Elements, Molecules, Compounds, and Mixtures: Seventh Grade Science
All matter across the universe can be classified into pure substances—elements and compounds—or physical combinations known as heterogeneous and homogeneous mixtures.
The Taxonomy of Chemical Matter
To analyze the physical and chemical world, scientists classify matter based on its chemical purity and composition. Matter is anything that possesses mass and occupies physical volume. The first major division separates pure substances from mixtures.
+-----------------------------------------------------------------------------+
| CLASSIFICATION OF MATTER TREE |
| |
| MATTER |
| / \ |
| PURE SUBSTANCES MIXTURES |
| (Definite Composition) (Physical Blend; Variable Comp.) |
| / \ / \ |
| ELEMENTS COMPOUNDS HOMOGENEOUS HETEROGENEOUS |
| (One type (Bonded (Uniform; (Non-uniform; |
| of atom) atoms) Solutions) Visible phases) |
| Ex: Iron Ex: H2O Ex: Saltwater Ex: Granite |
+-----------------------------------------------------------------------------+
Pure Substances: Elements and Compounds
A pure substance is a form of matter that possesses a constant, uniform chemical composition and distinct physical and chemical properties throughout:
- Elements: The simplest pure substances, composed of only one type of atom defined by its unique atomic number. Elements cannot be broken down into simpler substances by ordinary chemical reactions. While many elements exist as individual isolated atoms (monatomic gases like helium and neon), others exist naturally as molecules composed of two or more identical atoms chemically bonded together:
- Diatomic Elements: Seven elements exist naturally as diatomic molecules: Hydrogen (H2), Nitrogen (N2), Oxygen (O2), Fluorine (F2), Chlorine (Cl2), Bromine (Br2), and Iodine (I2).
- Compounds: Pure substances composed of two or more different chemical elements chemically bonded together in fixed, definite mass proportions. The properties of a compound differ fundamentally from the properties of the individual elements from which it was formed:
- Table Salt (Sodium Chloride, NaCl): Formed from sodium (a soft, highly reactive metal that explodes in water) and chlorine (a toxic, suffocating yellow-green gas). Chemically combined in a 1:1 ratio, they form an essential dietary mineral.
- Water (H2O): Formed from hydrogen (a flammable gas) and oxygen (a combustion-supporting gas). Chemically bonded in a 2:1 ratio, they form a liquid that extinguishes fires.
+-----------------------------------------------------------------------------+
| ELEMENTS, MOLECULES, AND COMPOUNDS CONTRAST |
| |
| MONATOMIC ELEMENT DIATOMIC MOLECULE CHEMICAL COMPOUND |
| (Argon, Ar) (Oxygen, O2) (Water, H2O) |
| (Ar) (O)---(O) (O) |
| / \ |
| Single atom Two identical atoms (H) (H) |
| of one element chemically bonded Two different elements |
| (Element & Molecule) chemically bonded |
+-----------------------------------------------------------------------------+
Molecules Versus Compounds: Clarifying Terminology
The terms molecule and compound are related but distinct:
- Molecule: Any chemical structure composed of two or more atoms held together by covalent chemical bonds. A molecule can consist of identical atoms (e.g., O2, N2, O3) or different atoms (e.g., CO2, CH4, H2O).
- Compound: Any substance formed from two or more different elements chemically bonded together. Compounds can be molecular (covalent compounds like H2O and CO2) or ionic lattices (crystalline networks of ions like NaCl and CaCO3).
All compounds are molecules (or formula units), but not all molecules are compounds: diatomic oxygen (O2) is a molecule because it contains two bonded atoms, but it is not a compound because it contains only one element.
Mixtures: Homogeneous and Heterogeneous
A mixture is a physical combination of two or more pure substances in which each substance retains its own individual chemical identity and properties. Mixtures can be blended in variable proportions:
- Heterogeneous Mixtures: Mixtures that lack uniform composition throughout. Different components remain physically separate and can be visually distinguished. Examples include granite rock, vegetable soup, salad dressing, and sand mixed with water.
- Homogeneous Mixtures (Solutions): Mixtures that have a completely uniform composition and physical appearance throughout down to the molecular level. The individual components are evenly distributed and cannot be visually distinguished. Examples include brass (an alloy of copper and zinc), air (a gaseous solution of nitrogen, oxygen, and trace gases), and dissolved saltwater.
