Chemical Change and Chemical Bond Class 8 Science Notes Maharashtra State Board
Ripening of mango, melting of ice, boiling of water, dissolution of salt in water, Ripening of banana, fragrance on ripening fruit, darkening of cut potato, bursting of an inflated balloon, sound of bursting fire cracker, foul smell from spoiled food. During any chemical change, the composition of the original substance changes to form a new substance with a different composition and properties. How to identify a chemical change?
Take the lemon juice in a clean glass. Take two drops of the lemon juice in a spoon and taste. Add a pinch of baking soda to the glass of lemon juice. Did you notice bubbling around the particles of soda? Did you hear a sound taking your ear near the glass? Now again taste it. Did it taste as sour as it was in the beginning? (The above activity is to be done using clean apparatus and edible material. Then only it is possible to test the ‘taste’, otherwise keep in mind that the testing of ‘taste’ cannot be done.)
Many perceivable observations are noticed during the above activity. A gas is seen to be liberating in the form of bubbles. A low sound is heard. The white solid particles of the baking soda disappear. The original sour taste becomes mild or diminishes. From this, it is understood that a new substance having a different taste is formed. At the end of the above change, the taste of the substance was different means its composition was different. Thus, during the above change, the composition of the original substance changed to form a new substance with different properties. Thus, the change that takes place on adding baking soda to lemon juice is a chemical change. Sometimes some characteristic observations are perceived during a chemical change. These enable us to know that a chemical change has taken place.
Chemical Change and Word Equation:
During a chemical change, the chemical composition of the original matter changes, and new substances having different properties and different chemical compositions are formed. A chemical equation can be written for a chemical change if the exact change in chemical composition is known. Names and chemical formulae of the original substance and newly formed substance are used while writing a chemical equation.
For example, when baking soda is added to lemon juice a chemical change takes place in the citric acid present in the lemon juice and the gas formed is carbon dioxide. The word equation can be written for this chemical reaction as follows:
Citric acid + Sodium bicarbonate → Carbon dioxide + Sodium citrate
Acid + Alkali → CO2 + Salt
This is a neutralization reaction.
The first step of writing a chemical equation is to write a word equation by using the names of the concerned substances. When the chemical formula is written in place of each of the names, it becomes a chemical equation. While writing a chemical equation, original substances are written on the left side newly formed substances are written on the right side and an arrow is drawn in between. The arrowhead points towards the substances formed. The arrow indicates the direction of the reaction. Substances written on the left side of the arrow are original substances that take part in the reaction. They are called reactants. New substances formed as a result of the reaction are called products. A place for the products of a reaction is on the right side of the arrow.
Chemical Changes in Everyday Life
We find many examples of chemical changes in our surroundings, body, home, and laboratory. Let us see some chemical changes for which words and chemical equations can be written easily.
Natural Chemical Changes
a. Respiration:
Respiration is a continuously occurring biological process. In this process, we inhale the air and exhale carbon dioxide and water vapor. After an in-depth study, it is learned that glucose in the cells reacts with oxygen in the inhaled air to form carbon dioxide and water. The word equation and the chemical equation of this chemical reaction are as follows. (Here, the chemical equation is not balanced.)
Word equation: Glucose + Oxygenrespiration \(\rightarrow{\text { respiration }}\) Carbon dioxide + Water
Chemical equation: C6H12O6 + O2 \(\rightarrow{\text { respiration }}\) CO2 + H2O
Take some freshly prepared lime water (solution of calcium hydroxide) in a test tube. Keep on blowing in it with a blow tube. What is seen after some time? Did the colorless lime water turn milky? After some more time, you will find that a white insoluble solid settles at the bottom of the test tube. This is a precipitate of calcium carbonate. The turning milky mill got lime water means that the blown gas mixed in it was carbon dioxide.
Carbon dioxide + Calcium hydroxide → Calcium carbonate + Water
Write a chemical equation for the above word equation.
b. Photosynthesis:
You know that green plants perform photosynthesis in sunlight. A word equation and a chemical equation (unbalanced) can be written for this natural chemical change as follows.
Word equation: Carbon dioxide + Water \(\rightarrow[\text { green plant }]{\text { Sunlight }}\) Glucose + Oxygen
Chemical equation: CO2 + H2O \(\rightarrow[\text { green plant }]{\text { Sunlight }}\) C6H12O6 + O2
Manmade Chemical Changes
We bring about many chemical changes for our use in everyday life. Let us see some of them. The chemical change that we saw in the first activity is used in the cold drink called ‘soda-lemon’. It means that it is a useful manmade chemical change.
a. Combustion of Fuels:
Wood, coal, petrol or cooking gas are burnt to get energy. The common substance that burns in all these fuels is ‘Carbon’. During the combustion process, carbon combines with oxygen in the air and the product carbon dioxide is formed. A common equation can be written for all these combustion processes as follows.
Word equation: Carbon + Oxygen → Carbon dioxide
Chemical equation: C + O2 → CO2
Combustion of fuel is a fast and irreversible chemical change.
b. Cleaning Shahabad tile with dilute hydrochloric acid:
The chemical composition of Shahabad tile is mainly calcium carbonate. During its cleaning with hydrochloric acid, the upper layer of the tile reacts with hydrochloric acid, and three products are formed. One of them is calcium chloride, which being soluble in water, gets washed away with water. The second product is carbon dioxide; its bubbles mix up in the air. The third product, water mixes with water. The following equation can be written for this chemical change.
