Question

Write a chemical equation that describes the dissolving of the following substances in water: (a) solid calcium hydroxide, \(\mathrm{Ca}(\mathrm{OH})_{2}\) (b) nitrogen gas, \(\mathrm{N}_{2}\) (c) liquid methanol, \(\mathrm{CH}_{3} \mathrm{OH}\)

Short answer

The equations for the dissolving of the mentioned substances in water are as follows: (a) \(\mathrm{Ca}(\mathrm{OH})_{2} \mathrm{(s)} \rightarrow \mathrm{Ca}^{2+} (\mathrm{aq}) + 2\mathrm{OH}^- (\mathrm{aq})\), (b) \(\mathrm{N}_{2} \mathrm{(g)} \rightarrow \mathrm{N}_{2}\mathrm{(g)}\), and (c) \(\mathrm{CH}_{3} \mathrm{OH} \mathrm{(l)} \rightarrow \mathrm{CH}_{3} \mathrm{OH} \mathrm{(aq)}\).

Step-by-step solution

Dissolving of calcium hydroxide

Calcium hydroxide is an ionic compound and soluble in water. When it dissolves, it disassociates into its ions. So, the equation of its dissolving in water will be: \(\mathrm{Ca}(\mathrm{OH})_{2} \mathrm{(s)} \rightarrow \mathrm{Ca}^{2+} (\mathrm{aq}) + 2\mathrm{OH}^- (\mathrm{aq})\)

Dissolving of nitrogen gas

Nitrogen gas is a non-polar molecule and doesn't react or dissolve in water. So, we can say that nitrogen stays as it is even if we try to dissolve it in water. Hence, the equation remains: \(\mathrm{N}_{2} \mathrm{(g)} \rightarrow \mathrm{N}_{2}\mathrm{(g)} \)

Dissolving of methanol

Methanol, a type of alcohol, dissolves in water due to the polar nature of both. But they don't undergo a chemical reaction upon dissolving so the structure of methanol remains intact. Therefore, the equation of the process will be: \(\mathrm{CH}_{3} \mathrm{OH} \mathrm{(l)} \rightarrow \mathrm{CH}_{3} \mathrm{OH} \mathrm{(aq)}\)

Key concepts

Dissolving Substances in Water

When a substance is mixed with water, it may dissolve to form a solution. This process involves interactions between water molecules and the molecules or ions of the substance. Not all substances dissolve in water, as the nature of the substance plays a crucial role.

• Ionic Compounds: These substances, like calcium hydroxide, dissociate into their respective ions when dissolved in water.
• Polar Molecules: Substances with a charge imbalance due to different electronegativities, like methanol, mix well with water.
• Non-polar Molecules: These molecules, such as nitrogen gas, do not dissolve because they lack attraction to water molecules.

Understanding these interactions helps predict solubility behaviors and write accurate chemical equations.

Calcium Hydroxide Dissociation

Calcium hydroxide, \(Ca(OH)_2\), is an ionic compound known for its solubility in water. When it dissolves, it undergoes dissociation. It splits into calcium ions \(Ca^{2+}\) and hydroxide ions \(OH^-\). These ions become surrounded by water molecules, making them stable in an aqueous solution.
This dissociation can be represented by the chemical equation:\[ Ca(OH)_2 \, (s) \rightarrow Ca^{2+} \, (aq) + 2 OH^- \, (aq) \]Understanding this process is essential for solving problems that involve these types of reactions in water.

Non-Polar Molecules in Water

Non-polar molecules, like nitrogen gas (\(N_2\)), exhibit uniform charge distribution, meaning they have no partial charges. This uniformity makes them incompatible with water, a polar solvent. Since water molecules are polar, they attract other polar substances and typically repel non-polar molecules.
When a non-polar molecule is introduced to water, it essentially "ignores" the water because there are no attractive forces strong enough to cause the molecule to dissolve. For instance, nitrogen gas remains unchanged and floats away in the water, reflecting essentially no interaction, thus maintaining: \\[ N_2 \, (g) \rightarrow N_2 \, (g) \]Here lies the critical concept that non-polar substances like fats and oils will not dissolve in water.

Polar Molecules

Polar molecules have areas with partial positive and negative charges due to the unequal sharing of electrons between different atoms. A classic example is methanol, \(CH_3OH\). In methanol, the oxygen atom is more electronegative than the hydrogen atoms, creating a polar region.
This charge separation allows polar molecules to interact strongly with water, another polar molecule. When methanol dissolves, it does not dissociate like ionic compounds, but it does become surrounded by water molecules, maintaining its molecular integrity:\[ CH_3OH \, (l) \rightarrow CH_3OH \, (aq) \]This capacity to dissolve without breaking apart is why so many biological substances, which are polar, interact readily in aqueous environments. Understanding polar interactions are crucial in predicting molecular behavior in various chemical and biological systems.