Question

A white substance melts with some decomposition at \(730^{\circ} \mathrm{C}\). As a solid, it does not conduct electricity, but it dissolves in water to form a conducting solution. Which type of solid (molecular, metallic, covalent- network, or ionic) might the substance be?

Short answer

Based on the given properties of the substance (melting point of 730°C, non-conducting in solid state, and conducting when dissolved in water), we can conclude that the substance is likely to be an ionic solid.

Step-by-step solution

Characteristics of Different Solid Types

To solve this problem, let's first briefly review the characteristics of different types of solids: 1. Molecular solids: Soft, low to moderate melting points, poor electrical conductivity. 2. Metallic solids: Hard, high melting points, good electrical conductivity. 3. Covalent-network solids: Hard, very high melting points, poor electrical conductivity. 4. Ionic solids: Hard, high melting points, poor electrical conductivity in solid state but good in dissolved or molten state. Now, let's compare these with the given properties of the unknown substance:

Melting Point

The substance has a melting point of 730°C, so it can either be a metallic, covalent-network, or ionic solid. This rules out molecular solids as they tend to have low to moderate melting points.

Electrical Conductivity in Solid State

The substance does not conduct electricity in solid form, so it can't be a metallic solid, as they are good conductors of electricity. Both covalent-network and ionic solids exhibit poor electrical conductivity in solid state, so it might be either one of them.

Conducting Solution when Dissolved in Water

When the substance is dissolved in water, it forms a conducting solution. Covalent-network solids do not dissolve in water and hence cannot form conducting solutions. Ionic solids, however, dissolve in water and form conducting solutions due to the presence of mobile ions.

Conclusion

Based on the given properties and the comparison with the characteristics of different solid types, we can conclude that the substance is likely to be an ionic solid.

Key concepts

Electrical Conductivity of Ionic Solids

Understanding the electrical conductivity of ionic solids is fundamental to identifying types of materials. Ionic solids consist of positively and negatively charged ions held together in a crystal lattice by electrostatic forces.

In their solid state, these ions are fixed in position and unable to flow, rendering the material an electrical insulator. However, when ionic solids dissolve in water, those ions become free to move. This mobility allows them to carry an electric charge, making the solution an electrolyte with good electrical conductivity.

Similarly, upon melting, the ionic compounds become molten and the ions are again free to move about, which also results in good electrical conductivity. This property is key in processes like electrolysis and is deeply connected to the behavior of the substance given in the exercise, indicating towards it being an ionic solid.

Melting Point of Ionic Solids

The melting point of a solid is an indication of the strength of the forces holding its particles together. Ionic solids have considerably high melting points; the extent of which depends on the charge and size of the ions that compose the solid.

The more charged the ions, and the smaller their size, the stronger the electrostatic forces between them are, leading to higher melting points. Because ionic bonds are quite strong, a significant amount of energy is required to overcome these forces and convert the solid into a liquid.

This characteristic correlates with the white substance mentioned in the exercise, which has a high melting point at 730°C. This high melting temperature suggests that the substance might be an ionic solid, as molecular solids typically have much lower melting points, and the condition of melting with some decomposition is also consistent with the behavior of many ionic compounds.

Solubility of Ionic Solids in Water

Solubility is the measure of how well a substance can dissolve in a solvent. Ionic solids tend to be soluble in polar solvents like water due to the polar solvent's ability to stabilize the ions that result from the dissolution process.

When an ionic solid dissolves, water molecules surround each ion, a process known as hydration. The positive end of the water molecules is attracted to negative ions (anions), and the negative end is attracted to positive ions (cations).

Hydration Energy and Lattice Energy

The process of dissolving involves two main energies: hydration energy and lattice energy. The hydration energy comes from the solvent molecules stabilizing the ions, while the lattice energy is the energy required to separate the ions from the crystal lattice.

If the hydration energy is greater than or equal to the lattice energy, the ionic solid will dissolve. The substance described in the textbook solution is soluble in water, further supporting the conclusion that it is likely an ionic solid because the water molecules are able to facilitate the dissolution of its ionic lattice.