Crystallisation is a process that separates a pure solid in the form of its crystals from a solution. A solid can also be separated from its solution by evaporation. However crystallisation technique is better than simple evaporation technique. We can get an impure sample of a solid even after evaporation. Some solids decomposes or some may get charred on heating to dryness during evaporation.
In which of the following processes, crystallisation is used.

Explanation

To determine in which processes crystallization is used, we need to understand the concept of crystallization and its applications. Here’s a step-by-step solution:

### Step 1: Understand Crystallization
Crystallization is a separation technique that allows us to obtain a pure solid in the form of its crystals from a solution. It is particularly useful for separating soluble solids from their solvents.

**Hint:** Remember that crystallization results in pure crystals, while other methods may not.

### Step 2: Identify the Context
In the context of the question, we are looking for processes where crystallization is applied. One common example is the separation of salt from seawater.

**Hint:** Think about common substances that can be separated from solutions, such as salt from water.

### Step 3: Consider the Evaporation Method
While evaporation can separate a solid from a solution, it often results in impurities remaining in the solid. This is because evaporation does not selectively remove impurities, and some solids may decompose or char when heated.

**Hint:** Evaporation may not yield pure substances, which is a key difference from crystallization.

### Step 4: Application of Crystallization
In the case of seawater, when salt is dissolved, the process of crystallization can be used to purify the salt. After evaporating the seawater, the salt obtained may be impure. By recrystallizing the salt, we can remove the impurities, resulting in pure salt.

**Hint:** Recrystallization is a crucial step in purifying substances obtained from solutions.

### Step 5: Conclusion
Based on the understanding of crystallization and its applications, we can conclude that crystallization is used in processes where the goal is to obtain pure solids from solutions, such as purifying salt from seawater.

**Final Answer:** Crystallization is used in the purification of salt obtained from seawater.