Question

Classify each substance as a pure substance or a mixture. If it is a pure substance, classify it as an element or a compound. If it is a mixture, classify it as homogeneous or heterogeneous. (a) helium gas (b) clean air (c) rocky road ice cream (d) concrete

Short answer

Helium gas is a pure substance and an element. Clean air is a homogeneous mixture. Rocky road ice cream is a heterogeneous mixture. Concrete is a heterogeneous mixture.

Step-by-step solution

Classify Helium Gas

Identify if helium gas is a pure substance or a mixture. Helium gas is a single type of atom and not combined with any other substance, making it a pure substance. Furthermore, being composed of only one type of atom, helium gas is classified as an element.

Classify Clean Air

Determine if clean air is a pure substance or a mixture. Clean air is a blend of several gases such as nitrogen, oxygen, argon and others in variable proportions; therefore, it is a mixture. Since its composition is uniform throughout and its components are not visually distinguishable, clean air is a homogeneous mixture.

Classify Rocky Road Ice Cream

Decide whether rocky road ice cream is a pure substance or a mixture. Rocky road ice cream contains chocolate, nuts, and marshmallows, which can be seen and picked out. Therefore, it is a mixture. The different components are visible and not uniform, making it a heterogeneous mixture.

Classify Concrete

Examine whether concrete is a pure substance or a mixture. Concrete is a combination of cement, water, sand, and gravel. These components can vary in proportion and are not uniform. Thus, concrete is a mixture. Due to the non-uniformity and the ability to see different parts within it, concrete is classified as a heterogeneous mixture.

Key concepts

Pure substance vs mixture

Understanding the difference between pure substances and mixtures is foundational in chemistry. A pure substance is a form of matter that has a constant composition and properties that are consistent throughout the sample. Examples of pure substances include elements like gold, and compounds such as water, where water's composition of hydrogen and oxygen is always in a 2:1 ratio.

A mixture, on the other hand, is a combination of two or more substances that are not chemically bonded together and thus can vary in composition. Mixtures can be physically separated into their components, like salt from saltwater through evaporation. This concept helps students to categorize substances they encounter daily, assisting them in developing a systematic approach to the study of materials and their properties.

Element and compound

Diving deeper into pure substances, we find two categories: elements and compounds. An element is a substance that cannot be broken down into simpler substances by chemical means. Each element is made of identical atoms, such as helium gas from our problem, which consists only of helium atoms.

A compound is a substance formed when two or more elements are chemically bonded together. Compounds have a fixed ratio of elements as seen in water (H2O), where two hydrogen atoms bond to one oxygen atom. Compounds have unique properties distinct from the elements that make them, a concept crucial for students to grasp as it lays the groundwork for understanding chemical reactions and molecular structures.

Homogeneous and heterogeneous mixtures

Mixtures can be either homogeneous or heterogeneous. A homogeneous mixture has a uniform composition throughout, meaning you cannot distinguish the different parts by looking at it. An example is clean air; it's a mixture of gases, but appears uniform to the eye and the proportions of the gases are evenly distributed.

In contrast, a heterogeneous mixture has a non-uniform composition, where you can see the individual components, such as in rocky road ice cream and concrete. By teaching these distinctions, we help learners to identify what type of mixture they are dealing with, an essential skill in fields such as material sciences and engineering.

Chemical education

In chemical education, the goal is to make complex concepts accessible and engaging. Exploring everyday examples, such as the ones provided in the exercise, helps bridge the gap between abstract concepts and the real world. Instructors should encourage active inquiry and critical thinking, as this will enable students to apply their knowledge to various scenarios.

Effective chemical education fosters problem-solving skills and lays a solid foundation for future learning. By classifying substances and understanding their properties, students gain the ability to predict behavior and reactions, which is central not only in chemistry but also in environmental science, pharmacology, and numerous other disciplines that involve material interaction.