Problem 9 - General Chemistry: Principles and Modern Applications

Question

Indicate whether each sample of matter listed is a substance or a mixture; if it is a mixture, indicate whether it is homogeneous or heterogeneous. (a) clean fresh air (b) a silver-plated spoon (c) garlic salt (d) ice

Step-by-Step Solution

Answer

(a) clean fresh air is a homogeneous mixture; (b) a silver-plated spoon is a heterogeneous mixture; (c) garlic salt is a homogeneous mixture; (d) ice is a substance.

Step 1: Analyzing 'clean fresh air'

Clean fresh air is made up of many components like nitrogen, oxygen, argon, etc. These are not chemically combined and the composition is uniform, which characterizes it as a homogeneous mixture.

Step 2: Analyzing 'a silver-plated spoon'

A silver-plated spoon is made of a layer of silver on top of another material. This suggests two different materials are involved. Therefore, it is a heterogeneous mixture as it doesn't have the same composition throughout.

Step 3: Analyzing 'garlic salt'

Garlic salt is a mixture of garlic and salt. These substances are not chemically combined but the mixture is uniform, characterizing it as a homogeneous mixture.

Step 4: Analyzing 'ice'

Ice is simply frozen water, meaning it has a uniform and definite composition. Therefore, it qualifies as a substance.

Key Concepts

Homogeneous Mixtures

Homogeneous mixtures are physical combinations of substances where each component is distributed evenly at a molecular level. These mixtures have a uniform composition throughout, meaning that any sample taken from them will have the same proportion of components. A common example would be clean fresh air. Fresh air is an amalgamation of nitrogen, oxygen, argon, and other gases. They mingle seamlessly to form a consistent mixture.
Such mixtures can also be found in solutions like sugar dissolved in water. Here are some key points about homogeneous mixtures:

• Components are not chemically bonded but are mixed uniformly.
• Every part of the mixture has the same properties and composition.
• They are often referred to as solutions.

Observing a homogeneous mixture requires considering its even texture and appearance, as well as its consistent chemical composition.

Heterogeneous Mixtures

Heterogeneous mixtures, unlike homogeneous ones, have components that do not blend into a uniform composition. Instead, they are distinct and easily identifiable, presenting visibly different substances or phases within the same mixture. For example, a silver-plated spoon consists of a layer of silver on top of another metal, making the materials separate and detectable, which classifies it as heterogeneous.
In everyday life, a salad is a typical example, where each component—lettuce, tomatoes, cucumbers—can be seen and separated physically.

• The components retain their individual properties.
• There is no uniformity in composition, allowing for distinct substances to be seen.
• May contain solids, liquids, and gases, existing in different phases.

Identifying a heterogeneous mixture involves noticing visible segregation of components and a lack of uniformity in composition.

Chemical Composition

Chemical composition refers to the identity and amount of substances that make up a material. It is a key factor in distinguishing between substances and mixtures, often determining whether matter will have a distinct, stable identity, such as a pure substance. Substances have a fixed chemical composition, meaning their molecular structure does not vary throughout.
Ice, for example, is a substance with a definite and consistent chemical composition of water (\(H_2O\)). It does not change regardless of the sample size.

• Substances are made of one kind of particle.
• The composition is not subject to change under physical separation methods.
• The identity of substances does not vary throughout different samples.

Understanding chemical composition allows us to differentiate substances from mixtures and appreciate the stability of pure substances.