Problem 62 - Master Resource Book in JEE Main Physics

Question

When a pinch of salt or any other salt which is soluble in water is added to water, its surface tension (a) increases (b) decreases (c) may increase or decrease depending upon salt (d) None of the above

Step-by-Step Solution

Answer

(a) increases

Step 1: Understand Surface Tension

Surface tension is a physical property that refers to the force per unit length acting along a liquid's surface, resulting from intermolecular forces. It causes the liquid surface to behave like a stretched elastic sheet.

Step 2: Consider the Role of Salt

When salt is dissolved in water, the salt dissociates into its constituent ions. This can affect the structure and intermolecular forces within the water, which in turn can impact surface tension.

Step 3: Analyze the Effect of Ions

The dissolved ions typically increase the number of interactions within the water, as ions attract water molecules and create a more structured water network. This generally leads to an increase in surface tension.

Step 4: Evaluate the Correct Option

Given that the addition of a soluble salt usually increases the structured interactions in water, it most often leads to a net increase in surface tension. Therefore, the answer is (a) increases.

Key Concepts

Intermolecular Forces

Intermolecular forces are the interactions that occur between molecules, playing a crucial role in determining many physical properties of substances. These forces include hydrogen bonds, van der Waals forces, and dipole-dipole interactions. In the context of surface tension, these forces are responsible for the cohesive behavior of water molecules at the surface.
When a soluble salt is added to water, ionic compounds dissociate into cations and anions. These ions influence the existing intermolecular forces among water molecules. Specifically, the ions attract water molecules due to electrostatic forces, enhancing the overall interaction network within the liquid.

• Hydrogen bonding becomes more pronounced as water molecules are drawn closer to ions.
• This increased interaction can strengthen the surface layer, impacting the surface tension.

Thus, understanding how intermolecular forces work is essential to explain the changes in surface tension observed upon salt dissolution.

Dissolution of Salts

The process of dissolution is where solute particles, like salt, disperse throughout the solvent, which is water in this case. Dissolution occurs when the ionic bonds in the salt are broken, allowing the ions to separate and interact with water molecules.
Once dissolved, the ions in the solution cause water molecules to rearrange. This rearrangement strengthens the network of hydrogen bonds and electrostatic interactions, influenced by the charge of the ions. This effectively impacts physical properties such as surface tension.

• Sodium and chloride ions, upon dissociation, influence water molecules, establishing a more ordered structure.
• This order increases the number of strong interactions at the liquid’s surface.

The dissolution of salts into water demonstrates a fascinating interaction between molecular forces and structural hierarchy in liquid solutions.

Water Structure

Water's structure is unique due to its polar nature and ability to form hydrogen bonds. Each water molecule consists of one oxygen atom bonded to two hydrogen atoms, creating a bent configuration with a partial negative charge on oxygen and a partial positive charge on hydrogens. This creates a dynamic, yet highly structured, liquid state.
When salt dissolves, the ions affect water's structure by inducing more interactions via ionic forces. As a result, water molecules align themselves around ions, enhancing the hydrogen bond network.

• The ions essentially "guide" water molecules into a tighter, more ordered configuration.
• This structured network increases resistance to external forces, like those acting at the surface, hence influencing the surface tension.

Understanding water's structural response to solute ions is key to comprehending changes in properties such as surface tension when salt is dissolved.