Amongst the following, the total number of compounds whose aqueous solution turns red litmus paper blue is \(\mathrm{KCN} \quad \mathrm{K}_{2} \mathrm{SO}_{4} \quad\left(\mathrm{NH}_{4}\right)_{2} \mathrm{C}_{2} \mathrm{O}_{4} \quad \mathrm{NaCl} \quad \mathrm{Zn}\left(\mathrm{NO}_{3}\right)_{2}\) \(\mathrm{FeCl}_{3} \quad \mathrm{~K}_{2} \mathrm{CO}_{3}\) 0 \(\mathrm{NH}_{4} \mathrm{NO}_{3} \quad \mathrm{LiCN}\)

Understanding the litmus paper test is essential when studying chemistry, especially when identifying the nature of a solution. The test uses a simple piece of paper treated with natural dyes derived from lichens, which change color in the presence of an acid or base. Red litmus paper is used to test for basicity. When a substance capable of altering the pH is introduced to red litmus paper and it turns blue, it indicates that the substance is basic.

Students often associate the test with a straightforward color change; however, the principle behind it is the reaction of the dye with ions in the solution. It's crucial to realize that not all compounds cause the litmus paper to change color. Neutral substances, which are neither acidic nor basic, will not affect the red litmus paper. In the exercise provided, the student's task was to identify which compounds in a given list would change red litmus paper to blue, pointing out which substances create a basic environment in aqueous solution.

Salts are chemical compounds consisting of positive ions (cations) and negative ions (anions). A common misconception is that salts are always neutral, but the properties of salts, including their pH level, can vary greatly depending on their constituent ions. To determine if a salt will produce an acidic, basic, or neutral solution, one must consider the strength of the acid and base from which the salt is derived.

For example, a salt formed from a strong acid and a strong base, like NaCl, yields a neutral solution, whereas salts from a strong base and a weak acid, such as KCN, will result in a basic solution. Conversely, salts from a strong acid and a weak base will form an acidic solution. In the context of the exercise provided, understanding these properties helps the student to analyze each given compound and decide whether it would create an acidic, neutral, or basic solution when dissolved in water.

A solution's acidity or basicity is determined by its pH level, which is a measure of the concentration of hydrogen ions present. Acidic solutions are those with a pH less than 7, while basic solutions have a pH greater than 7. A pH of 7 represents a neutral solution, like pure water.

An acid releases hydrogen ions (H+) when dissolved in water, and the more ions it releases, the stronger the acid. Bases can accept H+ ions or release hydroxide ions (OH-) in water, increasing the solution's pH. It's critical for students to understand the concept of acidity and basicity to predict and explain the behavior of various compounds in water, as showcased in the textbook exercise. Solutions resulting from salts of weak acids and strong bases, such as KCN, K2CO3, and LiCN, provided in the exercise, will be basic and thus capable of turning red litmus paper blue.