Problem 18 - Chemistry The Central Science

Question

Read the following description of the element zinc and indicate which are physical properties and which are chemical properties. Zinc is a silver-gray- colored metal that melts at \(420^{\circ} \mathrm{C}\). When zinc granules are added to dilute sulfuric acid, hydrogen is given off and the metal dissolves. Zinc has a hardness on the Mohs scale of 2.5 and a density of \(7.13 \mathrm{~g} / \mathrm{cm}^{3}\) at \(25^{\circ} \mathrm{C}\). It reacts slowly with oxygen gas at elevated temperatures to form zinc oxide, \(\mathrm{ZnO}\).

Step-by-Step Solution

Answer

Physical properties of Zinc: 1. Melting point: \(420^{\circ}\mathrm{C}\) 2. Hardness on Mohs Scale: 2.5 3. Density: \(7.13 \frac{g}{cm^3}\) at \(25^{\circ}\mathrm{C}\) Chemical properties of Zinc: 1. Reaction with dilute sulfuric acid to produce hydrogen gas 2. Reacts with oxygen gas at elevated temperatures to form Zinc Oxide (ZnO)

Step 1: 1. Melting Point

The given melting point of Zinc is \(420^{\circ}\mathrm{C}\). Melting points are physical properties; we can determine this property without changing the chemical composition of Zinc.

Step 2: 2. Reaction with Sulfuric Acid

Zinc reacting with dilute sulfuric acid to produce hydrogen gas is a chemical reaction. The zinc is dissolving and changing its chemical composition, which indicates that this is a chemical property.

Step 3: 3. Hardness on the Mohs Scale

The hardness of Zinc (2.5 on the Mohs scale) can be observed without changing the chemical composition of the substance. This makes it a physical property.

Step 4: 4. Density

Density is a physical property because it can be determined without changing the substance's chemical composition. The density of Zinc is given as \(7.13 \frac{g}{cm^3}\) at \(25^{\circ}\mathrm{C}\).

Step 5: 5. Reaction with Oxygen

The reaction of Zinc with oxygen gas at elevated temperatures to form Zinc Oxide (ZnO) is a change in its chemical composition. It is forming a new compound, Zinc Oxide, so this is a chemical property. Here we provide the physical properties and chemical properties of Zinc as required: Physical properties of Zinc: 1. Melting point: \(420^{\circ}\mathrm{C}\) 2. Hardness on Mohs Scale: 2.5 3. Density: \(7.13 \frac{g}{cm^3}\) at \(25^{\circ}\mathrm{C}\) Chemical properties of Zinc: 1. Reaction with dilute sulfuric acid to produce hydrogen gas 2. Reacts with oxygen gas at elevated temperatures to form Zinc Oxide (ZnO)

Key Concepts

Melting Point

The melting point of a substance is a key physical property, representing the temperature at which it changes from a solid to a liquid. For zinc, this temperature is precisely at \(420^{\circ}\mathrm{C}\). At this temperature, zinc transitions from its solid silver-gray appearance to a liquid form. This transformation does not involve any alteration in the chemical composition of zinc, making it a distinct physical property. Understanding melting points is vital, as it helps predict how substances will behave in different environments, such as extreme temperatures.

• Helps identify the substance by comparing known melting points.
• Assists in determining purity, as impurities typically change the melting point.
• Essential for applications involving heat, like melting metal for casting.

Density

Density is another important physical property, defined as the mass of a substance per unit volume. For zinc, this value is given as \(7.13 \mathrm{g/cm^3}\) at \(25^{\circ}\mathrm{C}\). Calculating density involves the formula: \[ \text{Density} = \frac{\text{Mass}}{\text{Volume}} \] This property is crucial because it can help identify a substance as well as characterize its packing efficiency at a given temperature.

• Density affects material selection for construction, as materials need different density characteristics depending on their application.
• Helps determine floatation – objects with densities lower than a fluid will float on it.
• Used in quality control, as consistent density indicates consistent material production.

Unlike chemical properties, assessing density doesn't require changing the chemical composition of zinc.

Chemical Reactions

Chemical reactions involve processes where substances combine or decompose, resulting in new material formations with different properties. Zinc participates in different reactions, demonstrating its chemical properties. One typical reaction is with diluted sulfuric acid, in which zinc dissolves and hydrogen gas is emitted. This reaction signifies a change where zinc transitions into another compound due to interacting elements.When zinc reacts with oxygen at higher temperatures, zinc oxide (\(\mathrm{ZnO}\)) is formed. This is another classic example of a chemical reaction where zinc combines with oxygen, altering the chemical structure to create a new product. Key aspects of chemical reactions include:

• They result in the formation of new substances.
• Are often irreversible and involve energy changes, such as heat or light.
• Can be influenced by factors like temperature, concentration, and presence of catalysts.

Understanding these properties is crucial in fields such as chemistry and material science, enabling the development of new materials and predicting substance behavior under various conditions.