Question

Specify what ions are present upon dissolving each of the following substances in water: (a) \(\mathrm{MgI}_{2}\), (b) \(\mathrm{K}_{2} \mathrm{CO}_{3}\), (c) \(\mathrm{HClO}_{4}\), (d) \(\mathrm{NaCH}_{3} \mathrm{COO}\).

Short answer

Upon dissolving each substance in water, the following ions are generated: (a) For \(\mathrm{MgI}_{2}\): \(\mathrm{Mg}^{2+}\) and \(2 \times \mathrm{I}^{-}\) (b) For \(\mathrm{K}_{2} \mathrm{CO}_{3}\): \(2 \times \mathrm{K}^{+}\) and \(\mathrm{CO}_{3}^{2-}\) (c) For \(\mathrm{HClO}_{4}\): \(\mathrm{H}^{+}\) and \(\mathrm{ClO}_{4}^{-}\) (d) For \(\mathrm{NaCH}_{3} \mathrm{COO}\): \(\mathrm{Na}^{+}\) and \(\mathrm{CH}_{3} \mathrm{COO}^{-}\)

Step-by-step solution

Identify the ions in each substance given

For each substance, we will identify the cations (positive ions) and anions (negative ions) that exist in the substance. Once we have those, we can determine the ions generated when they dissolve in water.

Analyze Substance (a): \(\mathrm{MgI}_{2}\)

Magnesium iodide, \(\mathrm{MgI}_{2}\), consists of a Magnesium ion, \(\mathrm{Mg}^{2+}\), and two iodide ions, \(\mathrm{I}^{-}\). When it dissolves in water, it will break down into its component ions: one magnesium ion and two iodide ions.

Analyze Substance (b): \(\mathrm{K}_{2} \mathrm{CO}_{3}\)

Potassium carbonate, \(\mathrm{K}_{2} \mathrm{CO}_{3}\), consists of two potassium ions, \(\mathrm{K}^{+}\), and a carbonate ion, \(\mathrm{CO}_{3}^{2-}\). When it dissolves in water, it will release two potassium ions and one carbonate ion.

Analyze Substance (c): \(\mathrm{HClO}_{4}\)

Perchloric acid, \(\mathrm{HClO}_{4}\), consists of a hydrogen ion, \(\mathrm{H}^{+}\), and a perchlorate ion, \(\mathrm{ClO}_{4}^{-}\). When it dissolves in water, it will break down into its component ions: one hydrogen ion and one perchlorate ion.

Analyze Substance (d): \(\mathrm{NaCH}_{3} \mathrm{COO}\)

Sodium acetate, \(\mathrm{NaCH}_{3} \mathrm{COO}\), consists of a sodium ion, \(\mathrm{Na}^{+}\), and an acetate ion, \(\mathrm{CH}_{3} \mathrm{COO}^{-}\). When it dissolves in water, it will break down into its component ions: one sodium ion and one acetate ion.

Present the Generated Ions for Each Substance

(a) For \(\mathrm{MgI}_{2}\), the ions generated are: \(\mathrm{Mg}^{2+}\) and \(2 \times \mathrm{I}^{-}\) (b) For \(\mathrm{K}_{2} \mathrm{CO}_{3}\), the ions generated are: \(2 \times \mathrm{K}^{+}\) and \(\mathrm{CO}_{3}^{2-}\) (c) For \(\mathrm{HClO}_{4}\), the ions generated are: \(\mathrm{H}^{+}\) and \(\mathrm{ClO}_{4}^{-}\) (d) For \(\mathrm{NaCH}_{3} \mathrm{COO}\), the ions generated are: \(\mathrm{Na}^{+}\) and \(\mathrm{CH}_{3} \mathrm{COO}^{-}\)

Key concepts

Cations and Anions

Understanding the difference between cations and anions is crucial in chemistry, especially when it comes to how substances interact with water. Cations are positively charged ions, which means they have more protons than electrons. Anions, on the other hand, are negatively charged ions with more electrons than protons.

When ionic compounds dissolve in water, they split into these individual cations and anions. For instance, when magnesium iodide, \(\mathrm{MgI}_{2}\), is introduced to water, it separates into one magnesium cation, expressed as \(\mathrm{Mg}^{2+}\), and two iodide anions, represented as \(\mathrm{I}^{-}\). Similarly, each of the given substances in the exercise will break down into its respective cations and anions upon dissolution. It's like a dance, where partners separate when they dive into the pool of water - they each go their own way as individual ions.

It's important to note that the subscripts in the chemical formula indicate the number of each ion. For example, in potassium carbonate, \(\mathrm{K}_{2} \mathrm{CO}_{3}\), there are two potassium cations and one carbonate anion. Placing emphasis on the balance of charges is essential when identifying the resulting ions.

Water Solubility of Ionic Substances

The water solubility of ionic substances is a topic that confuses many students, but it is quite logical once you grasp the basics. Water is known as a 'universal solvent' because it can dissolve a wide range of substances, and this includes many ionic compounds.

What makes water so effective is its polar nature. The water molecule has a positive end and a negative end, which allows it to interact with and stabilize the cations and anions when an ionic compound dissolves. This results in a process known as hydration, where water molecules surround the ions to keep them apart, effectively dissolving the substance.

Not all ionic substances dissolve in water, though. The rule of thumb is 'like dissolves like.' Polar solvents, such as water, are good at dissolving ionic and other polar substances. So, when approaching tasks like the textbook exercise, it's presumed that the given ionic compounds are soluble in water, otherwise, there would be no ions present in the solution. The solubility of a substance is a key factor that determines whether a substance will dissolve and to what extent.

Dissociation of Electrolytes in Solution

Dissociation of electrolytes in solution is essentially a chemical breakup story, where an ionic compound separates into its individual ions. Electrolytes are substances that conduct electricity when dissolved in water due to the presence of free-moving ions.

When you drop an ionic compound into water, the water molecules attack the solid structure, pulling away the cations and anions into the solution. This process is what we refer to as dissociation. For example, when sodium acetate, \(\mathrm{NaCH}_{3} \mathrm{COO}\), dissolves, it dissociates into sodium cations, \(\mathrm{Na}^{+}\), and acetate anions, \(\mathrm{CH}_{3} \mathrm{COO}^{-}\).

The resulting solution allows an electric current to flow, which is why such substances are called electrolytes. The ability of the solution to conduct electricity depends on the concentration of ions; more ions mean higher conductivity. When teaching this concept, it's beneficial to connect it with real-life applications, such as the function of electrolytes in sports drinks, which replenish the ions that the body loses through sweat.