion of a firework also produces heat, light, sound and unpleasant gases. These unpleasant gases pollute the air around us.
In a biogas plant, anaerobic bacteria digest (break down) the animal wastes (like cow-dung) and produce biogas whose major component is methane gas. The formation of biogas from animal wastes is a chemical change. This is because new substances like methane are produced from animal waste.- Biogas is burnt as a fuel.
The burning of biogas is also a chemical change. This is because burning of biogas produces new substances like carbon dioxide and water vapour along with the evolution of heat (and some light). Similarly, the burning of liquefied petroleum gas (LPG) in a gas stove is also a chemical change.
Note. We have already given the symbols of some elements and formulae of some compounds in the previous Chapter. In addition to those, we should also remember the following elements and their symbols as well as compounds and their formulae :
Element | Symbol | Element | Formula |
Magnesium | Mg | Oxygen (gas) | O2 |
Iron | Fe | ||
Copper | Cu | ‘ 4 | |
Compound | Formula | Compound | Formula |
Magnesium oxide | MgO | Acetic acid | CH3COOH |
Magnesium hydroxide | Mg(OH)2 | Sodium acetate | CH3COONa |
Calcium hydroxide | Ca(OH)2 | Copper sulphate | CuSO4 |
Carbon dioxide | CO2 | Iron sulphate | FeSO4 |
Calcium carbonate | CaCO3 | Iron oxide | Fe2SO3 |
We will now study some of the chemical changes by performing activities.
1. Burning of Magnesium Ribbon
Magnesium is a metal which burns easily on heating. A long and thin strip of magnesium metal is called magnesium ribbon. We usually use magnesium metal in the form of a magnesium ribbon to perform activities.
When a magnesium ribbon is heated, it burns in air with a brilliant white light to form a powdery ash called magnesium oxide. This magnesium oxide is an entirely new substance. Thus, a new substance is formed during the burning of magnesium ribbon. So, the burning of magnesium ribbon is a chemical change (see Figure). We can perform the magnesium ribbon burning activity as follows.
Activity 1
Take a small piece of magnesium ribbon and clean it by rubbing its surface with a sand paper. Hold the magnesium ribbon at one end with a pair of tongs and bring its other end over the flame of a burner [see Figure (a)]. The magnesium ribbon starts burning with a dazzling white light.
Hold the burning magnesium ribbon over a watch glass so that the powdery ash being formed by the burning of magnesium collects in the watch glass [see Figure (b)], Actually, when magnesium ribbon burns in air, then the magnesium metal combines with the oxygen (of air) to form a new substance called magnesium oxide. This change can be written in the form of a word equation as follows :
It is the magnesium oxide compound which appears as a white, powdery ash. So, magnesium oxide (ash) is the new substance formed when magnesium burns in air. The burning of magnesium ribbon is a chemical change because a new substance, magnesium oxide, is formed during this change.
We can carry out another chemical change by using the magnesium oxide ash formed by the burning of magnesium ribbon. This can be done as follows.
Activity 2
Take magnesium oxide (ash) in a hard glass test-tube and add a small amount of water to it. Stir the magnesium oxide and water mixture carefully with a glass rod to obtain an aqueous solution of magnesium oxide. Test the magnesium oxide solution with blue litmus paper and red litmus paper respectively, as follows:
- Take a strip of blue litmus paper and put a drop of magnesium oxide solution on it. The blue colour of litmus paper does not turn to red showing that magnesium oxide solution is not acidic.
- Now take a strip of red litmus paper and put a drop of magnesium oxide solution on it. The red litmus paper turns blue showing that magnesium oxide solution is basic in nature.
Actually, when we dissolve magnesium oxide in water, then magnesium oxide combines with water to form a new substance called magnesium hydroxide. This change can be written in the form of a word equation as follows:
The new substance magnesium hydroxide formed during this change is a base which turns red litmus paper to blue. The dissolving of magnesium oxide in water is a chemical change because a new substance magnesium hydroxide is produced during this change.
2. Reaction Between Baking Soda and Vinegar
When baking soda and vinegar are mixed together, then bubbles of carbon dioxide gas are formed (alongwith some other substances). This carbon dioxide is an entirely new substance. The reaction between baking soda and vinegar is a chemical change because it forms carbon dioxide as one of the new substances.
We can carry out the chemical change between baking soda and vinegar as follows: Take about 10 mL vinegar in a test-tube and add a pinch of baking soda to it. We will hear a hissing sound and see the bubbles of carbon dioxide gas coming out and rising in the test-tube (see Figure).
Baking soda is sodium hydrogencarbonate and vinegar contains acetic acid. So, when baking soda and vinegar are mixed together, then a chemical change takes place between sodium hydrogencarbonate and acetic acid to form three new substances : sodium acetate, carbon dioxide and water. This chemical change can be written in the form of a word equation as follows :
We can carry out another chemical change by u sing the carbon dioxide gas produced in the above chemical reaction between baking soda and vinegar. This can be done as follows.
