Question

List three examples of substances. Explain why each is a substance.

Short answer

Water (H2O), gold (Au), and carbon dioxide (CO2) are examples of substances. Water is a compound composed of 2 hydrogen atoms and 1 oxygen atom, having consistent properties such as boiling and freezing points. Gold is an element made of only gold atoms, known for its unique physical and chemical properties. Carbon dioxide is a compound of 1 carbon atom and 2 oxygen atoms, with properties like density and reactivity that make it identifiable as a substance.

Step-by-step solution

Example 1: Water (H2O)

Water is a well-known substance that is composed of two hydrogen (H) atoms and one oxygen (O) atom, chemically bonded together in a specific ratio (2:1). The distinct properties and well-defined composition of water make it a substance. Water exhibits consistent properties like a specific boiling point (100 degrees Celsius) and freezing point (0 degrees Celsius) under standard conditions, which makes it identifiable as a unique substance.

Example 2: Gold (Au)

Gold is an element, which means it is composed of only one type of atom – gold atoms. It is known for its distinct properties such as being a heavy, shiny, and ductile metal with a bright yellow color that does not tarnish. Gold has a specific atomic number (79) and a unique set of chemical and physical properties that distinguish it from other elements. These characteristics make gold a substance.

Example 3: Carbon dioxide (CO2)

Carbon dioxide is a compound, formed by the chemical combination of one carbon (C) atom and two oxygen (O) atoms in a specific proportion (1:2). It is a colorless, odorless gas under standard conditions and is a crucial component of Earth’s atmosphere. Carbon dioxide has consistent properties, such as a well-defined density, specific solubility in water, and reactivity with other chemical compounds, making it clearly identifiable as a substance.

Key concepts

Understanding Water as a Pure Substance

Water is a fundamental example of a pure substance due to its consistent chemical structure of two hydrogen atoms bonded with one oxygen atom, represented as H₂O. This specific 2:1 ratio remains constant in every water molecule, ensuring its classification as a chemical compound. Water's properties, such as boiling at 100°C and freezing at 0°C under normal atmospheric pressure, are predictable and consistent. These properties aid in recognizing water as a pure substance. Additionally, its importance in biological processes and environments further exemplifies its role as a pure substance essential for life.

The Purity of Gold

Gold, represented by the chemical symbol Au, is a noble metal known for its purity. It belongs to the group of elements called metals and is made entirely of only one type of atom—gold atoms. This elemental nature means that gold is a homogeneous material throughout, an important characteristic of a pure substance. It is recognized for several distinct traits such as ductility, malleability, and a resistance to tarnishing, which contribute to its uniqueness and value. These consistent properties are why gold is often found in high-purity forms, such as gold bullion, and confirm its status as a pure substance.

Carbon Dioxide's Role as a Pure Substance

Carbon dioxide (CO₂) is a chemical compound formed from one carbon atom and two oxygen atoms, always in the specific molar ratio of 1:2. This consistency marks it as a classic example of a pure substance. Although it exists as a gas at room temperature, carbon dioxide's attributes do not change. It possesses a distinct density and solubility characteristics in water that help differentiate it from other gases. Importantly, CO₂ is a critical greenhouse gas and is vital in processes like photosynthesis, showcasing its prominent place as a pure substance in various ecological and industrial contexts.