Question

Categorize each of the following as an element, a compound, or a mixture. (a) air (c) brass (b) fluorite (d) 18 -carat gold

Short answer

(a) Mixture, (b) Compound, (c) Mixture, (d) Mixture.

Step-by-step solution

Identify Air

Air is a substance that we breathe and is composed of various gases such as nitrogen, oxygen, carbon dioxide, and small amounts of other gases. Since it consists of multiple substances that are physically mixed but not chemically combined, air is classified as a mixture.

Identify Fluorite

Fluorite is a naturally occurring mineral composed of calcium fluoride (CaF2). Because it consists of molecules formed by chemical bonding of calcium and fluorine, fluorite is a pure chemical substance and therefore classified as a compound.

Identify Brass

Brass is an alloy made primarily of copper and zinc, where these metals are physically combined but not chemically bonded in specific proportions. Due to this physical mixture, brass is classified as a mixture.

Identify 18-carat Gold

18-carat gold is a type of alloy consisting of pure gold along with other metals such as copper and silver. Since it consists of different metals physically mixed together in set proportions without chemical bonding, 18-carat gold is classified as a mixture.

Key concepts

Elements

Elements are the simplest form of matter. They contain only one type of atom and cannot be broken down into simpler substances. This makes them the building blocks for all chemical structures.
They appear on the periodic table and are identified by their unique symbols.
For example:

• A carbon atom is an element labeled as 'C'.
• An oxygen atom, labeled 'O', is also an element.

Each element is distinct in its properties, like atomic weight and reactivity. These characteristics determine how elements will interact with each other.
Pure elements are rarely found in nature because they typically combine with others to form compounds.

Compounds

Compounds are substances formed when two or more elements are chemically bonded together. This bonding creates a new substance with entirely different properties from the original elements.
A classic example is water (H extsubscript{2}O), composed of hydrogen and oxygen. Individually, these gases are very different, but together they form liquid water.
Compounds are characterized by a fixed ratio of elements.
A few more examples:

• Table Salt (NaCl), which is made from sodium and chlorine.
• Carbon Dioxide (CO extsubscript{2}), formed from carbon and oxygen.

The formation of a compound involves chemical changes and bond formation, making it different from the physical mixtures of substances.

Mixtures

Mixtures are combinations of two or more substances where each retains its own properties, and they are not chemically combined. They can be separated by physical methods such as filtration, distillation, or magnetic separation.
Mixtures can be classified as homogeneous or heterogeneous:

• Homogeneous mixtures have a uniform composition throughout, like air and alloys such as brass.
• Heterogeneous mixtures do not have a uniform composition, such as salad or sandy water.

Mixtures are prevalent in everyday life, from the air we breathe (a mixture of gases) to our tap water (often mixed with minerals). Their components can vary in proportion.

Chemical Bonding

Chemical bonding is the process that allows atoms to combine and form compounds. It involves the interaction of electrons between atoms, leading to the formation of molecules. There are several types of chemical bonds:

• Ionic Bonds occur when one atom donates an electron to another, forming a bond between positively and negatively charged ions (e.g., sodium chloride).
• Covalent Bonds are formed when atoms share electrons. This is common in organic compounds, such as in the bonding of hydrogen and oxygen in water.

Chemical bonds are essential for creating complex substances and are fundamental to understanding chemical reactions and properties. The strength and type of bond affect the properties of the resulting compound, such as its melting point, solubility, and conductivity.