Question

You are presented with three white solids, $A,B$, and $C$, which are glucose (a sugar substance), $\mathrm{NaOH}$, and AgBr. Solid A dissolves in water to form a conducting solution. B is not soluble in water. C dissolves in water to form a nonconducting solution. Identify $\mathrm{A},\mathrm{B}$, and $\mathrm{C}$. [Section 4.2]

Short answer

A is $\mathrm{NaOH}$ because it dissolves in water to form a conducting solution. B is AgBr because it is insoluble in water. C is glucose because it dissolves in water to form a nonconducting solution.

Step-by-step solution

Analyze solubility

First, investigate each white solid's solubility in water. Glucose is a sugar, so it is soluble in water. Sodium hydroxide ($\mathrm{NaOH}$) is a strong base, which is also soluble in water. Silver bromide (AgBr), on the other hand, is an insoluble salt and does not dissolve in water.

Analyze conductivity

Now, analyze the conductivity of each solid. Glucose, being a covalent compound, dissolved in water will not form any ions, which makes it a non-conducting solution. Sodium hydroxide ($\mathrm{NaOH}$), dissolved in water, forms ions (Na+ and OH-) and conducts electricity, making it a conducting solution. Silver bromide (AgBr) is an ionic compound, although it does not dissolve in water, so it does not form a solution in which ions can conduct electricity.

Identify A, B, and C based on properties

Since solid A dissolves in water to form a conducting solution, A must be $\mathrm{NaOH}$. B is not soluble in water, which means B could only be the insoluble salt, AgBr. Finally, C dissolves in water and forms a nonconducting solution, so it must be glucose. So, A is $\mathrm{NaOH}$, B is AgBr, and C is glucose.

Key concepts

Conductivity

Conductivity refers to a solution's ability to conduct electricity. When dissolved in water, substances that form ions will generally conduct electricity well. This is due to the presence of charged particles, or ions, that move freely in the solution, allowing for the flow of electricity.

In solutions like those created with ionic compounds, such as sodium hydroxide ($\mathrm{NaOH}$), ions like ${\mathrm{Na}}^{+}$ and ${\mathrm{OH}}^{-}$ are generated when dissolved in water, resulting in high conductivity.

• Conducting Solutions: Contain ions, like in the case of sodium hydroxide in water.
• Non-conducting Solutions: Lack free ions, as seen with covalent compounds like glucose.

Thus, conductivity is a key indicator of the presence of ions in a solution.

Ionic Compounds

Ionic compounds are typically formed from a metal and a non-metal. They consist of positively and negatively charged ions. In solid form, they are held together by an electrostatic force known as ionic bonding.

For example, sodium hydroxide ($\mathrm{NaOH}$) and silver bromide ($\mathrm{AgBr}$) are ionic compounds due to their composition of metallic sodium or silver and non-metallic hydroxide or bromide, respectively.

• Solubility: Ionic compounds like $\mathrm{NaOH}$ often dissolve in water, releasing ions that can conduct electricity.
• Insolubility: Ionic compounds such as $\mathrm{AgBr}$ do not dissolve in water, meaning they won't produce ions to conduct electricity.

Understanding the solubility and conductivity properties of ionic compounds can help identify unknown substances.

Covalent Compounds

Covalent compounds are formed between non-metal atoms by sharing electron pairs, creating a strong bond. These compounds do not usually form ions in solution, as they retain their molecular structure when dissolved. This makes them typically non-conductive.

Glucose is a covalent compound, and when it dissolves in water, it disperses as intact molecules. This means no ions are present to carry an electrical current, resulting in a non-conducting aqueous solution.

• Bonding: Involves sharing of electrons between non-metals.
• Solubility: Often dissolves in water, but resulting solutions do not conduct electricity due to the absence of ions.

Recognizing whether a substance is covalent helps us predict its behavior in solution, especially its conductivity.

Aqueous Solutions

Aqueous solutions are created when a substance is dissolved in water. Water acts as a solvent, where the solute (the dissolved substance) interacts with water molecules.

These interactions can differ based on the nature of the solute. Ionic compounds dissolve to form solutions with free ions, increasing conductivity. In contrast, covalent compounds typically do not break down into ions, leading to solutions that do not conduct electricity.

• Ionic Solutes: Solutions can conduct electricity due to the movement of ions.
• Covalent Solutes: Solutions do not conduct electricity, as molecules are intact with no ions.

Through understanding aqueous solutions, the identification of different compounds based on their solubility and conductivity becomes easier.