At atmospheric pressure ice crystallises in __________ for.

Answer

To answer the question "At atmospheric pressure ice crystallises in __________ form," we can follow these steps:

### Step 1: Understand the Structure of Ice
Ice is a solid form of water (H₂O) and has a specific crystalline structure. The arrangement of water molecules in ice is crucial to understanding its properties.

**Hint:** Think about the arrangement of molecules in solids and how they can form different structures.

### Step 2: Identify the Common Forms of Ice
Ice can crystallize in different forms depending on the temperature and pressure. The two main forms of ice are:
- Hexagonal form (most common at atmospheric pressure)
- Cubic form (can occur at lower temperatures)

**Hint:** Recall the different crystalline forms of ice and their conditions of formation.

### Step 3: Atmospheric Pressure Conditions
At atmospheric pressure, the most stable and common crystalline form of ice is hexagonal. This is the form that ice takes when it freezes at standard conditions.

**Hint:** Consider the conditions under which ice typically freezes in nature.

### Step 4: Conclusion
Thus, at atmospheric pressure, ice crystallizes in a **hexagonal form**.

**Final Answer:** At atmospheric pressure, ice crystallizes in **hexagonal form**.