Problem 15 - Chemistry: The Central Science

Question

Specify what ions are present in solution upon dissolving each of the following substances in water: a) \(\mathrm{ZnCl}_{2}\), (b) \(\mathrm{HNO}_{3}\) (c) \(\left(\mathrm{NH}_{4}\right)_{2} \mathrm{SO}_{4}\), (d) \(\mathrm{Ca}(\mathrm{OH})_{2}\).

Step-by-Step Solution

Answer

Upon dissolving the substances in water, the following ions are present in the solution: a) \(\mathrm{ZnCl}_{2}\): \(\mathrm{Zn}^{2+}\) and \(\mathrm{Cl}^-\) b) \(\mathrm{HNO}_{3}\): \(\mathrm{H}^+\) and \(\mathrm{NO}_{3}^-\) c) \(\left(\mathrm{NH}_{4}\right)_{2} \mathrm{SO}_{4}\): \(\mathrm{NH}_{4}^+\) and \(\mathrm{SO}_{4}^{2-}\) d) \(\mathrm{Ca}(\mathrm{OH})_{2}\): \(\mathrm{Ca}^{2+}\) and \(\mathrm{OH}^-\)

Step 1: a) Dissolving \(\mathrm{ZnCl}_{2}\) in water

When \(\mathrm{ZnCl}_{2}\) is dissolved in water, it dissociates into its constituent ions: one zinc ion and two chloride ions. The equation representing this process can be written as follows: \[ \mathrm{ZnCl}_{2} \longrightarrow \mathrm{Zn}^{2+} + 2\mathrm{Cl}^-\] The ions present in the solution are \(\mathrm{Zn}^{2+}\) and \(\mathrm{Cl}^-\).

Step 2: b) Dissolving \(\mathrm{HNO}_{3}\) in water

When \(\mathrm{HNO}_{3}\) is dissolved in water, it dissociates into a hydrogen ion and a nitrate ion. The equation representing this process can be written as follows: \[\mathrm{HNO}_{3} \longrightarrow \mathrm{H}^+ + \mathrm{NO}_{3}^-\] The ions present in the solution are \(\mathrm{H}^+\) and \(\mathrm{NO}_{3}^-\).

Step 3: c) Dissolving \(\left(\mathrm{NH}_{4}\right)_{2} \mathrm{SO}_{4}\) in water

When \(\left(\mathrm{NH}_{4}\right)_{2} \mathrm{SO}_{4}\) is dissolved in water, it dissociates into two ammonium ions and one sulfate ion. The equation representing this process can be written as follows: \[\left(\mathrm{NH}_{4}\right)_{2} \mathrm{SO}_{4} \longrightarrow 2\mathrm{NH}_{4}^+ + \mathrm{SO}_{4}^{2-}\] The ions present in the solution are \(\mathrm{NH}_{4}^+\) and \(\mathrm{SO}_{4}^{2-}\).

Step 4: d) Dissolving \(\mathrm{Ca}(\mathrm{OH})_{2}\) in water

When \(\mathrm{Ca}(\mathrm{OH})_{2}\) is dissolved in water, it dissociates into one calcium ion and two hydroxide ions. The equation representing this process can be written as follows: \[\mathrm{Ca}(\mathrm{OH})_{2} \longrightarrow \mathrm{Ca}^{2+} + 2\mathrm{OH}^-\] The ions present in the solution are \(\mathrm{Ca}^{2+}\) and \(\mathrm{OH}^-\).

Key Concepts

Chemical Equations

Understanding chemical equations is fundamental in chemistry, as they are essentially the language that scientists use to describe reactions. A chemical equation shows the substances involved in the reaction — the reactants turning into products. They provide information on the compounds that react, the products formed, and, sometimes, the conditions under which the reaction occurs.

For example, when we write \( \mathrm{ZnCl}_2 \longrightarrow \mathrm{Zn}^{2+} + 2\mathrm{Cl}^- \), we're indicating that zinc chloride (\(\mathrm{ZnCl}_2\)) dissociates into zinc ions (\(\mathrm{Zn}^{2+}\)) and chloride ions (\(\mathrm{Cl}^-\)). The arrow signifies the direction in which the reaction occurs, and the coefficients (like the '2' in front of \(\mathrm{Cl}^-\)) tell us the ratio in which reactants form products. It's crucial to balance chemical equations to obey the law of conservation of mass, ensuring that the same number of each type of atom is present on both sides of the equation.

Ionic Compounds

Ionic compounds, such as \(\mathrm{ZnCl}_2\) and \(\left(\mathrm{NH}_4\right)_2 \mathrm{SO}_4\), are substances formed by the chemical bonding between metals and non-metals. These bonds are the result of the transfer of electrons from the metal atoms, which become positively charged cations, to the non-metal atoms, which become negatively charged anions.

When ionic compounds dissolve in water, a process known as dissociation occurs. Dissociation is the separation of an ionic compound into its individual ions, which can then move freely and conduct electricity in solution. For instance, \(\mathrm{Ca}(\mathrm{OH})_2\) in water separates into \(\mathrm{Ca}^{2+}\) and \(\mathrm{OH}^-\), becoming an electrolyte. This behavior is a characteristic property of ionic compounds in aqueous solutions and is essential for many biological and chemical processes.

Aqueous Solutions

Aqueous solutions are mixtures where water acts as the solvent, dissolving a substance, known as the solute. When an ionic compound dissolves in water, it typically separates into its constituent ions, making the solution conductive. The degree to which this occurs is described by the solute's solubility.

The dissociation of aqueous solutions is best observed with the strong acid \(\mathrm{HNO}_3\), which almost completely dissociates into \(\mathrm{H}^+\) and \(\mathrm{NO}_3^-\) when dissolved in water. This is unlike weak acids or bases, which dissociate only partially. The process of solvation involves interactions between the solute ions and the water molecules, which can significantly influence the properties of the solution such as boiling point, freezing point, and vapor pressure.