Prepare carbon dioxide gas by adding baking soda to vinegar in a test-tube. Take some freshly prepared lime water in another test-tube. Pass carbon dioxide gas through lime water by using a glass delivery tube as shown in Figure. We will see that lime water turns milky. This happens as follows.
Lime water is calcium hydroxide solution. When carbon dioxide gas is passed through lime water, then calcium hydroxide combines with carbon dioxide to form a white solid substance ‘calcium carbonate’ (which makes lime water appear milky). This chemical change can be written in the form of a word equation as follows:
The reaction between lime water and carbon dioxide gas is a chemical change because a new substance ‘calcium carbonate’ is formed during this change. The turning of lime water milky is used as a standard test for carbon dioxide gas. This is because only carbon dioxide gas can turn lime water milky (by forming a white precipitate of calcium carbonate). No other gas can turn lime water milky.
Note: To prepare lime water, dissolve some lime (choona) in water in a bottle. Stir the solution and keep it for some time. Pour a little clear solution from the top. This is lime water.
3. Reaction Between Copper Sulphate Solution and iron
When an iron object (like an iron nail, etc.) is kept immersed in the blue coloured solution of copper sulphate, then a chemical change takes place to form green coloured iron sulphate solution and a brown deposit of copper on the iron object (like nail). This change can be written in the form of a word equation as follows :
The reaction between copper sulphate (CuSO4) solution and iron (Fe) is a chemical change because it produces two new substances : iron sulphate (FeSO4) solution and copper (Cu). We can perform the reaction between copper sulphate solution and an iron nail as follows. Please note that the common name of copper sulphate is ‘blue vitriol’. It is called ‘neela thotha in Hindi.
Activity 3
Dissolve a little of copper sulphate in half test-tube of water. Add a few drops of dilute sulphuric acid to obtain a clear solution. This will give us a blue-coloured copper sulphate solution. Take a big iron nail and place it carefully in the test-tube containing copper sulphate solution [see Figure (a)].
Keep the test-tube containing copper sulphate solution and iron nail aside for a few hours. We will see that the blue colour of copper sulphate solution fades gradually and ultimately changes to light green colour and a brown deposit (or layer) is formed on the iron nail [see Figure (b)].
We can take out the iron nail from the test-tube to see the brown deposit on it clearly. These changes are due to a chemical reaction between copper sulphate solution and iron nail which produces two new substances: iron sulphate and copper. The formation of green coloured substance ‘iron sulphate’ makes the solution green. And the formation of’copper metal’deposits a brown layer on the iron nail.
Importance of Chemical Changes
Chemical changes are very important in our lives. All the new substances which we use in various fields of our life are produced as a result of chemical changes (or chemical reactions). Some of the examples of the importance of chemical changes are given below:
- Metals are extracted from their naturally occurring compounds called ‘ores’ by a series of chemical changes. For example, iron metal is extracted from the iron ore by chemical changes.
- Medicines are prepared by carrying out a chain of chemical changes.
- The materials such as plastics, soaps, detergents, perfumes, acids, bases, salts, etc., are all made by carrying out various types of chemical changes.
- Every new material is discovered by studying different types of chemical changes.
Differences Between Physical and Chemical Changes
The main differences between physical and chemical changes are given below :
Physical change | Chemical change | ||
1. | No new substance is formed in a physical change. | 1. | A new substance is formed in a chemical change. |
2. | A physical change is a temporary change. | 2. | A chemical change is a permanent change. |
3. | A physical change is easily reversible. | 3. | A chemical change is usually irreversible. |
4. | Very little energy (in the form of heat, etc.) is absorbed or given out in a physical change. | 4. | A lot of energy (in the form of heat, light, sound, etc.) is absorbed or given out in a chemical change. |
Please note that the same substance can undergo a physical change or a chemical change depending upon the conditions. For example:
- The tearing of a sheet of paper into pieces of paper is a physical change but the burning of a sheet of paper is a chemical change.
- The melting of wax is a physical change but the burning of wax is a chemical change. So, when a candle burns, then both physical and chemical changes take place. This is because when a candle burns, then some of the wax melts (physical change) and some of the wax burns (chemical change).
A Protective Shield of Ozone
Ozone is a gas. There is a layer of ozone gas high up in the atmosphere. The ozone layer in the upper atmosphere is very useful. The ozone layer protects us from the harmful ultraviolet radiations which come from the sun. This happens as follows :
Ozone absorbs ultraviolet radiations coming from the sun and breaks down to form oxygen. In this way, ozone layer absorbs harmful ultraviolet radiations coming from the sun and prevents them from reaching the earth. The breaking down of ozone into oxygen is a chemical change.
If ultraviolet radiations were not absorbed by ozone layer, they would reach the earths surface and cause harm to us and other living things. Ultraviolet radiations can cause skin cancer, damage our eyes and plants (including crops). The ozone layer in the atmosphere acts as a natural, protective shield against the harmful ultraviolet radiations coming from the sun.