Question

Imagine that you take an ice cube out of the freezer and place it on a counter at room temperature. a. Write a balanced equation for the process that occurs. b. Describe what you would do to reverse the process.

Short answer

Ice melts into water: \(\text{H}_2\text{O (s)} \rightarrow \text{H}_2\text{O (l)}\). Reverse it by freezing \(\text{H}_2\text{O (l)} \rightarrow \text{H}_2\text{O (s)}\).

Step-by-step solution

Identifying the Process

When you take an ice cube out of the freezer and place it at room temperature, it melts. This is a phase change from solid to liquid.

Writing the Balanced Equation

The balanced equation for the melting of ice is the conversion of solid water (ice) to liquid water. In equation form: \[\text{H}_2\text{O (s)} \rightarrow \text{H}_2\text{O (l)}\]Here, (s) denotes the solid state and (l) denotes the liquid state.

Reversing the Process

To reverse the melting process, you would need to freeze the liquid water back into ice. This means cooling the liquid water to below its freezing point, typically in a freezer, to facilitate the phase change from liquid to solid. The equation for the reverse process is: \[\text{H}_2\text{O (l)} \rightarrow \text{H}_2\text{O (s)}\]

Key concepts

Melting Point

The melting point of a substance is the temperature at which it changes from a solid to a liquid. This is a specific characteristic of each material. For ice, this point is at 0°C or 32°F under standard atmospheric pressure. At this temperature, the rigid structure of ice crystals breaks down, allowing water molecules to move more freely and form a liquid.

• Importance: Understanding the melting point is crucial for many applications, such as when preparing materials in the laboratory or determining the purity of a substance.
• Consistent Behavior: The melting point remains consistent under the same pressure, which makes it a reliable method to identify substances.

Primarily, achieving the melting point means providing enough energy, usually in the form of heat, to overcome the forces keeping the molecules in a fixed position. This is why ice melts when placed in warmer environments.

Freezing Point

The freezing point is the temperature at which a liquid becomes a solid. It is the reverse of melting and also occurs at 0°C (32°F) for water under normal atmospheric conditions. Freezing involves removing energy, often heat, from a liquid, allowing its molecules to lock into a structured form as a solid.

• At the freezing point, water molecules slow down and arrange into a more orderly structure, forming ice crystals.
• The energy removed during freezing is mainly in the form of heat, which is why freezing is often done in a cold environment like a freezer.

Understanding the concept of freezing is essential for various fields, from chemistry to culinary arts where precise temperature control affects texture and preservation.

Physical States of Matter

Matter exists in various states—solid, liquid, and gas—depending on temperature and pressure. These are called physical states of matter.

• In a solid state, particles are tightly packed in a fixed arrangement, leading to a definite shape and volume, like ice.
• In a liquid state, particles are more loosely packed compared to a solid, which allows them to take the shape of their container while maintaining a constant volume, such as water.
• In a gaseous state, particles are far apart and move freely, giving gases no fixed shape or volume.

The transition from one state to another is known as a phase change and involves adding or removing energy. For example, melting and freezing are both phase changes that alter the state of water based on temperature.

Balanced Chemical Equation

A balanced chemical equation shows the conversion of reactants to products with equal numbers of atoms for each element on both sides of the equation. In the given scenario, the melting of ice can be represented as a chemical equation:
\[\text{H}_2\text{O (s)} \rightarrow \text{H}_2\text{O (l)}\]

• Symbol Meaning: The '(s)' indicates a solid state, while '(l)' signifies a liquid state.
• Conservation of Mass: Balancing ensures that the law of conservation of mass is observed. The same quantity of water is present before and after melting.

In practice, balanced equations are essential for understanding chemical processes and performing calculations related to reactants and products. Writing such equations correctly ensures clarity in the study of chemistry and physics. In our ice cube scenario, reversing the process involves the equation:
\[\text{H}_2\text{O (l)} \rightarrow \text{H}_2\text{O (s)}\]