A solid ‘X’ conducts electricity in solid state as well as in molten state.its conductance decreases with increase in temperature. Identify the solid X.

Based on the description you've provided, the solid 'X' you are referring to is likely an ionic compound, specifically sodium chloride (NaCl), commonly known as table salt. Ionic compounds like NaCl exhibit unique electrical properties that align with your criteria.

Characteristics of Ionic Compounds

Ionic compounds are formed from the electrostatic attraction between positively and negatively charged ions. In the case of sodium chloride, sodium ions (Na⁺) and chloride ions (Cl^-) come together to form a stable lattice structure. This structure has significant implications for its electrical conductivity.

Conductivity in Different States

• Solid State: In the solid state, ionic compounds do not conduct electricity. This is because the ions are fixed in place within the crystal lattice and cannot move freely. Without the movement of charged particles, there is no electrical conductivity.
• Molten State: When an ionic compound is melted, the rigid lattice breaks down, allowing the ions to move freely. This mobility enables the molten ionic compound to conduct electricity effectively, as the free-moving ions can carry an electric current.

Temperature Dependence of Conductivity

As you mentioned, the conductivity of solid 'X' decreases with an increase in temperature. This behavior is somewhat counterintuitive, as many materials typically show increased conductivity with rising temperatures. However, in the case of ionic solids, the decrease in conductivity can be attributed to the following factors:

• Thermal Expansion: As temperature increases, the lattice structure of the ionic solid expands. This expansion can lead to a greater distance between ions, reducing the effectiveness of their interactions and thus their ability to conduct electricity.
• Increased Lattice Vibrations: Higher temperatures cause the ions in the lattice to vibrate more vigorously. These vibrations can disrupt the orderly arrangement of ions, making it harder for them to move and carry charge.

Example: Sodium Chloride

Taking sodium chloride as an example, when it is heated, the increased thermal energy causes the ions to vibrate more intensely. While this might seem like it would enhance conductivity, the resulting lattice disruption and increased distance between ions can actually impede the flow of electricity. Therefore, the conductivity of solid NaCl decreases as the temperature rises.

Conclusion

In summary, the solid 'X' you are inquiring about is most likely sodium chloride, an ionic compound that conducts electricity in its molten state but not in its solid state. Its conductivity decreases with rising temperature due to thermal expansion and increased lattice vibrations. Understanding these properties helps in grasping the fundamental concepts of ionic compounds and their behavior under different conditions.