Question

Predict whether each of the following solvents is miscible or immiscible with water. (a) ethanol, \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}\) (b) toluene, \(\mathrm{C}_{7} \mathrm{H}_{8}\) (c) trichloroethylene, \(\mathrm{C}_{2} \mathrm{HCl}_{3}\) (d) acetic acid, \(\mathrm{CH}_{3} \mathrm{COOH}\)

Short answer

(a) Miscible, (b) Immiscible, (c) Immiscible, (d) Miscible.

Step-by-step solution

Understanding Miscibility

Miscibility refers to the ability of two substances to mix in all proportions, forming a homogeneous solution. Water is a polar solvent, so it tends to be miscible with other polar substances or those capable of hydrogen bonding.

Evaluating Ethanol

Ethanol (C_{2} H_{5} OH) is a polar molecule with an OH group that can form hydrogen bonds with water, making it miscible with water.

Evaluating Toluene

Toluene (C_{7} H_{8}) is a nonpolar hydrocarbon. Nonpolar compounds do not mix well with polar solvents like water, so toluene is immiscible with water.

Evaluating Trichloroethylene

Trichloroethylene (C_{2} HCl_{3}) is mostly nonpolar due to the presence of chlorine atoms and lacks the ability to form hydrogen bonds, making it immiscible with water.

Evaluating Acetic Acid

Acetic acid (CH_{3} COOH) is polar and contains an OH group, allowing it to form hydrogen bonds with water, so it is miscible with water.

Key concepts

Polar Solvents

Polar solvents are a type of solvent characterized by molecules that have a separation of electric charge leading to partial positive and negative ends, or poles. This occurs due to the difference in electronegativity between the atoms that form the solvent molecule. Water is a classic example of a polar solvent. This polarity allows water molecules to interact and form hydrogen bonds with other polar molecules, which is why water is so effective at dissolving substances like salts and sugars.

When it comes to miscibility, polar solvents tend to mix well with other polar solvents. This is due to the 'like dissolves like' principle, which states that similar types of solvents (in terms of polarity) will dissolve in each other.

• Ethanol is a polar solvent as it has an OH group that makes it capable of hydrogen bonding, thus mixing well with water.
• Acetic acid is another polar solvent due to its ability to form hydrogen bonds, leading to high miscibility with water.

Hydrogen Bonding

Hydrogen bonding is a special type of interaction that occurs when a hydrogen atom, covalently bonded to a highly electronegative atom like oxygen, nitrogen, or fluorine, interacts with another electronegative atom. This creates a bridge between molecules, greatly influencing properties such as boiling and melting points, solubility, and miscibility.

In the context of miscibility, hydrogen bonding is a key factor. It allows molecules to interact more strongly, enabling the formation of homogeneous solutions. For example:

• Ethanol can form hydrogen bonds with water, thanks to its OH group, making it highly miscible.
• Acetic acid also forms hydrogen bonds with water, which helps it to dissolve and mix evenly.

Understanding this interaction helps explain why certain substances mix while others do not.

Nonpolar Compounds

Nonpolar compounds are composed of molecules with no separation of electric charge, resulting in no distinct poles. These molecules have similar electronegative atoms that share electrons fairly equally in their bonds. This lack of charge differential makes these substances unable to form hydrogen bonds with water, a polar solvent.

Nonpolar compounds generally do not dissolve well in water, adhering again to the principle 'like dissolves like'. They do, however, mix well with other nonpolar substances. In examples from the exercise:

• Toluene is a nonpolar hydrocarbon, leading it to be immiscible with water as it cannot form hydrogen bonds.
• Trichloroethylene is also mostly nonpolar because of its structure; despite having some electronegative elements, it fails to form hydrogen bonds, so it doesn't mix with water.

This helps explain why certain substances are immiscible with polar solvents like water.