Problem 9 - Chemistry and Chemical Reactivity

Question

Which part of the description of a compound or element refers to its physical properties and which to its chemical properties? (a) The colorless liquid ethanol burns in air. (b) The shiny metal aluminum reacts readily with orange-red bromine.

Step-by-Step Solution

Answer

For ethanol: 'colorless liquid' is a physical property, 'burns in air' is a chemical property. For aluminum: 'shiny' is a physical property, 'reacts readily' is a chemical property.

Step 1: Understanding Physical and Chemical Properties

First, define and distinguish between physical and chemical properties. Physical properties are characteristics observed without changing the substance's composition, like color, melting point, or density. Chemical properties describe a substance's ability to change into different substances, such as flammability or reactivity.

Step 2: Analyzing Ethanol's Description

Ethanol is described as a 'colorless liquid' and as a substance that 'burns in air.' The phrase 'colorless liquid' refers to its physical property, as it describes ethanol's appearance without altering its chemical makeup. The phrase 'burns in air' indicates its chemical property, as burning involves a chemical change with oxygen.

Step 3: Analyzing Aluminum's Description

Aluminum is described as a 'shiny metal' and as a metal that 'reacts readily with orange-red bromine.' The word 'shiny' describes a physical property because it refers to aluminum's appearance. The phrase 'reacts readily with orange-red bromine' refers to its chemical property, as it describes aluminum undergoing a chemical reaction with bromine.

Key Concepts

Ethanol

Ethanol, a common alcohol, is known for its various properties that play a significant role in both everyday life and industrial applications. One of its primary physical properties is its state and color. Ethanol is a colorless liquid which means it can be observed in this form without altering its composition. This is an example of a physical property, as it pertains to its appearance.
When it comes to chemical properties, ethanol displays a noteworthy ability, which is its flammability. When ethanol burns in air, it undergoes a chemical reaction with oxygen, producing heat, light, carbon dioxide, and water. This transformation from ethanol to different substances characterizes its chemical properties. A chemical property like flammability indicates how ethanol can change and react chemically, distinguishing it from physical traits.

Aluminum

Aluminum is a widely used metal known for its distinctive and useful characteristics. The mention of aluminum as a 'shiny metal' invites us to consider one of its primary physical properties. This shine is due to its ability to reflect light, a physical characteristic that can be observed without altering its internal structure.
Beyond its physical traits, aluminum also possesses fascinating chemical properties. One significant example is its reactivity with bromine, described in the problem as 'reacts readily with orange-red bromine.' This reaction suggests a chemical change where aluminum combines with bromine to form a new substance, aluminum bromide. Chemical properties like reactivity show how substances can change when they interact with specific agents or conditions.

Chemical Reactions

Chemical reactions are events where substances undergo transformations to form new substances. Such reactions can be identified when bonds are broken and formed to create new combinations of atoms.
A prime example in our review is ethanol burning in air, which involves a chemical reaction with oxygen. During this process, high energy release in the form of heat and light occurs, indicating a chemical change. Another instance is aluminum reacting with bromine, where these two substances combine to create aluminum bromide, illustrating a chemical transformation.
Chemical reactions are irreversible in most cases. Once the original substances undergo a change, such as a log burning in a fireplace to ash, you cannot revert to the initial state without additional complex steps.

Physical Characteristics

Physical characteristics refer to the traits of a substance that can be observed without changing its composition. These traits may include properties like color, state (solid, liquid, gas), melting point, and electrical conductivity.
For instance, ethanol being a 'colorless liquid' refers to its physical appearance. Similarly, describing aluminum as 'shiny' focuses on its light-reflective property, which is an inherent physical characteristic. These observations are crucial in identifying and differentiating materials in everyday applications.
Understanding the physical characteristics of substances helps in predicting their behavior in different contexts. They often serve as identification tools helping us distinguish one substance from another. Physical properties are typically reversible; for example, a shiny metal can tarnish, but it can be polished again to its original state.