Which substance is commonly obtained by the evaporation of seawater in salt pans?

Introduction / Context:
This question links everyday experience with basic chemistry and natural resources. Many coastal regions produce a common substance by evaporating seawater in shallow pans under the sun. Understanding what remains after water evaporates from seawater helps you connect ideas about solutions, evaporation and crystallisation to real industrial processes.

Given Data / Assumptions:

• Seawater is evaporated in large open pans.
• The process leaves behind a solid substance that people collect.
• Options include sugar, iron, salt, steel and petrol.
• We assume natural seawater with its typical dissolved salts.

Concept / Approach:
Seawater is a solution containing various dissolved salts, primarily sodium chloride, along with smaller amounts of magnesium, calcium and other ions. When seawater is evaporated, water vapour leaves and the dissolved salts crystallise out. The main product collected from salt pans is common salt, sodium chloride. Therefore, the correct answer is salt, not sugar, metals such as iron or steel, or fuels like petrol.

Step-by-Step Solution:
Step 1: Recall the composition of seawater. It contains water as the solvent and many dissolved salts, mainly sodium chloride. Step 2: When seawater is placed in shallow pans and exposed to sun and wind, water gradually evaporates. Step 3: As water evaporates, the concentration of dissolved salts increases until they begin to crystallise and separate as solids. Step 4: The primary crystalline solid obtained is common salt (sodium chloride), which is harvested and further purified for use. Step 5: Sugar is obtained from sugarcane or sugar beet, not from seawater. Step 6: Iron and steel are metallic materials produced from iron ore in blast furnaces and steel plants, not by evaporating seawater. Step 7: Petrol is a liquid hydrocarbon mixture refined from crude oil, again unrelated to evaporation of seawater. Step 8: Therefore, the correct answer is salt.

Verification / Alternative check:
In geography and chemistry lessons, the process of salt making in coastal salt pans is often described. Photographs show large white fields of salt crystals after seawater has evaporated. The collected salt is then transported to refineries. This process is a straightforward example of separation of a solute from a solution by evaporation and crystallisation. No other listed substances are typically associated with this method of production from seawater.

Why Other Options Are Wrong:
Sugar is obtained by extracting and crystallising sucrose from sugarcane juice or beet juice, not from seawater. Iron and steel are produced industrially from ores through smelting and alloying, processes that involve high temperatures and chemical reduction, not simple evaporation. Petrol is refined from crude oil in petroleum refineries. None of these processes involve evaporating seawater in salt pans, so these options cannot be correct.

Common Pitfalls:
The main risk in such a straightforward question is overthinking and suspecting a trick where none exists. Some students may briefly wonder about minerals other than salt in seawater, but the exam expects the familiar answer common salt. Remember that in basic science questions, simple real life examples like sea salt production are frequently used to illustrate concepts such as solubility and evaporation.

Final Answer:
By evaporating seawater in salt pans, the substance commonly obtained is salt.