Question

Briefly describe each of the following ideas or phenomena: (a) Henry's law; (b) freezing-point depression; (c) recrystallization; (d) hydrated ion; (e) deliquescence.

Short answer

Henry's law states that the solubility of a gas in a liquid is directly proportional to the pressure of the gas above the solution. Freezing-point depression refers to the lower freezing point of a solution compared to the pure solvent. Recrystallization is a purification process where a solid is dissolved in a solvent at high temperature and then allowed to cool and crystallize. A hydrated ion is an ion surrounded by water molecules in a solution. Deliquescence is the process by which a substance absorbs moisture from the atmosphere until it forms a solution.

Step-by-step solution

Describe Henry's Law

Henry's law is a principle in chemistry which states that the amount of gas which dissolves in a liquid to form a solution is directly proportional to the pressure of the gas above the solution. Increasing the pressure therefore causes more gas to dissolve.

Describe freezing-point depression

Freezing-point depression describes the phenomenon where the freezing point of a solution (which is a mix of solvent and solute) is lower than that of the pure solvent. It is a colligative property meaning that it depends on the amount of solute particles but not on their nature.

Describe recrystallization

Recrystallization is a process that is used to purify a solid substance. It involves dissolving the substance in an appropriate solvent at high temperature and then allowing it to cool. As the solution cools, the substance recrystallizes out of solution, leaving impurities behind.

Describe hydrated ion

A hydrated ion is a charged particle (ion) that is surrounded by water molecules. This occurs when an ion is put into a solution. The water molecules are attracted to the charged ion and surround it.

Describe deliquescence

Deliquescence is the process by which a substance absorbs moisture from the atmosphere until it dissolves in the absorbed water and forms a solution. Deliquescent materials like sodium hydroxide have very high affinity for water.

Key concepts

Henry's Law

Understanding the behavior of gases in liquid solutions is crucial for various applications in chemistry, from beverage carbonation to the bloodstream’s gas transport. Henry's Law aims to explain this aspect of gas-liquid interaction. It states that at a constant temperature, the concentration of a gas dissolved in a liquid is directly proportional to the partial pressure of that gas in contact with the liquid. In simpler terms, if you increase the pressure of a gas over a liquid, more of the gas will dissolve into the liquid. This principle is critical in fields such as pharmacology, environmental science, and the food industry, ensuring that we can predict and control how gases will behave when mixed with liquids.

The formula representing this law is: $C=kP$, where $C$ is the concentration of the gas, $k$ is Henry's law constant, and $P$ is the partial pressure of the gas.

Freezing-Point Depression

When you add antifreeze to your car's radiator or salt to ice on a wintery sidewalk, you’re taking advantage of freezing-point depression. This phenomenon reduces the temperature at which a liquid turns into a solid. The principle states that the freezing point of a solution is lower than that of the pure solvent by an amount that is proportional to the concentration of the solute. Therefore, the more solute particles present, the greater the depression of the freezing point. This is a colligative property, meaning it depends on the number of solute particles in the solution and not their specific identity. For solutions, the equation is $\Delta T_f=i{K}_{f}m$, where $\Delta T_f$ is the freezing point depression, $i$ is the van't Hoff factor, $K_f$ is the freezing-point depression constant, and $m$ is the molality of the solution.

To students studying properties of solutions, this concept is crucial for understanding how solutions behave differently from pure substances and is a pillar in physical chemistry.

Recrystallization

Purity is paramount in chemistry, and recrystallization is the go-to method for purifying solid compounds. The process involves first dissolving a solid material in a solvent at an elevated temperature, where the solubility of the solid is higher. As the solution cools, the solute's solubility decreases, and it begins to form pure crystals. Impurities remain dissolved in the solvent or form smaller, easily separable crystals. This technique is widely taught in organic chemistry labs and is essential for the preparation of pure samples for analysis or research.

The effectiveness of recrystallization depends on the proper choice of solvent and controlled cooling. The reasoning behind choosing an appropriate solvent includes understanding solubility trends and predicting the outcome of solvent-solute interaction on a molecular level.

Hydrated Ion

Water’s role in chemistry extends beyond being a mere solvent; its ability to hydrate ions is fundamental in both chemistry and biology. A hydrated ion is formed when water molecules coordinate to an ion, with their polar nature causing a sphere of hydration around it. This hydration sphere stabilizes the ion in solution and can impact the ion's reactivity and its transport across cell membranes. Knowing how ions interact with water is essential for studying solutions, understanding electrochemistry, and handling biochemical systems.

For students, visualizing how polar water molecules can shield and stabilize ions helps in grasping concepts like osmosis, electrolyte balance in the body, and the behavior of ions in various aqueous reactions.

Deliquescence

Imagine leaving a container of table salt open on a humid day and returning to find it has turned into a watery mess. This phenomenon is known as deliquescence, where a hygroscopic substance absorbs water from the air to the point it becomes a solution. This property is crucial in industries where moisture control is essential such as pharmaceuticals and food processing. Deliquescent compounds, like calcium chloride, are often used as desiccants to dry air streams or maintain dry conditions in packing.

It is worth noting for students that not all hygroscopic substances are deliquescent, as deliquescence refers specifically to absorbing enough moisture to form an aqueous solution. Understanding these nuanced differences can be vital in areas like materials science and chemical storage. Deliquescent materials are typically ionic compounds with high solubility in water that easily absorb moisture due to their strong ionic attraction to water molecules.