Problem 18 - A Complete Resource Book in Chemistry for JEE Main

Question

A saturated solution of non-radioactive sugar was taken and a little radioactive sugar was added to it. A small amount of it gets dissolved in solution and an equal amount of sugar was precipitated. This proves (a) the equilibrium has been established in the solution (b) radioactive sugar can displace non-radioactive sugar from its solution. (c) Equilibrium is dynamic in nature (d) none of the above

Step-by-Step Solution

Answer

The correct answer is (c) Equilibrium is dynamic in nature.

Step 1: Understanding the Scenario

We have a saturated solution of sugar where the amount of sugar dissolved in water is at its maximum. Upon addition of radioactive sugar, some amount dissolves while an equal amount precipitates out. This scenario involves the dissolution and precipitation process, indicating a state of balance.

Step 2: Applying Concepts of Equilibrium

In a saturated solution, the rates of dissolution and precipitation are equal when at equilibrium. Adding radioactive sugar doesn't change the overall concentration of dissolved sugar, but indicates the ongoing exchange between the dissolved and undissolved sugar components.

Step 3: Analyzing the Nature of Equilibrium

The dynamic nature of equilibrium implies that while the system appears static, microscopic exchanges (like molecules dissolving and precipitating) are constantly happening. The presence of radioactive sugar in both states confirms this dynamic exchange.

Step 4: Interpreting the Choices

(a) Equilibrium has been established because the solution manages to maintain equal rates of dissolution and precipitation with added sugar. (b) is incorrect because it does not lead to a displacement but a balance. (c) is correct because the presence of radioactive sugar in solution and the precipitated phase shows continuous dynamic equilibrium. (d) is incorrect as both (a) and (c) are viable conclusions.

Key Concepts

Saturated Solution

A saturated solution is a mixture in which no more solute can dissolve at a given temperature. It carries the maximum concentration of the solute in the given amount of solvent. Any additional solute will not dissolve unless the conditions of the solution, such as temperature or pressure, change.

When a solution is saturated, it reaches a balance known as dynamic equilibrium. This means the rate at which solute molecules dissolve in the solvent is equal to the rate at which they crystallize and return to the solid form.

• This balance ensures that the solution remains at a consistent concentration.
• The amount of undissolved solute remains constant despite the visible changes.

In our exercise, when radioactive sugar was added to a saturated sugar solution, some dissolved while an equal amount precipitated out, indicating that the solution was already at saturation and maintained this balance upon addition.

Dynamic Equilibrium

Dynamic equilibrium is a state in which a chemical system no longer changes in a macroscopic sense, even though microscopic processes continue to occur.

It occurs when the rates of the forward and reverse reactions are equal. The system may look like nothing's happening, but there's constant movement at the molecular level.

• This concept is essential in chemical reactions where reactants products seem to cease exchange but are actually in continuous exchange.
• In the saturation scenario, molecules from the solid form are dissolving and precipitating back at equal rates.

In our example, the addition of radioactive sugar helped visualize this dynamic nature. The dissolved and precipitated forms of sugar were constantly interchanging, upholding the equilibrium despite external changes.

Radioactive Isotopes

Radioactive isotopes are atoms with an unstable nucleus that emit radiation as they decay into more stable forms. They are used for tracing processes due to their detectable radiation emissions.

In chemistry, radioactive isotopes can help provide insight into dynamic equilibrium and reaction mechanisms, as their movement and participation in reactions can be detected:

• They allow non-visible processes to be observed by emitting detectable energy or particles.
• This property is crucial in confirming hypotheses in scientific investigations about chemical and physical transformations.

In the scenario from the exercise, the radioactive sugar functioned as a traceable marker, helping to demonstrate the ever-active nature of dynamic equilibrium by showing participation in both the soluble and precipitated phases of the solution.