+-----------------------------------------------------------------------------+
| TECHNIQUES FOR SEPARATING MIXTURES |
+---------------------+-----------------------+-------------------------------+
| Separation Method | Physical Property Used| Practical Application |
+---------------------+-----------------------+-------------------------------+
| Filtration | Particle size / state | Separating sand from water |
+---------------------+-----------------------+-------------------------------+
| Distillation | Boiling point | Desalinating seawater to |
| | differences | produce pure fresh water |
+---------------------+-----------------------+-------------------------------+
| Magnetism | Magnetic susceptibility| Extracting iron filings from |
| | | sulfur powder |
+---------------------+-----------------------+-------------------------------+
| Chromatography | Differential molecular| Separating dye pigments based |
| | solubility & affinity | on travel along paper strip |
+---------------------+-----------------------+-------------------------------+
| Evaporation | Volatility | Harvesting sea salt by boiling|
| | | off water solvent |
+---------------------+-----------------------+-------------------------------+
Because mixtures are formed by physical blending rather than chemical bonding, their components can be separated using physical techniques based on differences in physical properties (boiling point, particle size, solubility, magnetism) without breaking chemical bonds.
Exercises and Solutions
Exercise 1: Classifying Everyday Substances Across Chemical Categories. Classify each of the following six substances as an element, a compound, a homogeneous mixture, or a heterogeneous mixture, providing a scientific justification for each classification: (a) Pure liquid water (H2O); (b) Freshly squeezed orange juice containing pulp; (c) Atmospheric air inside a sealed scuba diving tank; (d) Solid 24-karat gold; (e) Granulated table sugar (sucrose, C12H22O11); (f) An iron nail coated in brown rust (iron oxide and iron metal).
Solution 1: (a) Pure liquid water (H2O) is a compound because it consists of hydrogen and oxygen atoms chemically bonded together in a fixed 2:1 ratio with constant properties throughout. (b) Freshly squeezed orange juice with pulp is a heterogeneous mixture because the suspended cellulose pulp particles create visible non-uniform phases that settle over time. (c) Scuba tank air is a homogeneous mixture (a gaseous solution) because its constituent gases (roughly 78 percent nitrogen, 21 percent oxygen, and trace noble gases) are evenly distributed at the molecular level, forming a single uniform phase. (d) Solid 24-karat gold is an element because it is composed exclusively of one type of atom (Au, atomic number 79) and cannot be broken down chemically. (e) Granulated table sugar (sucrose, C12H22O11) is a compound because it consists of carbon, hydrogen, and oxygen atoms chemically bonded in a fixed 12:22:11 ratio. (f) A rusted iron nail is a heterogeneous mixture because it consists of unreacted elemental iron metal physically intermixed with a crust of iron oxide compound (rust), displaying non-uniform physical composition and distinct visible phases.
Exercise 2: Designing a Physical Separation Protocol. A laboratory beaker contains an accidental mixture of four solid substances: coarse silica sand grains, sodium chloride (table salt), iron metal filings, and small pieces of wood sawdust. Design a sequential step-by-step laboratory protocol to separate this mixture into four pure, isolated components, identifying the specific physical property exploited at each step.
Solution 2: The four-component solid mixture can be separated sequentially through the following four steps: Step 1 (Magnetic Separation): Pass a rare-earth magnet wrapped in plastic wrap over the dry mixture. The iron filings are ferromagnetic and will be attracted to the magnet, leaving sand, salt, and sawdust behind. Step 2 (Density Separation via Flotation): Add distilled water to the remaining mixture of sand, salt, and sawdust, and stir thoroughly. Water dissolves the sodium chloride into an aqueous solution. Wood sawdust has a lower density than water and floats to the surface, while dense silica sand settles to the bottom. Skim the floating sawdust off the water surface and dry it. Step 3 (Filtration): Pour the remaining sand and saltwater mixture through a funnel lined with filter paper. The insoluble silica sand grains are larger than the filter pores and remain trapped on the paper (residue). Rinse the sand with distilled water and let it dry. Step 4 (Evaporation / Distillation): Pour the clear liquid filtrate (saltwater) into an evaporating dish and heat it over a Bunsen burner. Water boils at 100 degrees Celsius and evaporates into steam, leaving behind solid crystalline sodium chloride salt in the dish.