Understanding Matter: A Guide to Pure Substances, Mixtures, and Atomic Structure
Pure Substances and Mixtures
A pure substance is a type of matter with a defined and invariant composition and properties. Examples include table salt, gold, and oxygen.
A mixture consists of two or more substances, called components. Its composition is not defined, and its properties depend on the type and proportion of its components. Examples include seawater, air, and paint.
Types of Mixtures
Heterogeneous Mixtures
In a heterogeneous mixture, particles of different substances are visible with the naked eye or low-magnification optical instruments. Examples include sand in water, oil and water, and laundry detergent.
Homogeneous Mixtures
In a homogeneous mixture, the components are so small that they cannot be observed with any optical means. Examples include air, oxygen, water vapor, helium, and nitrogen.
Solutions
A solution is a homogeneous mixture of two or more substances.
- Solute: The substance present in a smaller amount.
- Solvent: The substance present in a larger amount.
The solubility of a solute in a solvent is the maximum amount of solute that can dissolve in a given amount of solvent at a certain temperature.
A substance is insoluble if it has very low solubility in a solvent.
Two liquids are immiscible if they have very low solubility in each other. For example, water and oil.
Types of Solutions
- Dilute solution: Contains a small amount of solute relative to the solvent.
- Concentrated solution: Contains a large amount of solute relative to the solvent.
- Saturated solution: Contains the maximum amount of solute that the solvent can dissolve at a given temperature. Excess solute will not dissolve and may settle at the bottom.
Factors Affecting Solubility
- Solubility of solids: Generally increases with temperature.
- Solubility of gases: Generally decreases with increasing temperature.
Concentration
The concentration of a solution is the amount of solute dissolved in a certain amount of solvent or solution.
Concentration is often expressed in grams per liter (g/L):
g/L = mass of solute (g) / volume of solution (L)
Separation Techniques
- Decanting
- Filtering
- Vaporization
- Distillation
- Chromatography
Atomic Structure
Early Models of the Atom
Dalton’s Atomic Theory
- All matter is made of atoms.
- Each chemical element is composed of identical atoms.
- Chemical compounds are formed by the joining of atoms of different elements.
Thomson’s Model
Atoms are made of negatively charged electrons distributed within a positively charged sphere, making the atom electrically neutral.
Rutherford’s Model
- Atoms are made up of protons, neutrons, and electrons.
- The number of protons and electrons in an atom is equal, making the atom electrically neutral.
- Protons and neutrons are located in a small, dense nucleus, while electrons orbit the nucleus in a much larger electron cloud.
Elements and Compounds
An element is a substance that cannot be broken down into simpler substances. It consists of only one kind of atom.
A compound is a substance that can be broken down into simpler substances. It is made up of atoms of more than one element.
Electrolysis can be used to break down a pure substance into its elements using electric current.
Atomic Number and Mass Number
The atomic number (Z) is the number of protons in an atom. For example, hydrogen (H) has 1 proton, and helium (He) has 2 protons.
The mass number (A) is the sum of protons and neutrons in an atom. Since atoms are electrically neutral, the number of protons equals the number of electrons.
A = Z + N, where N is the number of neutrons.
Isotopes
Isotopes are atoms of the same element with different masses. They have the same atomic number (Z) but different mass numbers (A).
Radioisotopes
Radioisotopes are radioactive isotopes. They emit different types of radiation:
- Alpha radiation: Helium nuclei consisting of two protons and two neutrons.
- Beta radiation: High-speed electrons.
- Gamma radiation: High-energy electromagnetic radiation similar to light or X-rays.
Nuclear Fission and Fusion
Nuclear fission occurs when a neutron strikes a uranium atom, causing it to split into smaller atoms and release a large amount of energy.
Nuclear fusion occurs when small nuclei combine to form larger nuclei, releasing a large amount of energy.
A chain reaction occurs when neutrons released during fission trigger further fission reactions.
Ions
Ions are atoms that have gained or lost electrons.
A cation is a positively charged ion formed when a neutral atom loses electrons.
An anion is a negatively charged ion formed when a neutral atom gains electrons.