ICSE Class 7 Chemistry Chapter 02 Physical and Chemical Changes

Physical and Chemical Changes

In the study of chemistry we come across two kinds of change - physical and chemical.

Physical Changes

A change in which no new substances are formed and which can be reversed by reversing the conditions is called a physical change.

The melting of ice, the freezing of water and the glowing of an electric bulb are some common examples of physical change. These changes are reversed by reversing the conditions. For example, ice when heated gives water, but water when cooled gives back ice. Similarly, on switching on the current, a bulb glows, but on switching off the current, the glow vanishes.

In all these cases, it is obvious that no new substances are formed and the mass of the substance also does not change.

Chemical Changes

A change in which new substances are formed and which cannot be reversed by reversing the conditions is called a chemical change.

The burning of coal or wood, the rusting of iron, the curdling of milk, the charring of sugar, photosynthesis in plants and the digestion of food are common examples of chemical change. All these changes are permanent. Coal when burnt gives carbon dioxide. But the carbon dioxide when cooled does not give back coal. When sugar is heated, it turns brown and ultimately gets charred. (See Figure 2.1.) If you cool charred sugar, you cannot get back sugar in its original form. Once milk forms curd, the curd cannot give back milk. You cannot reverse any of these changes by reversing the conditions. In every case, new substances are formed. As a result, the mass of any individual substance changes. The mass of coal decreases when it burns and that of iron increases when it rusts. However, we will soon learn that the total mass of all the substances taking part in a chemical change remains the same before and after the change.

Teacher's Note

When you boil water for tea, you see ice melting or water evaporating - these are physical changes happening in your kitchen every day. But when you burn wood in a fireplace or toast bread, those are chemical changes that create entirely new materials you cannot reverse.

Matter can neither be created nor destroyed, so the total quantity of matter, i.e., the total mass, before and after a change remains the same.

If you take 1 g of hydrogen and 8 g of oxygen and kindle the mixture, 9(= 1 + 8) g of water will be formed. All the hydrogen and oxygen will appear to have disappeared. But never think that they are destroyed. They have actually been changed into a new substance, water, which is different from hydrogen or oxygen.

Atoms rearrange themselves during a chemical change

Why are new substances formed in a chemical change? It is because the atoms rearrange themselves in such a change.

When burnt in air, carbon forms carbon dioxide

During the burning of coal, the carbon of coal combines with the oxygen of air to form carbon dioxide.

Carbon + Oxygen (molecules) → Carbon dioxide (molecules)

When burnt in air, hydrogen forms water

Again, the atoms contained in the molecules of hydrogen and oxygen rearrange themselves.

Hydrogen (molecules) + Oxygen (molecules) → Water (molecules)

If you could count the atoms of each kind before and after the chemical change, you would find that the number of atoms of each element remains the same.

So, the mass of the substances, taken together, before and after a chemical change also remains the same. This is in accordance with the law of conservation of mass, which is stated as follows.

Chemical Changes are Represented by Equations

You know that an element is represented by a symbol and a compound, by a formula. A chemical change is represented by an equation called a chemical equation. For example, the burning of carbon in oxygen to form carbon dioxide is represented by the following equation.

\[C + O_2 \rightarrow CO_2\]

The substances that react among themselves are called reactants and those that are formed are called products.

Remember that no atoms are lost or gained in a chemical reaction - the atoms only rearrange themselves.

A chemical equation in which the number of atoms of each element on the reactant side is the same as that on the product side is called a balanced chemical equation.

For example, the equation written above for the formation of carbon dioxide is a balanced chemical equation.

How to balance a chemical equation

You can balance an equation in the following steps.

Step 1: Write down the symbols and formulae of the reactants on the left-hand side and those of the products on the right-hand side of an arrow.

Step 2: Count the atoms of each kind on both sides of the arrow.

Step 3: Make the number of atoms of each kind equal on both sides by using proper coefficients.

Showing gases and precipitates on the product side by arrows

Any gases evolved in a reaction are shown by arrows pointing upwards in the equation and any precipitates, by arrows pointing downwards.

\[CaCO_3 \xrightarrow{\text{heat}} CaO + CO_2 \uparrow\]

\[BaCl_2 + Na_2SO_4 \rightarrow BaSO_4 \downarrow +2NaCl\]

Example 1: Hydrogen combines with chlorine to form hydrogen chloride. Write a balanced chemical equation for the reaction.

Solution

Step 1: The reactants and the products are written as follows.

\[H_2 + Cl_2 \rightarrow HCl\]

Step 2: The atom count shows that the equation is not balanced.

Step 3: Place the coefficient 2 before the product.

\[H_2 + Cl_2 \rightarrow 2HCl\]

Step 4: The numbers of atoms on the two sides are now as follows.

Therefore, the balanced chemical equation for the given reaction is

\[H_2 + Cl_2 \rightarrow 2HCl\]

Example 2: Hydrogen combines with oxygen to form water. Write a balanced chemical equation for the reaction.

Solution

Step 1: Write down the reactants and the products.

\[H_2 + O_2 \rightarrow H_2O\]

Step 2: Count the number of atoms of each element on both sides.

The atom count shows that the equation is not balanced.

Step 3: To balance the number of oxygen atoms on both sides, place the coefficient 2 before H2O.

\[H_2 + O_2 \rightarrow 2H_2O\]

Step 4: Again count the atoms of each element on both sides.

The atom count shows that the equation is still unbalanced.

Step 5: To balance the number of hydrogen atoms on both sides, place the coefficient 2 before H2 on the LHS.

\[2H_2 + O_2 \rightarrow 2H_2O\]

Step 6: Tally the number of atoms of each element on both sides.

The numbers tally. Therefore, the balanced chemical equation for the reaction is

\[2H_2 + O_2 \rightarrow 2H_2O\]

Teacher's Note

When you write a recipe, you need specific amounts of each ingredient to make the dish turn out right - balancing chemical equations works the same way, ensuring the right "recipe" of atoms for the reaction.

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