Problem 17 - Chemistry The Science in Context

Question

Which of the following is not a pure substance? (a) air; (b) nitrogen gas; (c) oxygen gas; (d) argon gas; (e) table salt (sodium chloride)

Step-by-Step Solution

Answer

Options: (a) Air (b) Nitrogen gas (N2) (c) Oxygen gas (O2) (d) Argon gas (Ar) (e) Table salt (sodium chloride, NaCl) Answer: (a) Air

Step 1: Understand the definition of a pure substance

A pure substance is either an element or a compound made up of only one type of particle, with a definite and constant composition and unique properties. It cannot be separated into its components by simple physical methods.

Step 2: Analyze each option

We will now analyze each of the given options to determine whether they are pure substances or not. (a) Air: Air is a mixture of different gases, mainly nitrogen, oxygen, carbon dioxide, and water vapor, with varying compositions depending on the location and conditions. Thus, air is not a pure substance as it does not have a constant composition. (b) Nitrogen gas (N2): Nitrogen gas is a pure substance as it is an element made up of only nitrogen atoms bonded together. It has a constant composition and distinct properties. (c) Oxygen gas (O2): Oxygen gas is a pure substance as it is an element made up of only oxygen atoms bonded together. It has a constant composition and distinct properties. (d) Argon gas (Ar): Argon gas is a pure substance as it is an element made up of only argon atoms. It has a constant composition and distinct properties. (e) Table salt (sodium chloride, NaCl): Table salt is a pure substance as it is a compound made up of sodium and chloride ions in a constant ratio (1:1) with distinct properties.

Step 3: Identify which option is not a pure substance

From the analysis of each option, we can determine that (a) air is not a pure substance, as it contains a mixture of gases with varying compositions. All other options are pure substances since they consist of either a single element or a compound with a constant composition and distinct properties.

Key Concepts

Elements

Elements are the simplest form of pure substances. They are made up of only one type of atom, which is why they have a fixed composition and specific properties. Each element is unique, and they cannot be broken down into simpler substances through chemical reactions. Examples of elements include nitrogen gas (\( \text{N}_2 \)), oxygen gas (\( \text{O}_2 \)), and argon gas (\( \text{Ar} \)). All these exist in nature as pure forms, meaning they are a single type of particle. Elements are represented in the periodic table and are the building blocks of all matter. They can exist in different states of matter, from solids, like iron, to gases, like helium.

• Elements have a constant composition: They are composed entirely of atoms of the same kind.
• Unique properties: Each element has a unique set of chemical and physical properties.

Compounds

Compounds are another category of pure substances. Unlike elements, compounds consist of two or more different types of atoms bonded together in definite proportions. This combination results in a substance with properties different from the individual elements. For example, table salt, or sodium chloride (\( \text{NaCl} \)), is a compound made from sodium and chlorine.

• Fixed composition: In a compound like sodium chloride, the ratio of sodium to chlorine is always 1:1.
• Distinct properties: Compounds often have properties that are very different from their component elements.
• Separation: Compounds can only be separated into their elements by chemical methods, not physical ones.

Mixtures

Mixtures are not pure substances because they are formed by physically combining two or more different substances. Each component retains its own chemical identity and properties. Mixtures can be homogeneous, where substances are uniformly distributed, or heterogeneous, where components are easily distinguishable. An example of a mixture is air.

• Varying composition: Unlike pure substances, mixtures do not have a fixed composition; their proportions can vary.
• Physical separation: Components of a mixture can often be separated by simple physical methods like filtration or distillation.
• Properties: The properties of a mixture are generally a blend of the properties of its components.

Chemical Composition

Chemical composition refers to the types of atoms and their ratios in a substance. It defines the nature of the substance and determines whether a material is an element, compound, or mixture. For pure substances, the chemical composition is fixed and clearly defined. For example, nitrogen gas (\( \text{N}_2 \)) and oxygen gas (\( \text{O}_2 \)) have simple compositions of nitrogen and oxygen atoms, respectively, while sodium chloride (\( \text{NaCl} \)) has a 1:1 ratio of sodium to chlorine ions.

• Identification: Chemical composition helps in identifying whether a substance is pure by its consistent atomic or molecular structure.
• Consistency in properties: The constant composition of pure substances leads to predictable and distinctive chemical and physical properties.
• Analysis: Understanding chemical composition is crucial in chemistry for predicting reactions and behavior of substances.