Understanding Chemical and Physical Processes

Processes can be classified as either physical changes or chemical changes. A physical change alters the form or appearance of a substance but does not change its chemical composition. Examples include changes of state (like melting, boiling, condensation, evaporation), dissolving, and changes in size or shape.

A chemical change, also known as a chemical reaction, results in the formation of new substances with different chemical properties from the original substances. This involves the breaking and forming of chemical bonds.

Analyzing the Given Processes

Let's examine each of the given processes to determine whether it involves a chemical reaction:

• Evaporation of petrol: Evaporation is the process where a liquid turns into a gas or vapor. Petrol changes from a liquid state to a gaseous state. The chemical composition of petrol molecules remains the same; they are just further apart and moving faster in the gaseous state. This is a physical change, not a chemical reaction.
• Dissolution of salt in water: When salt (like sodium chloride, $\text{NaCl}$) dissolves in water ($\text{H}_2\text{O}$), the ionic compound $\text{NaCl}$ separates into sodium ions ($\text{Na}^+$) and chloride ions ($\text{Cl}^-$) which become surrounded by water molecules. The water and the salt ions are still present, and their chemical identities are retained (though the salt is now in dissociated form). If the water is evaporated, the salt will remain. No new chemical substances are formed. This is a physical process, commonly called dissolving or solvation.
• Storing of oxygen gas under pressure in a gas cylinder: Storing a gas under pressure changes its physical state (it might become liquid at high pressure) and its density. However, the oxygen molecules ($\text{O}_2$) themselves do not change into a different type of molecule. The chemical composition remains oxygen. This is a physical process involving changes in pressure and potentially state, but not a chemical reaction.
• Burning of magnesium ribbon in air: Burning is a chemical process that typically involves rapid reaction with oxygen, releasing heat and light. When magnesium ribbon burns in air, the magnesium metal ($\text{Mg}$) reacts with oxygen gas ($\text{O}_2$) from the air to form a new substance, magnesium oxide ($\text{MgO}$). The chemical equation for this reaction is:

  \(2\text{Mg(s)} + \text{O}_2\text{(g)} \rightarrow 2\text{MgO(s)}\)

  Magnesium oxide is a white powder with chemical properties entirely different from magnesium and oxygen. The formation of a new substance indicates that a chemical reaction has occurred.

Conclusion: Identifying the Chemical Reaction

Based on the analysis, the only process among the given options that involves the formation of new chemical substances is the burning of magnesium ribbon in air. This process is a clear example of a chemical reaction.