Question

We have learned in this chapter that many ionic solids dissolve in water as strong electrolytes, that is, as separated ions in solution. What properties of water facilitate this process? Would you expect ionic compounds to be soluble in elemental liquids like bromine or mercury, just as they are in water? Explain.

Short answer

Water's highly polar nature, due to the electronegativity difference between oxygen and hydrogen atoms, enables it to dissolve ionic solids as strong electrolytes. Its positively charged hydrogen end attracts negatively charged ions, and its negatively charged oxygen end attracts positively charged ions, resulting in the separation of ions and their incorporation into the water. In contrast, ionic compounds are not expected to be soluble in elemental liquids like bromine or mercury, as these lack the polar nature and strong interactions with ions needed to create electrolyte solutions.

Step-by-step solution

Understanding the properties of water that help dissolve ionic solids as strong electrolytes

Water is a highly polar molecule due to the high electronegativity difference between oxygen and hydrogen atoms. The oxygen atom carries a slight negative charge, and the hydrogen atoms carry a slight positive charge. This polarization leads to the formation of hydrogen bonds with other polar or ionic substances. When water comes into contact with ionic solids, the positively charged end of the water molecule (hydrogen) gets attracted to the negatively charged ions (anions) of the solid, and the negatively charged end of the water molecule (oxygen) gets attracted to the positively charged ions (cations) of the solid. This process leads to the separation of ions from the solid and their incorporation into the water, forming a strong electrolyte solution.

Discussing the solubility of ionic compounds in elemental liquids like bromine or mercury

Before comparing the solubility of ionic compounds in elemental liquids like bromine and mercury to water, we need to consider the properties of these elemental liquids. They both lack properties that make water an ideal solvent for ionic compounds. 1. Bromine (Br2): Bromine is a nonpolar, covalent liquid, which would not form strong interactions with ionic compounds. Since it lacks the polar nature of water, it is unable to dissolve ionic compounds to create electrolyte solutions efficiently. 2. Mercury (Hg): Mercury is a metal and exists as a liquid at room temperature. Even though it can dissolve some metals to form amalgams, it does not efficiently dissolve ionic compounds. The interactions between mercury and ions in the ionic compound are not strong enough to break the ionic bonds as water does. In conclusion, ionic compounds are not expected to be soluble in elemental liquids like bromine or mercury as they are in water. Water's polar nature and strong interactions with ions make it an excellent solvent for ionic compounds, creating strong electrolyte solutions, while bromine and mercury lack these essential properties.

Key concepts

Properties of Water

Water is often referred to as the 'universal solvent' due to its remarkable ability to dissolve a vast array of substances. This unique quality arises from the intrinsic properties of the water molecule. Each water molecule consists of one oxygen atom covalently bonded to two hydrogen atoms, resulting in a bent structure. This structure, alongside the oxygen atom's higher electronegativity, leads to an uneven distribution of charge, rendering the molecule highly polar.

Hydrogen bonds are another critical feature of water's properties, which contribute to its high solvent powers. These bonds occur due to the attractive force between the slightly positive hydrogen atoms of one water molecule and the slightly negative oxygen atom of another. The real magic happens when these two features come together: water's polarity allows it to interact with and stabilize ions or other polar molecules when they enter the solution, while hydrogen bonding maintains the integrity of the liquid itself, providing an environment for the molecules to be supported while they separate and disperse.

It's this combination of polarity and hydrogen bonding that enables water to break down ionic compounds into their constituent ions, resulting in what are known as strong electrolyte solutions. In essence, water's properties make it an unmatched medium for facilitating the dissolution and interaction of various substances.

Strong Electrolytes in Water

When discussing strong electrolytes, we refer to substances that completely disassociate into ions when dissolved in water. This complete disassociation is crucial for the substance to conduct electricity, hence the term 'electrolyte'.

Ionic compounds, which are made up of positively and negatively charged ions held together by strong ionic bonds, typically dissolve in water to form strong electrolytes. Water's polar nature means that its molecules surround each individual ion, separating and stabilizing them so they can move freely in the solution. The oxygen end of the water molecule, with its slight negative charge, is attracted to cations, while the hydrogen end, with its slight positive charge, is attracted to anions. This results in a solution where the ions are evenly dispersed and can conduct electricity.

It's important to highlight that not every soluble compound in water creates a strong electrolyte. Only those substances that disassociate entirely into ions do. Common table salt (sodium chloride, NaCl) is a classic example; when dissolved in water, NaCl separates into Na+ and Cl- ions, each being completely surrounded by water molecules, creating a solution that is an excellent conductor of electricity.

Polar and Nonpolar Solvents

Polar and nonpolar solvents are distinguished by their ability to dissolve certain types of substances. This distinction has significant implications for the solubility of ionic compounds.

Polar solvents, like water, possess a separation of charge that allows them to interact with ionic and other polar substances. Nonpolar solvents, on the other hand, lack this charge separation and are characterized by a balanced distribution of electrons. As a result, they are better suited for dissolving nonpolar substances, such as oils and fats.

The phrase 'like dissolves like' is key here; polar solvents typically dissolve polar substances, and nonpolar solvents dissolve nonpolar substances. Given the strongly polar nature of ionic compounds, they are particularly soluble in polar solvents like water. This is due to the fact that the solvent molecules can surround and interact with the ions, similar to how water molecules facilitate the dissolution of salts.

In contrast, nonpolar solvents such as bromine or mercury, which were mentioned in the original exercise, lack the ability to interact effectively with ionic compounds. This mismatch means that ionic compounds will not dissolve well in nonpolar solvents, highlighting the specialized role of polar solvents like water in the dissolution of ionic substances.