Word equation: Calcium carbonate + Hydrochloric acid → Calcium chloride + Carbon dioxide + water
Write a chemical equation (unbalanced) for the above reaction.
c. Softening of Hard Water:
Some wells or tube wells have hard water. It is brackish to taste and does not form a lather with soap. This is because of hard water contains the chloride and sulfate salts of calcium and magnesium in a dissolved state. To soften the hard water, a solution of washing soda is added to it. This results in a chemical reaction to form a precipitate of insoluble carbonate salts of calcium and magnesium. As the dissolved salts of calcium and magnesium go out in the form of a precipitate of the carbonate salts, the water is softened. The following equation can be written for this chemical change.
Word equation: Calcium chloride + Sodium carbonate → Calcuim carbonate+ Sodium chloride
Chemical equation (unbalanced): CaCl2 + Na2CO3 → CaCO3 + NaCl
Write word and chemical equations for the chemical change taking place in magnesium salts during the softening of hard water.
We saw that the composition of matter changes during a chemical change and new substances having different properties are formed. When this happens, some chemical bonds in the reactants break and new substances called products are formed by the formation of new chemical bonds. We have also seen in the chapter ‘Composition of Matter’ that the number of chemical bonds formed by one atom is its valency. Now let us see what is a chemical bond.
Chemical Bond
We have seen the relationship between electronic configuration and valency of an element in the chapter ‘Inside the Atom’. Noble gases do not form any chemical bond as their electron octet/duplet is complete while the atoms with incomplete electron octet/duplet form chemical bonds. The reason for this is that an atom uses its valence electrons during the formation of a chemical bond. Moreover, on forming chemical bonds equal to its valency, the atom attains the electronic configuration of a complete octet/duplet.
1. Ionic Bond:
Let us first see how the compound sodium chloride is formed from the atoms of the constituent elements sodium and chlorine. For this purpose let us see the electronic configuration of sodium and chlorine.
11Na: 2, 8, 1; 17Cl: 2, 8, 7
We have seen the correlation that the valency of sodium is one as it has one electron in its valence shell and the valency of chlorine is one as its valence shell is short of one electron to have a complete octet. On loss of a valence electron from ‘M’shell, the penultimate shell ‘L’ of the sodium atom becomes an outermost shell. It has eight electrons in it. Effectively, sodium attains an electron octet state.
However, the electron number becomes 10. Hence the positive charge +11 on the sodium nucleus is imbalanced and a Na+ cation, carrying a net positive charge +1 is formed. On the other hand valence shell of the chlorine atom contains an electron less than the octet state. On accepting an electron from the outside, an octet of chlorine is completed. However, the charge balance is disturbed due to the addition of an electron to the neutral chlorine atom. This results in the formation of an anion Cl–, carrying a net negative charge -1. When the elements sodium and chlorine combine, an atom of sodium gives its valence electron to a chlorine atom, whereby the cations Na+ and anions Cl– are formed. Due to the electrostatic force of attraction between opposite charges, the oppositely charged ions get attracted to each other and a chemical bond is formed between them.
The chemical bond formed due to an electrostatic force of attraction between the oppositely charged cation and anion is called an ionic bond or an electrovalent bond. The compound formed using one or more ionic bonds is called an ionic compound. The formation of an ionic compound sodium chloride from the elements sodium and chlorine is shown with the help of a diagrammatic representation of electronic configuration in the figure.
One ionic bond is formed due to the electrical charge +1 or -1 on an ion. The valency of an ion is equal to the magnitude of positive or negative charge on it. An ion forms the same number of ionic bonds as its valency.
The figure shows how the ionic compound magnesium chloride is formed from the elements magnesium and chlorine. Show the formation of the following ionic compounds from the corresponding elements using two methods namely, numerical and diagrammatic representation of the electronic configuration.
- K+F–, from 19K and 9F
- Ca2+O2- from 20Ca and 8O
2. Covalent Bond:
Generally, a covalent bond is formed when atoms of two elements having similar properties combine. Such atoms cannot exchange electrons. Instead, these atoms share electrons. The shared electrons become a common property of both the atoms and thereby the electron octet/duplet of both the atoms becomes complete. Let us first consider an example of the hydrogen molecule (H2).
We have seen in the chapter ‘Inside an Atom’ that a hydrogen atom contains one electron, its duplet is short of one electron and therefore the valency of hydrogen is one. The two atoms of hydrogen are identical and have similar tendencies therefore they share their electrons. As a result, the electron duplet of both the hydrogen atoms is complete and a chemical bond is formed between them. The chemical bond formed by the sharing of valence electrons of two atoms with each other is called a covalent bond. One covalent bond is formed by the sharing of two valence electrons.
The figure shows the formation of the H2 molecule from two hydrogen atoms, using a diagrammatic representation of the electronic configuration. A covalent bond between two atoms is also represented by a dash joining their symbols.
Now let us see how an H2O molecule of a covalent compound is formed from hydrogen and oxygen atoms. There are six electrons in the valence shell of the oxygen atom. It means that the electron octet in oxygen is short of two electrons and the valency of oxygen is ‘2’. In the H2O molecule, the oxygen atom completes its octet by forming two covalent bonds, one each with the two hydrogen atoms. While this happens, the duplets of the two hydrogen atoms also are completed.
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