On fusion of a mixture of `Na_2CO_3` and ` CaCO_3` with silica at `1500^@C`, a liquid consisting of silicates of sodium and calcium is formed. On cooling . Liquid become viscous and eventually ceases to flow . It becomes solid and is known as glass. By varying the proportions of the three basic ingredients and by adding other substances, the properties of glass can be altered, Glass can be represented as `R_2 O. MO.6SiO_2`, where `R= Na` or `K, M = CaBa, Zn` or `PbSiO_2` may be replaced by `Al_2 O_3 , B_2 O_3` or `P_2 O_5`. Coloured glasses are obtained by adding certain metallic oxides or salts in the fused mass, Glass is attached by `HF` and this property is used to make marking on the glass , This is known as etching . The glass on rapid cooling becomes brittle and fragile . The articles of glass are cooled neither slowly nor very rapidly . The process of gradual cooling of glass is called annealing .
Glass is .

Options

A. Solid
B. Liquid
C. Supercooled liquid
D. Colloidal solution

Answer

To solve the question regarding the nature of glass, we will analyze the information provided step by step.

### Step-by-Step Solution:

1. **Understanding the Formation of Glass**:
- When a mixture of sodium carbonate (Na₂CO₃) and calcium carbonate (CaCO₃) is fused with silica at a high temperature (1500°C), a liquid is formed. This liquid consists of silicates of sodium and calcium.

**Hint**: Remember that the high temperature is essential for the fusion process to occur.

2. **Cooling Process**:
- Upon cooling, this liquid does not solidify immediately. Instead, it becomes viscous and eventually ceases to flow. This indicates that the material is transitioning from a liquid state to a more solid-like state without crystallizing.

**Hint**: Focus on the behavior of the liquid as it cools; it becomes less fluid but does not form a crystalline solid.

3. **Defining Glass**:
- The final product, which is formed after the cooling process, is known as glass. It is important to note that glass retains some liquid-like properties even in its solid state.

**Hint**: Consider the characteristics of glass that differentiate it from typical solids.

4. **Properties of Glass**:
- The question mentions that glass can be represented by the formula R₂O·MO·6SiO₂, where R and M can be various elements. This indicates that glass can have varying compositions, affecting its properties.

**Hint**: The formula suggests that glass is a complex mixture rather than a simple compound.

5. **Nature of Glass**:
- The key point to consider is that glass, while solid, behaves like a supercooled liquid. It does not have a definite crystalline structure and can flow very slowly over time.

**Hint**: Think about the definition of a supercooled liquid and how it applies to the properties of glass.

6. **Conclusion**:
- Based on the above analysis, glass is best described as a supercooled liquid. It is not a typical solid or liquid, nor is it a colloidal solution.

**Final Answer**: Glass is a supercooled liquid.

### Summary:
Glass is formed from the fusion of Na₂CO₃, CaCO₃, and silica at high temperatures. Upon cooling, it transitions into a viscous state and eventually becomes solid, but it retains properties of a supercooled liquid.