Problem 97 - Chemistry and Chemical Reactivity

Question

Suggest a method for separating a precipitate consisting of a mixture of solid CuS and solid Cu (OH) \(_{2}\)

Step-by-Step Solution

Answer

React with dilute HCl to dissolve Cu(OH)_2, then filter to separate solid CuS.

Step 1: Understanding Properties

Before we separate the salts, we need to consider the properties of each compound. Copper (II) sulfide (CuS) is insoluble in water and generally remains as a solid, whereas copper (II) hydroxide (Cu(OH)_2) also does not dissolve in water but can react with acids.

Step 2: Acid Reaction

React the mixture with a dilute acid, such as dilute hydrochloric acid (HCl). This will dissolve the Cu(OH)_2, converting it into soluble CuCl_2, while CuS will remain as an undissolved solid because CuS does not react with dilute acids.

Step 3: Filtration

After adding the dilute HCl and allowing the reaction to complete, filter the mixture. The CuS will stay on the filter paper as it is insoluble, while the CuCl_2 solution will pass through, separating the two components.

Step 4: Rinse and Dry the Precipitate

Wash the CuS residue on the filter paper with distilled water to remove any remaining CuCl_2 solution, and then dry the CuS for pure solid form.

Key Concepts

Precipitation

Precipitation is a crucial separation technique in chemistry. It involves forming a solid from a solution during a chemical reaction. In the context of separating CuS and Cu(OH)_2, CuS remains as a solid because it does not dissolve in water or react with the chosen dilute acid. This stability allows it to be separated as a precipitate. Precipitates are usually formed when the product of the concentration of the ions in solution exceeds the solubility product. They can also occur due to a change in temperature or the addition of a precipitating agent, such as an acid in our example, where Cu(OH)_2 is transformed into a soluble substance, leaving behind the insoluble CuS.

Filtration

Filtration is a practical method to separate solids from liquids or gases. After the Cu(OH)_2 reacts with the acid to form a soluble product, the mixture needs to be filtered to isolate CuS. This technique involves pouring the mixture over a porous material, like filter paper, which allows the liquid component to pass through while retaining the solid material. Key points include:

• CuS is retained on the filter paper as it is insoluble and remains as a solid.
• The dissolved CuCl_2 passes through the filter paper with the liquid, effectively separating the two components.

Filtration is ideal for separating components based on their solubility and physical state, making it invaluable in similar scenarios.

Chemical Properties

Chemical properties tell us how substances interact with other chemicals. Knowing these properties is key to choosing the right separation method. For example, CuS’s low reactivity with dilute acids and insolubility in water guide the decision-making process. Cu(OH)_2, on the other hand, reacts with dilute acids, changing its solubility and making it possible to separate these compounds using chemical reactions. Understanding these properties allows us to predict how substances will behave in different chemical environments, enabling effective separation and purification techniques.

Reaction with Acids

When substances react with acids, they can undergo changes that facilitate separation. In our example, dilute hydrochloric acid ( HCl) is used to react with Cu(OH)_2, transforming it into a soluble form. Here’s how this reaction assists in the process:

• The acid reacts with Cu(OH)_2, forming copper chloride ( CuCl_2), which dissolves in water.
• CuS does not react with the acid, remaining as an insoluble solid in the mixture.

These reactions with acids are essential for dissolving certain compounds while leaving others intact, providing a precise means to isolate components from a mixture.