Question

You may have noticed that when water boils, you can see bubbles that rise to the surface of the water. a. What is inside these bubbles? i. air ii. hydrogen and oxygen gas iii. oxygen gas iv. water vapor v. carbon dioxide gas b. Is the boiling of water a chemical or physical change? Explain.

Short answer

a. The bubbles that rise to the surface of the water when it boils primarily contain iv. water vapor. b. Boiling water is a physical change because no new substance is formed during the process. The water simply changes its phase from liquid to gas, and the composition of water remains the same (H2O) in both phases.

Step-by-step solution

Identify the contents of bubbles

When water boils, the bubbles that rise to the surface of the water primarily contain water vapor, which is the gaseous phase of water. Therefore, the correct answer is option iv. water vapor. #Step 2: Determine if boiling of water is a chemical or physical change#

Determine the type of change during boiling

To determine whether the boiling of water is a chemical or physical change, we need to consider if there is any new substance formed during the process. When water boils, there is no new substance formed; instead, the water simply changes its phase from liquid to gas (water vapor). This indicates that boiling water is a physical change rather than a chemical change. #Step 3: Provide an explanation for the type of change#

Explain the type of change

Boiling water is a physical change, as no new substance is formed during the process. The boiling of water involves breaking intermolecular forces (specifically, hydrogen bonds) between water molecules in the liquid phase, but it does not involve breaking or forming any chemical bonds within the water molecules themselves. The composition of water remains the same (H2O) whether it is in liquid or gaseous form. So, the answers to the exercise are: a. iv. water vapor b. The boiling of water is a physical change because no new substance is formed during the process, and the water simply changes its phase from liquid to gas.

Key concepts

Boiling

Boiling is the process that occurs when a liquid turns into a gas. It happens when the liquid is heated to its boiling point. For water, this temperature is usually about 100°C (212°F) at sea level. Boiling is an interesting phenomenon because it involves a phase change from liquid to gas, but there's no change in chemical composition. This is because boiling is a physical change. During boiling, heat energy is absorbed by the liquid, causing the molecules to move more energetically. This energy is used to break the intermolecular forces rather than chemical bonds.

It's important to note that boiling occurs throughout the bulk of the liquid. This is different from evaporation, which only happens at the surface. The bubbles you see during boiling are made of the liquid in vapor form. As the energetic molecules break away from the rest of the liquid, they form bubbles that rise to the surface where they release the vapor into the air.

Water Vapor

Water vapor is the gaseous form of water. It is part of the water cycle—an essential natural process on Earth. During boiling, water molecules escape from the liquid state to become water vapor. Because water vapor is less dense than liquid water, it forms bubbles. These bubbles are what you observe rising to the surface when water is boiling.

Water vapor is crucial in maintaining the Earth's climate and weather patterns. It acts as a greenhouse gas, trapping heat and contributing to the warming of the planet. Additionally, water vapor is invisible to the naked eye. The white "steam" we often associate with boiling water isn’t water vapor but is rather tiny droplets of liquid water suspended in the air.

Intermolecular Forces

Intermolecular forces are the attractions between molecules that hold them together, especially important in liquids and solids. In water, the most significant intermolecular force is the hydrogen bond. This occurs because of the polarity of the water molecules. Each water molecule has partial positive charges near the hydrogen atoms and a partial negative charge near the oxygen atom.

• Hydrogen bonds are relatively strong compared to other intermolecular forces. This is why water has high boiling and melting points for a molecule of its size.
• When boiling occurs, these hydrogen bonds must be broken to allow water molecules to transition from liquid to vapor.

Although intermolecular forces are disrupted during boiling, chemical bonds within the molecules remain intact. Thus, no new substances are formed during the process, which is why boiling is classified as a physical change. Understanding the nature of intermolecular forces helps explain why different substances have various boiling points and physical properties.