Calculate the percentage of water in the soil sample given initial mass and final dry mass.

$$\text{Initial combined volume} = \text{Volume of soil used} + \text{Volume of water added}$$ $$\text{Initial combined volume} = 40 \text{ cm}^3 + 50 \text{ cm}^3 = 90 \text{ cm}^3$$

The difference between the initial combined volume and the final volume represents the volume of air displaced by water.

$$\text{Volume of air} = \text{Initial combined volume} - \text{Final volume of water + soil after mixing}$$ $$\text{Volume of air} = 90 \text{ cm}^3 - 80 \text{ cm}^3 = 10 \text{ cm}^3$$

The volume of air in the soil sample is $\boxed{\text{10 cm}^3}$.

$$\text{Percentage of air} = \frac{\text{Volume of air}}{\text{Volume of soil used}} \times 100\%$$ $$\text{Percentage of air} = \frac{10 \text{ cm}^3}{40 \text{ cm}^3} \times 100\%$$ $$\text{Percentage of air} = 0.25 \times 100\% = 25\%$$

The percentage of air in the soil sample is $\boxed{\text{25\%}}$.

In another investigation, 80g of a fresh soil sample was heated in a crucible. After repeated heating and cooling, the constant mass of the soil was found to be 65g. Calculate the percentage of water in the soil sample and explain the factors that affect the water content of a given soil.

$$\text{Mass of water} = \text{Mass of fresh soil} - \text{Mass of dry soil}$$ $$\text{Mass of water} = 80 \text{ g} - 65 \text{ g} = 15 \text{ g}$$

$$\text{Percentage of water} = \frac{\text{Mass of water}}{\text{Mass of fresh soil}} \times 100\%$$ $$\text{Percentage of water} = \frac{15 \text{ g}}{80 \text{ g}} \times 100\%$$ $$\text{Percentage of water} = 0.1875 \times 100\% = 18.75\%$$

The percentage of water in the soil sample is $\boxed{\text{18.75\%}}$.

Factors that affect the water content of a given soil:

• Soil texture: The proportion of sand, silt, and clay particles determines pore size and surface area, influencing how much water the soil can hold and how quickly it drains. Clay soils generally retain more water than sandy soils.
• Organic matter content: Organic matter acts like a sponge, significantly increasing the soil's water-holding capacity.
• Soil structure: The arrangement of soil particles into aggregates affects pore space distribution, influencing water infiltration and retention.
• Climate: Factors like rainfall, temperature (affecting evaporation), and humidity directly impact the amount of water present in the soil.