Problem 55 - General Chemistry

Question

Which of the following are pure substances and which are mixtures? For each, list all of the different phases present. a) bromine liquid and its vapor b) paint, containing a liquid solution and a dispersed solid pigment. c) partially molten iron d) baking powder containing sodium hydrogen carbonate and potassium hydrogen tartrate

Step-by-Step Solution

Answer

Bromine and partially molten iron are pure substances; paint and baking powder are mixtures. Bromine has liquid & vapor phases, paint has liquid & solid, molten iron has solid & liquid.

Step 1: Identify Pure Substances and Mixtures

A pure substance has a uniform and consistent composition and can either be an element or a compound. A mixture is composed of two or more substances that are not chemically combined. - **Bromine Liquid and Its Vapor**: While it might seem like a mixture due to two phases (liquid and gas), both are made of the same substance (bromine), making it a pure substance. - **Paint**: It consists of a liquid solution and dispersed solid pigments, indicating it is a mixture. - **Partially Molten Iron**: As it includes both solid and liquid iron, which is the same element, it is still considered a pure substance. - **Baking Powder**: It contains chemically distinct substances, sodium hydrogen carbonate and potassium hydrogen tartrate, which identifies it as a mixture.

Step 2: Analyze Present Phases

Next, determine the phases present in each substance or mixture. - **Bromine Liquid and Its Vapor**: Contains two phases, liquid and gas. - **Paint**: Contains two phases, liquid (the solution) and solid (the dispersed pigment). - **Partially Molten Iron**: Contains two phases, solid and liquid iron. - **Baking Powder**: Since it is a mixture of different compounds, it mainly exists in a powdered solid phase; however, if it reacts (for example, in baking), it may temporarily create gas bubbles.

Key Concepts

Phases of Matter

Phases of matter are the distinct forms that different substances take. The primary phases include solid, liquid, and gas, with plasma considered a fourth, more extreme state. Each phase is characterized by its physical properties, such as shape and volume. For example:

• Solid: Has definite shape and volume. Particles are closely packed in a fixed structure.
• Liquid: Has definite volume but takes the shape of its container. Particles are close but can move past each other.
• Gas: Has neither definite shape nor volume, filling the entire container. Particles are far apart and move freely.
• Plasma: An ionized state of matter, found in stars, with properties distinct from the above three.

These phases are what we observe in many natural and synthetic materials. A change in temperature or pressure can transition matter from one phase to another, such as melting, freezing, or boiling. Understanding these transitions is key to analyzing substances like bromine, which may exist in both liquid and gas phases simultaneously.

Chemical Composition

Chemical composition refers to the identity and proportion of elements that make up a substance. A pure substance is uniform in its composition, meaning every molecule is the same throughout. A pure substance will always consist of either a single element or compound.

• Elements: These are substances that consist of only one type of atom, such as oxygen or iron. Elements cannot be broken down further by chemical means.
• Compounds: These consist of two or more elements chemically bonded together, such as water (\( ext{H}_2 ext{O} \)) or carbon dioxide (\( ext{CO}_2\)). Compounds have unique properties that are different from their constituent elements.

Knowing the chemical composition helps in differentiating pure substances from mixtures. For instance, paint is a mixture because its composition includes various substances like liquid components and solid pigments. Baking powder is another example of a mixture, containing multiple chemical compounds.

Elements and Compounds

Understanding the distinction between elements and compounds is fundamental in chemistry. An element is a basic substance that cannot be broken down further, while a compound results from the chemical combination of two or more elements.
- In nature, elements like gold, oxygen, or bromine occur naturally in either singular atomic forms or combined in structures such as molecular or lattice forms. Bromine, as noted in the exercise, can exist in different phases but remains a single element.
- On the other hand, compounds result from elements chemically combining in fixed proportions. For example, water is a compound made from hydrogen and oxygen. Compounds hold unique properties that differ from their elements, sometimes dramatically—think of how unreactive water is compared to its explosive component, hydrogen.
Knowing these basics can help you determine if a material is a pure substance or a mixture. This is particularly useful when examining substances like baking powder, which features multiple compounds.

Types of Mixtures

Mixtures are combinations of two or more substances where each retains its own chemical identity. They can be classified based on their uniformity and phase. The main types include:

• Homogeneous Mixtures: These are mixtures with a consistent composition throughout, such as saltwater. They are well mixed, so you cannot distinguish the components visually.
• Heterogeneous Mixtures: These have a non-uniform composition, like a salad or granite. You can often see and separate the different substances physically.

Analyzing mixtures involves observing the phases they exhibit. Paint is a classic heterogeneous mixture, with liquid and solid phases present. Understanding the phase distribution in mixtures can help in industrial applications and everyday situations alike. When you encounter a mixture, its classification is crucial as it dictates the methods you might use for separation or analysis.