Question

Choose the best answer to each of the following. Explain your reasoning with one or more complete sentences. Frozen lakes often have liquid water beneath their icy surfaces primarily because (a) Earth's internal heat keeps the water liquid; (b) ice floats and provides insulation to the water below; (c) sunlight penetrates the ice and warms the water below.

Short answer

The best answer is (b) ice floats and provides insulation to the water below.

Step-by-step solution

Analyze the Situation

Consider why lakes freeze on the surface while remaining liquid beneath. Frozen lakes form a solid ice layer on the surface when the air temperature is below freezing. The properties of water and ice play a vital role in this situation.

Understand Ice Properties

Ice is less dense than liquid water, which causes it to float. As a result, when lakes freeze, ice forms on the top surface, while water remains liquid underneath.

Consider Insulation Effect

The ice layer on the lake's surface acts as an insulating barrier. This insulation slows the rate at which the heat in the water escapes into the colder air above. This process helps maintain the water beneath the ice in a liquid state, even when air temperatures are very low.

Evaluate External Heat Sources

While the Earth’s internal heat does contribute to keeping underground water sources warmer, in the case of a lake, this is a minor factor compared to the insulating effect of ice. Also, sunlight penetration is limited through ice and does not significantly affect the temperature of the water below the ice.

Choose the Best Answer

Based on the analysis of the ice's insulating effect and its role in preventing the lake from completely freezing, the most accurate answer is (b) ice floats and provides insulation to the water below.

Key concepts

Ice Properties

Ice is a fascinating substance with some unique properties that play a crucial role in the natural world. One of the key characteristics of ice is its density. Unlike most substances, ice is less dense than its liquid form, water.
This means that ice will float on water, rather than sink. This happens because when water freezes, it expands and the molecules arrange themselves in a crystalline structure that occupies more space.
As a result, when a lake freezes, the ice forms on the upper surface, while the denser liquid water stays underneath. This effect is essential for aquatic life as it prevents entire bodies of water from freezing solid during cold months.

Thermal Insulation

The ice layer on top of a frozen lake serves as more than just a surface crust; it acts as an important insulating shield for the water beneath. This insulation property means that ice slows down the rate of heat transfer from the liquid water to the cold air above it.
When ice forms, it creates a barrier that keeps the temperature of the water below relatively stable, even as air temperatures plummet below freezing. This insulating effect ensures that the ecosystem in the water can still survive by maintaining liquid water beneath.

• This insulation is crucial during winter, creating a habitat where aquatic life can continue to thrive.
• It can prevent further heat loss, which might otherwise lead to deeper freezing of the lake.

Water Density

Water is unique in that its maximum density occurs at a temperature of 4°C (39.2°F). This is a result of a molecular structure that packs water molecules most efficiently at this temperature.
When water is cooled down from this point, it becomes less dense until it freezes. Because of the anomaly of water's density behavior, when lakes cool down, the colder water (less dense) stays on the top and eventually freezes, while the slightly warmer, denser water remains at the bottom.
This stratification by density is unique to water and allows organisms, such as fish, to survive in the liquid layers beneath the ice during cold periods. This underlies why simply freezing temperatures aren't enough to freeze a lake entirely from top to bottom.

Heat Transfer

Heat transfer is a fundamental concept that explains how thermal energy moves from one place to another. In the context of frozen lakes, we are mainly concerned with how heat moves from the warmer water below the ice to the colder air above it.
There are three primary mechanisms of heat transfer: conduction, convection, and radiation. For a frozen lake, conduction is particularly important because it describes how heat moves through the ice layer.

• Conduction happens when heat travels through materials, like ice, which is why the ice layer influences how heat escapes from the lake.
• Convection would normally move water around, but since the ice adds a solid cap, it limits this process to the water beneath the ice.

Radiation from the sun has limited impact, as ice can reflect much of the solar energy away. Understanding these processes is key to knowing how a lake might lose its heat during the winter.