Question

Which of the following oxides would yield an acidic solution when they react with water? Which would give a basic solution? (a) \(\mathrm{P}_{4} \mathrm{O}_{10},\) (b) \(\mathrm{K}_{2} \mathrm{O},\) (c) \(\mathrm{SeO}_{3},\) (d) \(\mathrm{Cl}_{2} \mathrm{O}_{7}\)

Short answer

Acidic solutions: (a) \(\mathrm{P}_{4}\mathrm{O}_{10}\), (c) \(\mathrm{SeO}_{3}\), (d) \(\mathrm{Cl}_{2}\mathrm{O}_{7}\). Basic solution: (b) \(\mathrm{K}_{2}\mathrm{O}\).

Step-by-step solution

Identify the type of oxides

Oxides can be classified into acidic, basic, or amphoteric based on the characteristics of the parent element. Oxides of non-metals are acidic, whereas oxides of metals are generally basic.

Categorize each oxide

Categorize each given oxide based on the metal or non-metal nature of the central element. For example, phosphorus, selenium, and chlorine are non-metals which form acidic oxides. Potassium is a metal that forms a basic oxide.

Determine the pH of the solutions

Upon reaction with water, acidic oxides yield acidic solutions (pH < 7) and basic oxides yield basic solutions (pH > 7).

Key concepts

Chemical Properties of Oxides

Understanding the chemical properties of oxides is essential for comprehending the behavior of different compounds when they interact with water. Oxides are binary compounds composed of oxygen and another element. Based on the second element, oxides can exhibit varying chemical properties. For instance, oxides that contain metal elements usually display basic properties, meaning they can react with acids to produce salt and water. This reaction is characteristic of a broader group of compounds known as bases or alkaline substances. On the other hand, non-metal oxides tend to react with bases to form salts and water, classifying them as acidic.
When dissolved in water, an oxide can affect the solution's pH. If the oxide is acidic, such as sulfur dioxide (SO2), it will produce an acidic solution, causing the pH to fall below 7. Conversely, a basic oxide like sodium oxide (Na2O) will yield a basic solution, increasing the pH above 7. Some oxides, such as zinc oxide (ZnO), are amphoteric, meaning they can react either as an acid or a base depending on the reacting substance. This distinctive behavior is crucial for many industrial processes, such as metal extraction and environmental management.

pH of Solutions

The pH scale is a measure of acidity or alkalinity of a solution on a scale of 0 to 14. It is an important concept in chemistry, as it helps predict the behavior and reactivity of solutions. A pH value below 7 indicates an acidic solution, while a value above 7 indicates a basic (also called alkaline) solution. A pH of exactly 7 is considered neutral, which is the pH of pure water at 25°C.
When an oxide reacts with water, it forms a solution which can significantly alter the pH. Acidic oxides, such as carbon dioxide (CO2), will form carbonic acid when dissolved in water, leading to a pH drop. Basic oxides like calcium oxide (CaO) will produce hydroxide ions in solution, thus increasing the pH. Being able to determine the likely pH of a solution formed by reacting an oxide with water is valuable in many applications, from industrial processes to environmental science. Furthermore, understanding pH is crucial for safety, since extreme pH levels can be corrosive or damaging to tissues and materials.

Metal and Non-metal Oxides

The distinction between metal and non-metal oxides is pivotal to grasping their different reactions and the resulting pH of solutions. Metals typically form basic oxides, which yield basic solutions when dissolved in water. These oxides are generally ionic in nature, meaning they form when a metal cation interacts with the oxide anion. For example, magnesium oxide (MgO), produced by a metal, is ionic and will react with water to form magnesium hydroxide, a basic solution.
Conversely, non-metal oxides tend to be acidic. They are typically covalent compounds, where electrons are shared between the oxygen and the non-metal atom. A common example is sulfur trioxide (SO3), which reacts with water to form sulfuric acid, yielding an acidic solution. Besides being essential in predicting the behavior of oxides in water and their influence on pH, recognizing whether an oxide is metallic or non-metallic is also influential in understanding global processes, such as the formation of acid rain from the atmospheric reaction of non-metal oxides with water vapor.