Physical and Chemical Changes
Chapter: Physical and Chemical Changes
Understanding Basic Definitions
- A Physical Change involves a change in the form of a substance, not in its chemical composition. Substances retain their original properties during physical change.
- Chemical Change, on the other hand, alters the substance’s chemical composition, leading to the formation of an entirely new substance with different properties.
- Changes in state—like melting, freezing, vaporisation, and condensation—are examples of physical changes.
- Burning, rusting, and digestion are examples of chemical changes.
Identifying Chemical Changes
- Chemical changes are often accompanied by an energy change. Energy is either absorbed (endothermic reaction) or released (exothermic reaction).
- Observing a colour change, the formation of a gas (often seen as bubbles), or the formation of a precipitate (solid) can indicate that a chemical reaction has occurred.
- A souring of milk or baking of bread entails chemical changes.
Identifying Physical Changes
- The characteristics of a physical change include change in shape, size, and state of matter. These changes are reversible.
- For instance, water can change from liquid (water) to solid (ice) when the temperature drops, and then to gas (steam) when the temperature increases.
- Crumpling of paper, slicing of an apple, and breaking of a glass are all examples of physical changes.
Chemical Reactions
- In a chemical reaction, the substances that react are called reactants and the new substances formed are called products.
- A chemical reaction involves the breaking of old bonds in the reactants to form new bonds in the products.
- Chemical reactions are represented by chemical equations.
Conservation of Mass during Changes
- The Law of Conservation of Mass states that mass is neither created nor destroyed during chemical or physical changes.
- It means the total mass of reactants involved in a reaction will always equal the total mass of products formed.
- In terms of equations, the number of atoms of each element on the reactants’ side of a chemical equation must equal the number of atoms of those elements on the products’ side.
Energy Changes
- Chemical reactions often involve energy changes. Energy is either taken in (endothermic) or given out (exothermic) during a chemical reaction.
- For example, photosynthesis is an endothermic reaction as plants absorb light energy to convert carbon dioxide and water into glucose and oxygen.
- The combustion of fuels (like coal, petrol, etc.) is an example of an exothermic reaction as energy is released during the reaction.
Importance of Understanding Chemical and Physical Changes
- Understanding the difference between physical and chemical changes helps us recognise and understand the numerous changes we observe in our everyday life.
- It aids in predicting the properties of substances involved in changes and thus plays a crucial role in chemical industries, environmental science, and pharmacology.