Question

Amongst the following, the total number of compounds whose aqueous solution turns red litmus paper blue is $\mathrm{KCN}{\mathrm{K}}_2{\mathrm{SO}}_4{\left({\mathrm{NH}}_4\right)}_2{\mathrm{C}}_2{\mathrm{O}}_4\mathrm{NaCl}\mathrm{Zn}{\left({\mathrm{NO}}_3\right)}_2$ ${\mathrm{FeCl}}_3{\mathrm{K}}_2{\mathrm{CO}}_3$ 0 ${\mathrm{NH}}_4{\mathrm{NO}}_3\mathrm{LiCN}$

Short answer

Three compounds - KCN, K2CO3, and LiCN will turn red litmus paper blue.

Step-by-step solution

Understanding Litmus Paper Indicator

Litmus paper is an indicator used to test whether a substance is acidic or basic. Red litmus paper turns blue under basic conditions.

Identifying Basic Compounds

To find the compounds that turn red litmus paper blue, identify the compounds that are bases or will form basic solutions when dissolved in water. Bases typically contain hydroxide ions or are formed from weak acids and strong bases.

Evaluating Each Compound

Analyze each compound, determining if it forms a basic solution in water:- KCN (a salt from a weak acid HCN and a strong base KOH) will form a basic solution.- K2SO4 (a salt of a strong acid H2SO4 and a strong base KOH) is neutral.- (NH4)2C2O4 (a salt of a weak acid H2C2O4 and a weak base NH4OH) is neutral to slightly acidic.- NaCl (a salt of a strong acid HCl and a strong base NaOH) is neutral.- Zn(NO3)2 (a salt from a strong acid HNO3 and a weak base Zn(OH)2) is slightly acidic.- FeCl3 (a salt from a strong acid HCl and a weak base Fe(OH)3) is slightly acidic.- K2CO3 (a salt from the weak acid H2CO3 and a strong base KOH) will form a basic solution.- NH4NO3 (a salt from a strong acid HNO3 and a weak base NH4OH) is neutral to slightly acidic.- LiCN (a salt from a weak acid HCN and a strong base LiOH) will form a basic solution.

Counting the Basic Compounds

From the list, KCN, K2CO3, and LiCN will form basic solutions and turn red litmus paper blue.

Key concepts

Litmus Paper Test

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.

Properties of Salts

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.

Acidic and Basic Solutions

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.