Physical and Chemical Changes - Class 7 Notes & Olympiad Questions

Physical Changes

a) Physical changes refer to alterations that occur in a substance without forming any new substances.
b) These changes involve modifications in the physical properties of the substance, such as its shape, size, colour, state (solid, liquid, gas), or temperature.
c) It's important to note that the original substance retains its identity throughout the process, and its chemical properties and composition remain unchanged.
d) In many cases, physical changes are reversible, meaning that the substance can return to its original state. However, it's worth mentioning that some physical changes may not be easily reversible or may require specific conditions to revert back.
e) Whether heat is released or absorbed during a physical change depends on the particular circumstances and is not a universal rule.

Some examples of physical changes that you may encounter in daily life are:

1. Changes in Size and Shape: Cutting a piece of paper into smaller squares or stretching a rubber band are examples of physical changes. Although the paper or rubber band may change in shape or size, they can still be brought back to their original forms.

2. Change in State: Melting ice to obtain water or boiling water to produce steam are examples of physical changes. These changes involve alterations in the state of a substance, from solid to liquid or from liquid to gas. The original substance can be recovered by reversing the process.

3. Some other examples: Observing the glow of an electric bulb, mixing sand with water, etc.

Chemical Changes

a) Chemical changes, also known as chemical reactions, involve the formation of one or more new substances that possess completely different properties compared to the original substance.
b) These changes result in the transformation of the chemical composition and structure of the substance involved.
c) Unlike physical changes, chemical changes are usually irreversible and permanent. The original substance undergoes alteration and loses its identity in the process.
d) Both the physical and chemical properties of the substance are modified during a chemical change.
e) Examples of chemical changes include the rusting of iron, extraction of iron from iron ore, cooking food, germination of a seed, and the ripening of fruit. These processes involve the formation of new substances with different properties, marking a chemical change.

Several indicators can accompany a chemical change, which include:

1. Radiation or absorption of heat: Heat may be released or absorbed during a chemical reaction, leading to a change in temperature.

2. Production of sound: Some chemical reactions may produce sound or noise as a byproduct.

3. Change in the colour of the substance: The colour of the substances involved may change during a chemical reaction, indicating the formation of new compounds.

4. Change in the smell of the substance: Certain chemical reactions result in the release of gases or compounds with distinct odours, leading to a change in smell.

5. Formation of a gas: The production of a gas, such as the release of bubbles, is often observed during chemical reactions.

6. Formation of a solid as residue: A chemical reaction may produce a solid residue or sediment, which settles at the bottom of the reaction mixture.

Examples of Chemical Changes

1. Burning Magnesium Ribbon

Burning a small strip of magnesium ribbon and observing its transformation into powdery ash demonstrates a chemical change. The burning process involves the combination of magnesium with oxygen from the air, forming magnesium oxide.

2. Reaction of Copper Sulphate and Iron

Mixing copper sulphate solution with iron and observing the colour change from blue to green, along with the formation of iron sulphate and copper, exemplifies a chemical change. New substances are produced during the reaction.

3. Vinegar and Baking Soda Reaction

Combining vinegar (acetic acid) and baking soda, resulting in the release of carbon dioxide gas and the formation of other substances, represents a chemical change. The production of bubbles and the change in colour and smell indicate a chemical reaction.

Different Types of Chemical Reactions

1. Combination Reaction: This type of reaction occurs when two or more substances combine to form a single compound.

It can be represented by the general equation: A + B → AB.

2. Decomposition Reaction: In a decomposition reaction, a single compound breaks down into two or more simpler substances, which can be elements or compounds.

The general equation for a decomposition reaction is AB → A + B.

3. Displacement Reaction: This type of reaction occurs when a more reactive element displaces a less reactive element from its compound.

The general equation for a displacement reaction is A + BC → AC + B.

4. Precipitation Reaction: In a precipitation reaction, two soluble compounds react to form an insoluble product, which is called a precipitate. It typically occurs when ions in solution combine to form an insoluble compound.

5. Neutralisation Reaction: A neutralisation reaction involves the reaction between an acid and a base to form salt and water.
The general equation for a neutralisation reaction is acid + base → salt + water.

6. Exothermic Reaction: An exothermic reaction releases energy in the form of heat or light during the reaction. It is often accompanied by a temperature increase in the surroundings. Combustion reactions, such as the burning of fuel, are examples of exothermic reactions.

7. Endothermic Reaction: In contrast to exothermic reactions, endothermic reactions absorb heat energy from the surroundings to proceed. They typically result in a decrease in temperature in the surroundings.

Rusting

a) Rusting of iron is a chemical change that occurs when iron comes into contact with oxygen and water. It results in the formation of a reddish-brown substance known as iron oxide, commonly referred to as rust.
b) The chemical equation for the rusting process can be represented as follows: 4Fe + 3O₂ → 2Fe₂O₃
c) The chemical formula for rust is Fe₂O₃.nH2O, indicating that rust is a hydrated form of iron oxide. The rate of rust formation is influenced by the amount of moisture present in the air. In environments with higher humidity or moisture levels, rust tends to form more quickly.

Methods to Prevent Rusting

Painting: Applying a layer of paint creates a barrier between the iron surface and the surrounding oxygen and moisture, preventing direct contact with the iron.

Oiling and greasing: Coating the iron surface with oil or grease helps to create a protective layer that inhibits the interaction of iron with oxygen and water.

Chromium plating: The process of chromium plating involves depositing a layer of chromium onto the iron surface, providing a protective coating that prevents rust formation.

Galvanising: Galvanisation is a specific method of rust prevention that involves coating the surface of iron with zinc. The zinc layer acts as a barrier, protecting the iron from direct contact with oxygen and moisture, thereby preventing rust formation.

Alloying: Mixing iron with other metals to form alloys, such as stainless steel, can enhance its resistance to rusting. Stainless steel, for example, contains chromium and nickel, which provide a protective layer and prevent rust formation.

Crystallisation

a) Crystallisation is a process that involves obtaining crystals of a substance from its solution. It is commonly used to purify substances and separate them from impurities.
b) During crystallisation, a solution containing the dissolved substance is allowed to cool or evaporate slowly under controlled conditions. As the temperature decreases or the solvent evaporates, the solute molecules come together and arrange themselves in a highly ordered, repeating pattern, forming solid crystals.
c) It is important to note that crystallisation is a physical change because it does not involve any chemical reactions. The original substance remains the same, and its chemical composition does not change during the process.
d) One practical application of crystallisation is the preparation of pure salt from seawater. Sea salt typically contains various impurities, including other minerals and substances dissolved in the water. By evaporating the water from the seawater solution, the salt molecules start to come together and form salt crystals. The impurities remain in the remaining liquid or can be separated from the crystals through filtration or other purification methods, resulting in the production of pure salt crystals.