When ZnO is heated to elevated temperatures which is incorrect statement ?

Options

A. Conductivity increases
B. It becomes non stoichiometric
C. It acquires metal deficient defect
D. It acquires colour due to the trapped electrons

Answer

To determine which statement about zinc oxide (ZnO) when heated to elevated temperatures is incorrect, we can analyze the properties and behavior of ZnO under such conditions. Here’s a step-by-step breakdown of the solution:

### Step 1: Understanding the Heating of ZnO
When ZnO is heated to high temperatures, it undergoes a change in its structure and properties. The heat causes the zinc oxide to lose some of its oxygen content.

**Hint:** Consider how heating affects the composition of metal oxides.

### Step 2: Formation of Metal Excess Defect
As ZnO is heated, it tends to lose oxygen, leading to an increase in the concentration of zinc ions (Zn²⁺). This results in a metal excess defect, where the ratio of cations (Zn²⁺) to anions (O²⁻) is no longer stoichiometric.

**Hint:** Think about how the loss of oxygen affects the balance of cations and anions in the compound.

### Step 3: Conductivity Increase
The removal of oxygen creates "trapped electrons" in the lattice, which are free to move. This increase in mobile electrons enhances the electrical conductivity of ZnO.

**Hint:** Reflect on how the presence of free electrons influences conductivity in materials.

### Step 4: Color Change Due to Trapped Electrons
The trapped electrons can absorb energy and get excited to higher energy states. When they return to their ground state, they emit energy in the form of light, which gives ZnO a yellow color when heated.

**Hint:** Consider the relationship between electron excitation and the emission of light.

### Step 5: Identifying the Incorrect Statement
The question asks for the incorrect statement regarding the behavior of ZnO when heated. The statement claiming that ZnO acquires a "metal deficient defect" is incorrect. Instead, it exhibits a metal excess defect due to the increase in zinc ions from the loss of oxygen.

**Hint:** Pay attention to the terminology used to describe defects in solids; "metal excess" and "metal deficient" have specific meanings.

### Conclusion
The incorrect statement regarding ZnO when heated to elevated temperatures is that it acquires a metal deficient defect. In reality, it exhibits a metal excess defect due to the loss of oxygen and the resulting increase in zinc ions.

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