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 when water boils contain water vapor (option iv). b. The boiling of water is a physical change, as it involves a phase change from liquid to gas without altering the chemical composition of water (H2O). No new substance is formed during this process.

Step-by-step solution

Q1: Identify the contents of the bubbles

When water boils, the bubbles you see are formed due to the water molecules gaining enough energy to break their intermolecular bonds and change from the liquid phase to the gaseous phase. This gaseous phase of water is known as water vapor. So, the correct answer is: iv. water vapor

Q2: Determine if boiling water is a chemical or physical change

To answer this question, we need to understand the difference between chemical and physical changes. In a chemical change, the original substance reacts to form a new substance with a different chemical composition. In a physical change, the original substance undergoes a change in its state (such as solid, liquid, or gas) or its appearance, but its chemical composition remains the same. In the case of boiling water, the water just changes from its liquid state to its gaseous state (water vapor) without any alteration in its chemical composition (still H2O). There is no reaction taking place to form a new substance. Therefore, the boiling of water is a physical change.

Q2: Explanation

The boiling of water is a physical change because the change that occurs is a phase change (from liquid to gas). The chemical composition of water remains the same (H2O) before and after boiling. No new substance is formed during this process, so it is not a chemical change.

Key concepts

Water Vapor

When you put a pot of water on the stove and turn up the heat, you eventually see bubbles forming and rising to the surface. These bubbles are not air, nor are they purely hydrogen and oxygen gas. The bubbles are filled with water vapor, which is simply water in its gas phase.

Water vapor is formed when water molecules gain enough energy from the heat to overcome the attractive forces that hold them together in the liquid state. These energy-infused molecules move faster and farther apart, turning into gas that you see as water vapor within the bubbles. In essence, the transition of water from liquid to gas is what produces water vapor, which is an integral part of processes like boiling and evaporation.

Physical Change

Understanding the nature of changes that substances undergo is crucial in science. When boiling water, it is essential to determine whether this transformation is a physical or chemical change. A physical change is characterized by an alteration in the form or state of a substance without any change in its chemical identity. When water boils and turns into water vapor, it undergoes a physical change.

The boiling process doesn't produce a new substance; it's still water, simply in a different phase. This distinguishes a physical change from a chemical change, where the chemical identity of the substance is altered, and a new substance forms. The importance of recognizing this distinction lies in comprehending conservation of mass and energy as well as the reversibility of physical processes.

Intermolecular Bonds

Molecules in substances are held together by forces known as intermolecular bonds. These forces are the glue that keeps molecules close enough to one another to maintain a certain state—solid, liquid, or gas. In water, these are primarily hydrogen bonds; a type of strong dipole-dipole attraction that occurs between the positively charged hydrogen atom of one water molecule and the negatively charged oxygen atom of another.

When water is heated during boiling, these intermolecular bonds are strained and eventually overcome by the molecular motion brought on by thermal energy. Once the temperature reaches boiling point, the molecules have enough kinetic energy to break free from these bonds, leading to a phase transition from liquid to gas.

Phase Change

A phase change is a transition of matter from one state (phase) to another due to a variation in environmental conditions, most commonly temperature and pressure. In the boiling of water, heat is the driving force that propels water molecules from the liquid phase to the gas phase. This transformation is achieved by providing energy to the water, which, in turn, increases the motion and energy of the water molecules.

During boiling, the phase change occurs when the vapor pressure of the water equals atmospheric pressure, facilitating the movement of molecules into the air as water vapor. This is a captivating concept because it showcases matter's ability to transform while retaining its intrinsic properties. Recognizing the mechanisms behind phase changes helps us understand a broad range of natural and technological processes.