Question

Explain the terms soluble and insoluble. Use the solubility rules to write the formula of an insoluble ionic compound.

Short answer

An insoluble compound is one that does not dissolve in water; lead(II) sulfate (PbSO₄) is an example.

Step-by-step solution

Define Soluble

Soluble substances are compounds that can dissolve in a solvent, typically water, resulting in a homogeneous solution. This means the solute particles are uniformly dispersed at the molecular or ionic level within the solvent.

Define Insoluble

Insoluble substances are compounds that do not readily dissolve in a solvent like water. They may partially dissolve, but most of the solid remains undissolved, resulting in no homogeneous solution.

Understanding Solubility Rules

Solubility rules help predict whether an ionic compound will dissolve in water. Generally, compounds containing alkali metal ions (e.g., Li⁺, Na⁺, K⁺) and ammonium (NH₄⁺), as well as nitrates (NO₃⁻), acetates (CH₃COO⁻), and most chlorides and sulfates, are soluble. Exceptions apply, such as compounds containing lead (Pb²⁺), mercury (Hg₂²⁺), and silver (Ag⁺).

Apply Solubility Rules

Using the solubility rules, identify ions that typically form insoluble compounds. For instance, sulfates (SO₄²⁻) are soluble except when combined with Ba²⁺, Pb²⁺, Ca²⁺, and Sr²⁺, making lead(II) sulfate an example of an insoluble compound.

Write the Chemical Formula

Lead(II) sulfate can be expressed with the chemical formula PbSO₄. Based on solubility rules, PbSO₄ is generally considered an insoluble compound in water.

Key concepts

Soluble and Insoluble Substances

Solubility is a term that refers to how well a substance can dissolve in a solvent. When we say a substance is soluble, we mean that it can dissolve in a solvent, like water, and form a homogeneous solution. This is akin to sugar dissolving in hot tea, where the sugar particles spread evenly throughout the liquid.
On the other hand, insoluble substances do not dissolve well in water. They may not dissolve at all, or only a small amount dissolves, leaving the solution cloudy or with a residue. Imagine trying to dissolve sand in water; most of it will just sink to the bottom.
Understanding the difference between soluble and insoluble substances is key in chemistry because it helps us predict how substances will behave in solutions. Knowing which substances can dissolve helps us identify their reactions or applications in different chemical processes.

Solubility Rules

Solubility rules are guidelines that help determine whether an ionic compound will dissolve in water. These rules are like a checklist that allows chemists to predict solubility outcomes without conducting laborious experiments each time. Here are some simplified solubility rules to consider:

• Compounds containing alkali metals, like sodium (Na⁺) and potassium (K⁺), are always soluble.
• Ammonium compounds (NH₄⁺) are also soluble.
• Nitrates (NO₃⁻) and acetates (CH₃COO⁻) dissolve in water without exception.
• Most chlorides (Cl⁻) and sulfates (SO₄²⁻) are soluble, except for those with lead (Pb²⁺), mercury (Hg₂²⁺), and silver (Ag⁺).
• Some ions, like hydroxides (OH⁻) and carbonates (CO₃²⁻), are generally insoluble, except when paired with the highly soluble cations like alkali metals.

By applying these rules, chemists can predict and explain if a compound can dissolve, which helps in forming solutions for various reactions.

Ionic Compounds

Ionic compounds are formed when atoms transfer electrons, resulting in a metal cation and a non-metal anion that attract each other due to opposite charges. These compounds often consist of a lattice structure, where positive and negative ions are held together tightly.
This ionic bond is what gives many ionic compounds their characteristic properties, such as high melting points and the ability to conduct electricity when melted or dissolved in water.
However, not all ionic compounds dissolve in water perfectly. Whether or not an ionic compound dissolves depends on the "competition" between the attraction of the ions for each other and the attraction of the ions for the water molecules. When ionic attraction is stronger, the compound does not dissolve easily, making it insoluble, as per the solubility rules we discussed.
Understanding ionic compounds and their behavior in different solvents is crucial, as it dictates how they can be used in practical applications, from creating medications to designing new materials.