Problem 56 - General Chemistry

Question

Which of the following are pure substances and which are mixtures? For each, list all of the different phases present. A a sugar solution with sugar crystals at the bottom B ink containing a liquid solution with fine particles of carbon C a sand containing quartz (silicon dioxide) and calcite (calcium carbonate) D liquid water and steam at \(100^{\circ} \mathrm{C}\)

Step-by-Step Solution

Answer

A, B, C are mixtures; D is a pure substance.

Step 1: Understand Definitions

A pure substance is composed of only one type of particle, either element or compound, and has a fixed composition throughout. A mixture contains two or more different substances that are not combined chemically and retain their individual properties.

Step 2: Analyze Sugar Solution (A)

The sugar solution with sugar crystals at the bottom is a mixture. The presence of two phases can be identified: the liquid solution (solvent and dissolved sugar) and the solid phase (sugar crystals).

Step 3: Analyze Ink (B)

Ink containing a liquid solution with fine particles of carbon is a mixture. It consists of at least two phases: a liquid phase (solvent) and a solid phase (carbon particles suspended in the liquid).

Step 4: Analyze Sand (C)

Sand containing quartz and calcite is a mixture with multiple solid phases. Each component (quartz and calcite) is a separate phase since they do not chemically bond or dissolve into each other.

Step 5: Analyze Water and Steam (D)

Liquid water and steam at 100°C is a pure substance with two phases: liquid and gas. Both phases are composed entirely of water molecules.

Key Concepts

Phases of Matter

The phases of matter refer to the distinct forms that different regions of matter take on. Typically, we come across three primary phases:

• Solid: Matter retains a fixed shape and volume.
• Liquid: Matter retains a fixed volume but can change shape.
• Gas: Matter can change both shape and volume, expanding to fill its container.

A single substance can exist in multiple phases under certain conditions. A great example is water, which can exist simultaneously as ice, liquid water, and water vapor. The exercise shows liquid water coexisting with steam at 100°C, illustrating the liquid and gas phases. Each phase represents a distinct state with its own properties and characteristics. Understanding phases is crucial as it helps us identify different forms and behaviors of substances in various environments.

Chemical Composition

Chemical composition describes the types and amounts of elements or compounds in a substance. It defines whether a substance is a pure or a mixture. Pure substances exhibit a uniform chemical composition throughout and can be either:

• Elements: Consisting of a single type of atom, such as oxygen.
• Compounds: Made up of two or more atoms bonded together, like water (\( \text{H}_2\text{O} \)).

For example, the pure substance in the exercise, water and steam, contains only water molecules. In mixtures, components retain their original chemical identity and can be separated by physical means. By understanding chemical composition, we can determine whether a substance is pure or a mixture, providing insight into its properties and potential uses.

Mixtures

Mixtures are combinations of two or more substances that retain their individual properties. They can be classified into homogeneous and heterogeneous mixtures. This distinction depends on how uniformly the components are distributed:

• Homogeneous Mixtures: Components are evenly distributed, such as saltwater.
• Heterogeneous Mixtures: Components are unevenly distributed, like sand containing quartz and calcite.

The exercise mentions various mixtures:

• The sugar solution with sugar crystals at the bottom is heterogeneous because you can distinctly see both sugar crystals and the liquid solution.
• Ink is another example, where carbon particles disrupt uniformity, forming a heterogeneous mixture.

Recognizing the type of mixture helps in understanding how substances interact and can assist in selecting appropriate methods for separating them.

Pure Substances

Pure substances are made up of only one type of particle and have a consistent structure. These consist of either:

• Elements: A single type of atom, like hydrogen.
• Compounds: A fixed ratio of chemically bonded atoms, such as carbon dioxide (\( \text{CO}_2 \)).

In the exercise, water in both liquid and gas (steam) phases is a pure substance since it comprises only water molecules throughout. Unlike mixtures, pure substances maintain uniform characteristics regardless of the sample size or source. Understanding pure substances can lead to better predictions of chemical behavior and reactions.