Question

Is each of the following very soluble in water? Explain. (a) Benzene, \(\mathrm{C}_{6} \mathrm{H}_{6}\) (b) Sodium hydroxide (c) Ethanol, \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\) (d) Potassium acetate 4.15 Is cach of the following very soluble in water? Explain. (a) Lithium nitrate (b) Gilycine, \(\mathrm{H}_{2} \mathrm{NCH}_{2} \mathrm{COOH}\) (c) Pentane (d) Ethylene glycol, \(\mathrm{HOCH}_{2} \mathrm{CH}_{2} \mathrm{OH}\)

Short answer

Benzene and pentane are not very soluble in water. Sodium hydroxide, ethanol, potassium acetate, lithium nitrate, glycine, and ethylene glycol are very soluble in water.

Step-by-step solution

- Benzene, \(\mathrm{C}_{6} \mathrm{H}_{6}\)

Benzene (\(\mathrm{C}_{6}\mathrm{H}_{6}\)) is a non-polar molecule due to its symmetrical structure. Water is a polar solvent, and according to the principle 'like dissolves like,' non-polar substances do not dissolve well in polar solvents. Therefore, benzene is not very soluble in water.

- Sodium hydroxide

Sodium hydroxide (NaOH) is an ionic compound. Ionic compounds typically dissolve very well in water due to the electrostatic interactions between the ions and the polar water molecules. Thus, sodium hydroxide is very soluble in water.

- Ethanol, \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\)

Ethanol (\(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{OH}\)) has both a polar hydroxyl (OH) group and a non-polar ethyl group. The polar OH group allows ethanol to form hydrogen bonds with water, making ethanol very soluble in water.

- Potassium acetate

Potassium acetate (KCH\(\mathrm{O}_2\)) is an ionic compound. Ionic compounds dissolve well in polar solvents like water due to the strong attraction between the ions and water molecules. Consequently, potassium acetate is very soluble in water.

- Lithium nitrate

Lithium nitrate (LiNO\(\mathrm{_3}\)) is an ionic compound. Ionic substances generally dissolve well in water because the ions interact strongly with water molecules. As a result, lithium nitrate is very soluble in water.

- Glycine, \(\mathrm{H}_{2} \mathrm{NCH}_{2} \mathrm{COOH}\)

Glycine (\(\mathrm{H}_{2}\mathrm{NCH}_{2}\mathrm{COOH}\)) is an amino acid that contains both amino and carboxyl groups, which can form hydrogen bonds with water. This property makes glycine very soluble in water.

- Pentane

Pentane (C\(\_5\)H\(\_{12}\)) is a non-polar hydrocarbon. Non-polar substances do not dissolve well in polar solvents like water. Hence, pentane is not very soluble in water.

- Ethylene glycol, \(\mathrm{HOCH}_{2} \mathrm{CH}_{2} \mathrm{OH}\)

Ethylene glycol (\(\mathrm{HOCH}_{2}\mathrm{CH}_{2}\mathrm{OH}\)) has two hydroxyl (OH) groups, which can form multiple hydrogen bonds with water molecules. This makes ethylene glycol very soluble in water.

Key concepts

Non-polar Molecules

Non-polar molecules do not have a significant charge separation. This means their electrons are distributed more or less evenly. A good example is benzene \((\text{C}_6\text{H}_6)\), which consists of a ring of carbon and hydrogen atoms. Because it has a symmetrical structure, its electrons are evenly shared. This makes benzene non-polar.
In chemistry, the principle 'like dissolves like' helps predict solubility. Non-polar molecules do not mix well with polar solvents like water, which has partial positive and negative charges. So, non-polar substances such as benzene and pentane are not very soluble in water.
To summarize, non-polar molecules do not interact favorably with water molecules. As a result, they do not dissolve well in water.

Ionic Compounds

Ionic compounds are made up of positively and negatively charged ions. These ions are held together by strong electrostatic forces, known as ionic bonds. Sodium hydroxide \((\text{NaOH})\), lithium nitrate \((\text{LiNO}_3)\), and potassium acetate \((\text{KCH}_3\text{O}_2)\) are all examples of ionic compounds.
Water is a polar solvent, meaning it has partial positive and negative charges. These charges allow it to interact effectively with the ions in an ionic compound. The positive and negative ends of water molecules attract the respective ions, helping to separate and dissolve them.
This process makes ionic compounds generally very soluble in water. Even those with complex structures, like lithium nitrate and potassium acetate, dissolve well due to these electrostatic interactions with water.

Hydrogen Bonding

Hydrogen bonding occurs when a hydrogen atom, which is covalently bonded to a highly electronegative atom like oxygen or nitrogen, experiences an attraction to another electronegative atom. This kind of bonding is critical for the solubility of many organic molecules in water.

• Ethanol \((\text{CH}_3\text{CH}_2\text{OH})\) has a polar hydroxyl group \((\text{-OH})\).
• Glycine \((\text{H}_2 \text{NCH}_2 \text{COOH})\) has both amino \((\text{-NH}_2)\) and carboxyl groups \((\text{-COOH})\).
• Ethylene glycol \((\text{HOCH}_2 \text{CH}_2 \text{OH})\) has two hydroxyl groups \((\text{-OH})\).

All these groups can form hydrogen bonds with water molecules, significantly increasing their solubility. Hydrogen bonds are a specific type of dipole-dipole interaction that is stronger and more specific. Because water molecules can form several hydrogen bonds, they effectively 'surround' and interact with these solute molecules, facilitating their dissolution in water.