Problem 27 - Chemistry The Molecular Science

Question

Which substance conducts electricity when dissolved in water? (a) \(\mathrm{NaCl}\) (b) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{3}\) (propane) (c) \(\mathrm{CH}_{3} \mathrm{OH}\) (methanol) (d) \(\mathrm{Ca}\left(\mathrm{NO}_{3}\right)_{2}\)

Step-by-Step Solution

Answer

(a) \( \mathrm{NaCl} \) and (d) \( \mathrm{Ca}\left(\mathrm{NO}_{3}\right)_{2} \) conduct electricity in water.

Step 1: Identify Substances

First, identify the type of each substance: - (a) \( \mathrm{NaCl} \) is an ionic compound. - (b) \( \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{3} \) is a non-polar covalent compound (hydrocarbon). - (c) \( \mathrm{CH}_{3} \mathrm{OH} \) is a polar covalent compound (alcohol). - (d) \( \mathrm{Ca}\left(\mathrm{NO}_{3}\right)_{2} \) is an ionic compound.

Step 2: Understand Conductivity

Recall that substances that conduct electricity when dissolved in water (electrolytes) typically dissociate into ions. Ionic compounds, when dissolved, separate into ions that move freely and conduct electricity. On the other hand, covalent compounds do not dissociate into ions and therefore usually do not conduct electricity.

Step 3: Analyze Sodium Chloride

\( \mathrm{NaCl} \) dissolves in water to form \( \mathrm{Na}^+ \) and \( \mathrm{Cl}^- \) ions, conducting electricity. Thus, \( \mathrm{NaCl} \) is an electrolyte.

Step 4: Analyze Propane

\( \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{3} \) does not ionize in water as it is a non-polar hydrocarbon. Therefore, it does not conduct electricity.

Step 5: Analyze Methanol

\( \mathrm{CH}_{3} \mathrm{OH} \) is a polar covalent compound and does not dissociate into ions in water. Therefore, it cannot conduct electricity.

Step 6: Analyze Calcium Nitrate

\( \mathrm{Ca}\left(\mathrm{NO}_{3}\right)_{2} \) dissociates in water to form \( \mathrm{Ca}^{2+} \) and \( \mathrm{NO}_{3}^- \) ions, allowing it to conduct electricity. Thus, it is an electrolyte.

Step 7: Conclusion

From the analysis, \( \mathrm{NaCl} \) and \( \mathrm{Ca}\left(\mathrm{NO}_{3}\right)_{2} \) conduct electricity when dissolved in water as they both are ionic compounds that dissociate into ions.

Key Concepts

Electrolytes

Electrolytes are substances that can conduct electricity when they are dissolved in water. This happens because electrolytes dissociate into ions, which are charged particles. These ions, positively charged cations and negatively charged anions, move freely in the solution. Their movement allows electric current to flow. This is why electrolytes are crucial in many biological and chemical processes.
Examples of electrolytes include common salt (\(\mathrm{NaCl}\)), which dissociates into \(\mathrm{Na}^+\) and \(\mathrm{Cl}^-\) ions, and calcium nitrate (\(\mathrm{Ca}\left(\mathrm{NO}_{3}\right)_{2}\)), which forms \(\mathrm{Ca}^{2+}\) and \(\mathrm{NO}_{3}^-\) ions when dissolved.

• Aqueous solutions of electrolytes can conduct electricity.
• The strength of an electrolyte solution is determined by its ion concentration.
• Electrolytes can be strong or weak depending on their ability to dissociate in water.

Covalent Compounds

Covalent compounds are formed when atoms share electrons, resulting in the creation of molecules. Unlike ionic compounds, these do not dissociate into ions in water. Therefore, most covalent compounds do not conduct electricity in aqueous solutions.
Examples include propane (\(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{3}\)), which is non-polar and does not dissolve in water, and methanol (\(\mathrm{CH}_{3} \mathrm{OH}\)), which is polar but still does not break into ions.

• Covalent compounds can be polar or non-polar.
• Non-polar covalent compounds do not dissolve in water.
• The absence of ions in these solutions is why they generally do not conduct electricity.

Conductivity in Water

Conductivity in water refers to the ability of a substance to conduct electric current when dissolved or present in water. This primarily depends on the presence of charged particles or ions in the solution.
Ionic compounds, when dissolved in water, release ions which move freely, thus facilitating the flow of electricity. This is why substances like sodium chloride (\(\mathrm{NaCl}\)) and calcium nitrate (\(\mathrm{Ca}\left(\mathrm{NO}_{3}\right)_{2}\)) are able to conduct electricity.

• Only substances that dissociate into free ions can enhance water's conductivity.
• High ionic concentration usually leads to greater conductivity.
• Covalent compounds do not contribute ions, thus often result in poor conductivity.

In contrast, covalent compounds tend to either stay intact in water or form molecules that don't carry a charge, contributing very little to the conductivity.