Problem 28 - Chemistry and Chemical Reactivity

Question

A soft, white waxy solid melts over a temperature range from \(120^{\circ} \mathrm{C}\) to \(130^{\circ} \mathrm{C}\). It doesn't dissolve in water and it doesn't conduct electricity. These properties are consistent with its identity as (a) a network solid (c) an amorphous solid (b) an ionic solid (d) a metallic solid

Step-by-Step Solution

Answer

The substance is an amorphous solid.

Step 1: Identify Key Properties

First, identify the key properties of the substance: it melts over a range from 120°C to 130°C, does not dissolve in water, and does not conduct electricity.

Step 2: Understand Property Implications

Melting over a range of temperatures typically indicates an irregular structure, which is characteristic of amorphous solids. Being non-soluble in water and non-conductive further supports this classification.

Step 3: Analyze Each Option

(a) Network solids typically have very high melting points and are non-conductive but do not melt over a range. (b) Ionic solids generally dissolve in water and conduct electricity in molten form. (c) Amorphous solids melt over a range and fit the described properties well. (d) Metallic solids conduct electricity, which does not match the given properties.

Step 4: Select the Most Likely Option

Based on the characteristics provided, the substance is an amorphous solid, as it melts over a range, does not dissolve in water, and does not conduct electricity.

Key Concepts

Melting Point Range

When we talk about a substance melting over a temperature range, it's essential to consider its physical structure. An amorphous solid, unlike a crystalline solid, lacks a well-defined, orderly arrangement of atoms or molecules. This irregular structure causes it to melt not at a single temperature but rather over a range. This range occurs because different parts of the amorphous material require varying amounts of energy to transition from solid to liquid.

• Crystalline solids melt at a specific temperature due to their uniform structure.
• An amorphous solid's melting point range is crucial in identifying it, as seen in materials like glass and certain types of wax.

Understanding this concept is vital because it helps differentiate between substances when other properties, such as conductivity and solubility, are also considered.

Electrical Conductivity

Electrical conductivity is a material's ability to allow the flow of electrically charged particles. In the context of solids, this property relies heavily on the types of bonds and the arrangement of atoms within the solid. Metals are typically good conductors because they have free-moving electrons that carry electrical charge across the material.

• Amorphous solids, however, generally do not conduct electricity well.
• This is due to the lack of free electrons or mobile ions needed for the current to pass through.

Some substances like ionic solids will conduct electricity only in molten or dissolved form, unlike amorphous solids, which maintain their non-conductive nature. Therefore, the lack of electrical conductivity helps to confirm the amorphous status of certain solids, as observed in this exercise.

Solubility in Water

Solubility refers to a substance's ability to dissolve in a solvent, like water. For a substance to dissolve, its molecules or ions must interact more favorably with water molecules than with each other. This interaction is often influenced by the polarity of the substance and the solvent.

• Ionic solids tend to be water-soluble because their charged particles can be stabilized by the polar water molecules.
• Amorphous solids, on the other hand, often exhibit low solubility in water.

This low solubility is due to the non-polar characteristics of many amorphous solids, which do not attract the polar water molecules effectively. Thus, a solid which does not dissolve in water aligns with the properties of an amorphous material, confirming its identity in the given scenario.