Problem 160 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following substances will precipitate the negatively changed emulsions? (a) Glucose (b) Urea (c) \(\mathrm{NaCl}\) (d) \(\mathrm{KCl}\)

Step-by-Step Solution

Answer

Substances (c) NaCl and (d) KCl will precipitate the negatively charged emulsions.

Step 1: Understand the Problem

The problem asks which substance will cause a negatively charged emulsion to precipitate. These are typically colloids with small particles suspended and stabilized by charge.

Step 2: Clarify the Concept of Precipitation

Precipitation in this context means the aggregation of particles in a colloid, causing them to settle out of the dispersion. This can happen when the charges that stabilize an emulsion are neutralized.

Step 3: Identify Substances Affecting Charge

Among the given substances, ionic compounds like NaCl and KCl can dissociate into ions in solution. These ions can potentially neutralize the charge causing an emulsion to precipitate.

Step 4: Analyze the Effect of Ions

NaCl dissociates into Na⁺ and Cl⁻, and KCl dissociates into K⁺ and Cl⁻. These ions can neutralize the negative charge on the emulsions, leading to precipitation.

Step 5: Consider Nonionic Substances

Glucose and urea are covalent compounds and do not dissociate into ions in solution. They will not affect the charge of the emulsion and thus do not lead to precipitation.

Step 6: Conclusion

Given the ability of Na⁺ and K⁺ ions to neutralize the negatively charged particles, either NaCl or KCl will cause precipitation of the emulsion.

Key Concepts

Colloids

Colloids are fascinating mixtures where microscopic particles are dispersed throughout a medium, but they don't settle out like in a suspension. The particles in a colloid are larger than in a solution but too small to settle out.
Think about milk or fog; these are everyday examples of colloids. The particles, made up of clusters of molecules or atoms, remain suspended due to their small size and interactions with the surrounding molecules. Each colloidal particle carries a charge that helps it stay suspended by repelling other particles, preventing them from clumping together.
In the context of precipitation, these charged colloids can be stabilized by the electrical charges on their surfaces, often gained from adsorbed ions or from their own ionic nature. The stability of a colloid can be disrupted when the charges are neutralized, leading to precipitation.

Ions and Charges

Ions are charged particles that form when atoms gain or lose electrons. They are critical in many chemical processes, including the precipitation of colloids. When a colloid needs to be stabilized or destabilized, ions play the role of adjusting surface charges.

• Cations are positively charged ions, like Na⁺ and K⁺, and play a role in neutralizing negative charges on colloidal particles.
• Anions are negatively charged ions, like Cl⁻, and they balance the charge of cations in ionic compounds.

In emulsions, negatively charged particles can be neutralized through the introduction of cations, leading to reduced repulsion among particles and leading to potential aggregation and precipitation.
This is often achieved by adding salts, which dissociate into ions and disrupt the electrostatic stability of the colloid.

Chemical Precipitation

Chemical precipitation is the process where dissolved ions come together to form solid particles that settle out of the solution. In colloidal emulsions, this is a crucial step where particles aggregate to lose their dispersion.
A precipitate forms when the charges stabilizing the colloid are neutralized, often by oppositely charged ions. This process causes the suspended particles to clump together and form larger aggregates that can settle. Neutralizing the charges leads to decreased repulsion between particles, allowing them to bond.
In the context of negatively charged emulsions, the addition of cations like Na⁺ or K⁺, which are provided by ionic compounds such as NaCl or KCl, can destabilize the colloid and result in precipitation. This change is an excellent example of how small ion exchanges can significantly impact the stability of colloids.

Ionic Compounds

Ionic compounds are made up of positive and negative ions bonded together by strong electrostatic forces. These materials can dissolve in water, releasing ions that influence chemical reactions, including precipitation.

• NaCl (table salt) dissolves into Na⁺ and Cl⁻ ions.
• KCl dissolves into K⁺ and Cl⁻ ions.

The ability of these compounds to dissociate into ions makes them perfect candidates to alter the charge balance in a colloid.
When added to a colloid, the ions from these compounds can interact with the charged particles of the emulsion, neutralizing their charge and resulting in the precipitate formation. This is a fundamental principle in chemistry, showcasing how ionic compounds are not just simple salts but are active players in many reactions and processes.