Problem 20 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

Which of the following is soluble in water (a) \(\mathrm{CS}_{2}\) (b) \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}\) (c) \(\mathrm{CCl}_{4}\) (d) \(\mathrm{CHCl}_{3}\)

Step-by-Step Solution

Answer

\(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}\) is soluble in water.

Step 1: Understand Polar and Non-Polar Solubility

Review the rule 'like dissolves like,' which means polar substances dissolve in polar solvents, and non-polar substances dissolve in non-polar solvents. Water is a polar solvent.

Step 2: Analyze Structure of Each Compound

Look at the chemical makeup of each option: (a) \(\mathrm{CS}_{2}\) is a non-polar molecule. (b) \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}\) is polar due to the \(-OH\) group. (c) \(\mathrm{CCl}_{4}\) is non-polar. (d) \(\mathrm{CHCl}_{3}\) is slightly polar but less so than \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}\).

Step 3: Determine Solubility in Water

Since water is a polar solvent, it will dissolve polar substances. Among the given options, \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}\) is highly polar and will be soluble in water.

Key Concepts

Polar and Non-Polar Substances

In chemistry, the distinction between polar and non-polar substances is crucial for understanding solubility. Molecules are classified as either polar or non-polar based on their distribution of electrical charge.

• Polar substances have an uneven distribution of electrons, leading to a molecule with a positive end and a negative end. This is often due to certain functional groups like hydroxyl (\(-OH\)) groups , as seen in ethanol ( \(\mathrm{C}_{2}\mathrm{H}_{5}\mathrm{OH}\) ).
• Non-polar substances, meanwhile, have electrons distributed more symmetrically, resulting in no distinct electrical poles. Molecules such as carbon disulfide (\(\mathrm{CS}_{2}\)) and carbon tetrachloride (\(\mathrm{CCl}_{4}\)) fall into this category.

Understanding the polarity of a substance helps predict its solubility behavior. Polar molecules can attract other polar molecules, whereas non-polar molecules stick together. This behavior plays a significant role in chemical processes.

Like Dissolves Like

The principle of "like dissolves like" is a helpful guideline in chemistry to predict solubility. Essentially, it indicates that polar solvents are more effective at dissolving polar solutes, while non-polar solvents dissolve non-polar solutes. This concept arises from the interactions between molecules. When a solvent and a solute are similar in polarity, they can interact more effectively. Polar molecules often engage in hydrogen bonding or dipole-dipole interactions, both of which increase solubility. For instance, in our exercise example, ethanol (\(\mathrm{C}_{2}\mathrm{H}_{5}\mathrm{OH}\)) is soluble in water because both are polar. Their interactions are enhanced by the hydrogen bonds formed between the \(-OH\) groups in ethanol and water, making them "like" each other. On the other hand, non-polar substances like \(\mathrm{CS}_{2}\) (and often \(\mathrm{CCl}_{4}\) )do not dissolve in water as there are minimal attractions between them.

Water as a Solvent

Water is known as the "universal solvent" due to its remarkable capacity to dissolve a wide range of substances. This ability is largely influenced by its polar nature.

• Each water molecule has a bent shape, with a partial positive charge near the hydrogen atoms and a partial negative charge near the oxygen atom. This leads to strong hydrogen bonding capabilities.
• Such interactions allow water to pull apart the components of polar solutes, effectively leading to dissolution. This is why polar substances like salt or alcohol are easily dissolved in water.

It's important to note that while water excels at dissolving polar compounds, it is not effective with non-polar substances. Non-polar molecules like oils tend to repel water, leading to insolubility. This is why oil and water do not mix well. Understanding these principles helps in explaining why certain compounds dissolve in water and others do not.