Problem 40 - Chemistry Matter and Change

Question

Boiling Water At what temperature would 250 \(mL\) of water boil? 1000 \(mL\) ? Is the boiling point an intensive or extensive property? Explain.

Step-by-Step Solution

Answer

Both 250 mL and 1000 mL of water will boil at the same temperature of 100 °C, as boiling point is an intensive property. It means that it does not depend on the amount of material or water, but depends on the type of material and remains constant under specific conditions, such as pressure. In the case of water, boiling point is determined by the hydrogen bonds within the water molecules and their interactions, and is independent of the amount of water.

Step 1: Recall the standard boiling point of water

The standard boiling point of water is 100 °C (212 °F) at the sea-level atmospheric pressure (1 atm or 101.325 kPa).

Step 2: Determine boiling point at 250 mL and 1000 mL

Since the boiling point is independent of the amount of water, both 250 mL and 1000 mL of water will boil at the same temperature, which is 100 °C.

Step 3: Classify boiling point property

The boiling point is an intensive property, as it does not depend on the amount of the material (in our case, it is the amount of water) and remains constant for a given substance under specific conditions (such as pressure).

Step 4: Explain why boiling point is an intensive property

An intensive property (such as boiling point, melting point, or density) is a property that only depends on the type of material and not on its amount. In this case, the boiling point of water is determined by the hydrogen bonds within the water molecules and their interactions. The boiling point does not change if we increase or decrease the amount of water. Therefore, boiling point is concluded to be an intensive property.

Key Concepts

Intensive Property

An intensive property is an attribute of a substance that remains unchanged regardless of the quantity of the material. This concept is crucial in distinguishing between properties that depend solely on the nature of the substance itself rather than its mass or volume. When we talk about water's boiling point, it is an example of an intensive property.
For instance, whether you have 250 mL or 1000 mL of water, the temperature at which it boils remains the same under a given atmospheric pressure—this is because boiling point is an intensive property. Similarly, other intensive properties include:

• Density
• Refractive index
• Temperature
• Melting point

These can vary depending on factors like pressure or temperature but not on the mass or volume of the material.

Standard Boiling Point

The standard boiling point of a substance is the temperature at which it transitions from a liquid to a vapor under a pressure of 1 atmosphere (atm). For water, this is typically 100 °C at sea level, where the pressure is about 101.325 kPa.
The concept of a standard boiling point is crucial in science because it provides a reference point from which we can compare changes that occur under different conditions. While the standard boiling point for water is pegged at 100 °C at sea level, it should be noted that this can vary with different atmospheric pressures. For example, at higher altitudes where atmospheric pressure is lower, water boils at a lower temperature.
By understanding the standard conditions, predictions and calculations can be made for boiling points at various pressures, enhancing our grasp of thermodynamic concepts.

Hydrogen Bonds

Hydrogen bonds play a vital role in determining the physical properties of water, including its boiling point. A hydrogen bond is a weak attraction between the positive hydrogen atom of one water molecule and the negative oxygen atom of another.
These interactions are strong enough to provide water with its high boiling point relative to other molecules of similar size, such as methane or oxygen. Water's hydrogen bonds require more energy—in the form of heat—to break, explaining why water has to be heated to 100 °C to transform from liquid to vapor at sea-level pressure.
In general, substances with strong intermolecular forces such as hydrogen bonds will have a higher boiling point. This is why understanding molecular interactions like hydrogen bonds is essential in explaining why water behaves uniquely compared to other substances.

Atmospheric Pressure

Atmospheric pressure is the force that air exerts on objects—a critical factor affecting the boiling point of liquids. Under normal conditions, at sea level, this pressure is approximately 101.325 kPa or 1 atm. Alterations in atmospheric pressure can dramatically change the temperature at which a liquid boils.
At decreased atmospheric pressures, like at higher altitudes, the boiling point of water reduces. That’s why water boils at temperatures below 100 °C atop mountains. Conversely, in a pressure cooker, atmospheric pressure is increased, raising the boiling point above 100 °C and cooking food faster.
Understanding atmospheric pressure helps us predict and manipulate boiling points, important in fields like cooking, engineering, and even physiochemical studies.