The correct Answer is 1, 2, 3 and 4
Explanation
A chemical reaction involves the rearrangement of atoms and molecules, resulting in the formation of new substances with different properties. Several observable changes can indicate that a chemical reaction has occurred.
Statement-wise Analysis
- Statement 1: Change in state
Correct. A change in state (e.g., from liquid to gas, or solid to liquid) can indicate a chemical reaction. For instance, when hydrogen gas reacts with oxygen gas to form liquid water, a change in state occurs. While physical changes also involve changes in state, a chemical reaction often accompanies or causes such a change as new substances are formed.
- Statement 2: Change in colour
Correct. A change in colour is a common indicator of a chemical reaction. For example, when iron rusts, its colour changes from silvery-grey to reddish-brown. Similarly, the reaction between lead nitrate solution and potassium iodide solution produces a yellow precipitate of lead iodide.
- Statement 3: Evolution of a gas
Correct. The production of a gas (often observed as bubbles) is a strong indicator of a chemical reaction. For instance, when an acid reacts with a metal (e.g., zinc with dilute sulfuric acid), hydrogen gas is evolved. Similarly, when baking soda reacts with vinegar, carbon dioxide gas is produced.
- Statement 4: Change in temperature
Correct. Chemical reactions often involve energy changes, which manifest as a change in temperature. Exothermic reactions release heat, causing the temperature to rise, while endothermic reactions absorb heat, causing the temperature to drop. For example, the reaction between quicklime and water (slaking of lime) is highly exothermic, releasing significant heat.
Key Takeaway
Observable changes such as change in state, change in colour, evolution of a gas, and change in temperature are common indicators that a chemical reaction has taken place.