Problem 17 - Chemistry and Chemical Reactivity

Question

You make a saturated solution of \(\mathrm{NaCl}\) at \(25^{\circ} \mathrm{C}\) No solid is present in the beaker holding the solution. What can be done to increase the amount of dissolved NaCl in this solution? (See Figure 13.11.) (a) Add more solid NaCl. (b) Raise the temperature of the solution. (c) Raise the temperature of the solution, and add some NaCl. (d) Lower the temperature of the solution, and add some NaCl.

Step-by-Step Solution

Answer

Raise the temperature of the solution, and add some NaCl (option c).

Step 1: Understanding Saturated Solutions

A saturated solution is one where no more solute can dissolve at a given temperature; any additional solute will remain undissolved. This means that at 25°C, the maximum amount of NaCl has already dissolved in the solution.

Step 2: Effect of Adding More NaCl

If we add more solid NaCl to a saturated solution, it will not dissolve because the solution is already at its carrying capacity for the solute at that temperature. The added NaCl will remain as a solid.

Step 3: Effect of Raising Temperature

Increasing the temperature of the solution generally increases the solubility of solids, including NaCl. This means that by raising the temperature, more NaCl can potentially dissolve in the solution.

Step 4: Evaluation of Option (c)

If the temperature is raised and more NaCl is added, the increased temperature allows additional NaCl to dissolve. However, this is effective only if the temperature is sufficient to increase solubility. Therefore, option (c) will work to increase dissolved NaCl in combination with raised temperature.

Step 5: Consequences of Lowering Temperature

Lowering the temperature usually decreases the solubility of solutes like NaCl. This means that at a lower temperature, less NaCl would dissolve in the solution, making option (d) ineffective.

Key Concepts

Solubility

Solubility is a fundamental concept in chemistry that describes how much of a solute can dissolve in a solvent at a given temperature. In simple terms, it is the property of a solid, liquid, or gaseous chemical substance to dissolve in a solvent and form a homogeneous solution. It is usually expressed in terms of concentration like grams per liter (g/L).
When you look at common salt or sodium chloride (NaCl), its solubility in water is a well-studied example. At room temperature, which is around 25°C, NaCl has a specific level of solubility. Once this level is reached, the solution is said to be saturated. This means no additional salt can dissolve in the water, and any extra NaCl added will simply sit there as a solid. By understanding the solubility of materials, chemists can predict how substances will behave in various solutions.
For students trying to understand saturated solutions, it’s critical to know that temperature plays a significant role. Each solute has different solubility properties, which means that under the right conditions, more can be dissolved.

NaCl Solutions

Sodium chloride solutions, often referred to as saline solutions, are mixtures of salt and water. These solutions are commonly used not only in chemistry labs but also in everyday life. The process of dissolving NaCl in water involves breaking the ionic bonds between sodium and chloride ions and forming attractions with water molecules.
The ions separate and become surrounded by water molecules, preventing them from reassociating. This interaction is a classic example of solution chemistry, demonstrating how ionic compounds dissolve in polar solvents like water.

• NaCl dissociates into Na⁺ and Cl⁻ ions.
• Water molecules, being polar, interact differently with each ion.
• The positive end of water molecules is attracted to Cl⁻ ions, while the negative end to Na⁺ ions.

Understanding NaCl solutions is crucial for comprehending more complex solution chemistry concepts. It lays the groundwork for understanding things like electrical conductivity in solutions and the formation of precipitates in reactions.

Temperature Effects

Temperature has a profound influence on the solubility of substances. In most cases, raising the temperature increases a solvent's capacity to dissolve more solute. This happens because higher temperatures usually mean more energy is available to separate solute molecules, breaking intermolecular forces and allowing more solute to enter the solution.
For NaCl, when you heat the solution, you generally see an increase in solubility, allowing more salt to dissolve. This is why option (b) in our exercise suggests raising the temperature to dissolve more NaCl. It recommends increasing the kinetic energy of the molecules, creating more space between water molecules, which then can accommodate additional salt molecules.
Though increasing solubility with temperature is common with solids, not all substances behave the same way. For gases, solubility typically decreases with higher temperatures. Always consider the nature of the substance when thinking about temperature effects on solubility.

Solution Chemistry

Solution chemistry involves studying how substances interact and dissolve in solvents to form solutions. It's a pillar of chemistry because it helps explain how and why reactions occur in liquid environments.
When dealing with solutions, like NaCl in water, various factors influence how much of a solute dissolves:

• Concentration: The amount of solute present. Solution saturation is when no more solute can dissolve.
• Temperature: Affects solubility as previously described. Higher temperatures generally increase solubility for solids.
• Pressure: Typically affects gases more than solids or liquids when dissolved in solvents.

When studying solution chemistry, it's important to understand these variables and how they affect chemical behavior. This knowledge helps in predicting outcomes in both academic settings and practical applications, such as pharmaceuticals or environmental science. Understanding these interactions can aid in producing desired concentrations for different uses or analyzing solution behaviors under varying conditions.