Question

The concentration of copper in the surface organic layer of a forest soil is \(37 \mathrm{ppm}\), and in the underlying mineral layer it is 17 ppm. The bulk densities of these two layers are \(0.36\) and \(1.22 \mathrm{gmL}^{-1}\) respectively, The very low density of the surface material is because it consists in large part of partially degraded organic material and there is very little of the heavier mineral matter. Which of these two layers has the larger amount of copper per unit volume?

Short answer

The mineral layer has a larger copper mass per unit volume.

Step-by-step solution

Identify the Quantity to Calculate

We need to find which layer has a larger amount of copper per unit volume. This means calculating the amount of copper (mass) in a unit volume for each layer.

Formula for Copper Mass per Unit Volume

To find the mass of copper per unit volume, we use the formula: \[ \text{Copper Mass per Unit Volume (mg/cm}^3\text{)} = \text{Copper Concentration (ppm)} \times \text{Bulk Density (g/cm}^3\text{)} \] where 1 ppm is equivalent to 1 mg of substance per kg of the material.

Calculate for Organic Layer

For the surface organic layer: \[ \text{Copper Mass} = 37 \text{ ppm} \times 0.36 \text{ g/mL} = 37 \text{ mg/kg} \times 0.36 \text{ g/cm}^3 \] Convert units: 1 g/mL = 1000 g/L = 1 g/cm³. Then, \[ \text{Copper Mass} = 37 \text{ mg/kg} \times 0.36 \text{ g/cm}^3 \approx 13.32 \text{ mg/cm}^3 \]

Calculate for Mineral Layer

For the underlying mineral layer: \[ \text{Copper Mass} = 17 \text{ ppm} \times 1.22 \text{ g/mL} = 17 \text{ mg/kg} \times 1.22 \text{ g/cm}^3 \] \[ \text{Copper Mass} = 17 \text{ mg/kg} \times 1.22 \text{ g/cm}^3 \approx 20.74 \text{ mg/cm}^3 \]

Compare the Two Layers

Now compare the two: The copper mass per unit volume is \(13.32 \text{ mg/cm}^3\) for the organic layer and \(20.74 \text{ mg/cm}^3\) for the mineral layer. Thus, the mineral layer has a larger copper mass per unit volume.

Key concepts

Forest Soil Layers

When examining forest soils, it is important to note that they are structured into distinct horizontal layers or 'horizons'. These layers include a surface organic layer and a deeper mineral layer, each with unique characteristics.
The **surface organic layer** is primarily composed of decomposed plant material like leaves, branches, and other organic matter. This layer plays a crucial role in nutrient cycling, storing essential nutrients that feed plant roots and support microbial life.
On the other hand, the **mineral layer** lies beneath the organic layer and contains a higher concentration of inorganic materials such as sand, silt, and clay. It provides a key structural foundation and influences water movement and retention in the soil. Together, these layers create a dynamic environment that supports forest ecosystems.

Bulk Density

Bulk density of soil is a key measure of soil health and structure. It represents the mass of soil in a given volume, usually expressed in grams per cubic centimeter (g/cm³).
- **Organic Layer Bulk Density:** This layer typically has a lower bulk density, often around 0.36 g/cm³, due to its composition of light and airy decomposed organic matter. Its low density helps maintain soil aeration and porosity.
- **Mineral Layer Bulk Density:** In contrast, the mineral layer has a higher bulk density, such as 1.22 g/cm³ in our example. This is due to its compact nature, containing denser materials like minerals and clay, making it heavier and more compacted.

• Bulk density provides insights into the soil's porosity and compaction levels, impacting root penetration and water movement.

Unit Volume Calculation

Calculating the copper mass per unit volume involves understanding volume and mass relationships, specifically within soil contexts. The calculation helps determine the amount of a substance, like copper, in a specific volume of soil.
The formula for copper mass per unit volume is: \[ \text{Copper Mass per Unit Volume (mg/cm}^3\text{)} = \text{Copper Concentration (ppm)} \times \text{Bulk Density (g/cm}^3\text{)} \]
Using this formula, students can calculate how much copper is present in a cubic centimeter of soil by taking into account both the ppm (parts per million) concentration of the substance and the bulk density of the soil layer. This mathematical tool can reveal significant differences in copper content between layers, aiding soil management and environmental studies.

Organic vs Mineral Layer

Organic and mineral layers in forest soils differ in their composition, texture, and role in the ecosystem.
- **Organic Layer:** Predominantly made up of decomposed organic matter, this layer is rich in nutrients but typically has a lower copper concentration when expressed per unit volume due to its lighter density. Organic matter acts as a reservoir for nutrients and aids in retaining moisture.
- **Mineral Layer:** Comprising mainly mineral particles, it has less organic matter but a higher density, contributing to a higher copper concentration per unit volume. This layer provides firmness and stability to the soil structure.

• These differences underscore the unique functions each layer plays in sustaining forest life, with minerals imparting strength and structure, and organics enhancing fertility and moisture retention.

Environmental Chemistry Problem Solving

Environmental chemistry often involves analyzing natural substances and their interrelation with the environment, using problem-solving skills to assess issues like soil contamination.
In this context, recognizing how different factors such as bulk density and concentration levels influence copper content in soil layers is crucial. **Problem-solving** may involve identifying pollutants, assessing risks to plant and animal life, and devising remediation strategies.
By calculating and comparing chemical concentrations in various soil layers, students can understand how pollutants distribute across environments. This understanding is vital for tasks such as:

• Determining potential sources of soil contamination.
• Designing methods to manage or remediate contaminated sites.
• Predicting the environmental impact of pollutants on ecosystems.

Such analyses support informed decision-making and sustainable environmental practices.