Pure Substances, Compounds & Mixtures: Sixth Grade Science
All matter in our world can be classified into pure substances (elements and compounds) or complex physical combinations known as mixtures.
The Matter Classification Hierarchy
To analyze materials, chemists organize all matter using a systematic branching taxonomy based on whether substances can be separated by physical or chemical means.
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| THE TAXONOMY OF ALL MATTER |
| |
| [ MATTER ] |
| | |
| +-----------------------+-----------------------+ |
| | | |
| v v |
| [ PURE SUBSTANCES ] [ MIXTURES ] |
| Definite Composition; Variable Composition;|
| Cannot separate physically Can separate physically
| | | |
| +-----+-----+ +-----+-----+ |
| | | | | |
| v v v v |
| [ ELEMENTS ] [ COMPOUNDS ] [ HETEROGENEOUS ] [ HOMOGENEOUS]|
| 1 type of 2+ elements Non-uniform; Uniformly |
| atom only chemically visible phases mixed |
| (Fe, O2, Au) bonded (H2O, NaCl) (Salad, Granite) (Saltwater, Air)
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Pure Substances: Elements and Compounds
A pure substance is matter that has a uniform, definite chemical composition and identical characteristic properties throughout every sample:
- Elements: Pure substances consisting of only one type of atom (e.g., pure iron, pure copper, sulfur powder). They cannot be broken down into simpler substances by any chemical or physical reaction.
- Compounds: Pure substances composed of two or more different elements chemically bonded together in a fixed, definite mathematical ratio. Examples include pure water (H2O, always 2 hydrogen atoms to 1 oxygen atom), table salt (NaCl, always 1 sodium to 1 chlorine), and carbon dioxide (CO2).
The Magic of Chemical Synthesis: New Properties Emerge
When elements chemically react to form a compound, their original physical and chemical properties vanish completely. In their place, an entirely new substance emerges with brand-new properties:
- Elemental Sodium (Na) is an extremely soft, shiny, silvery metal that reacts explosively with water and burns flesh.
- Elemental Chlorine (Cl2) is a choking, greenish-yellow, poisonous gas that was used as a chemical weapon.
- When sodium and chlorine react chemically, their violent properties are destroyed, producing sodium chloride (NaCl)—ordinary table salt, a harmless, crystalline white mineral that is essential for human life and health!
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| THE TRANSFORMATION OF SODIUM AND CHLORINE |
| |
| [ Sodium Metal (Na) ] + [ Chlorine Gas (Cl2) ] |
| (Violent, toxic metal) (Suffocating poisonous gas) |
| | |
| v (Violent Chemical Synthesis) |
| [ Sodium Chloride (NaCl) ] |
| (Harmless, edible table salt) |
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Mixtures: Physical Combinations of Matter
A mixture is a physical combination of two or more substances that are physically blended together but not chemically bonded. In a mixture:
- Each individual substance retains its own original chemical identity and physical properties.
- The components can be combined in any variable ratio (you can make sugar water with 1 spoon of sugar or 5 spoons of sugar).
- The components can be separated using physical techniques based on differences in their physical properties (such as boiling point, density, solubility, or magnetism).
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| HETEROGENEOUS VERSUS HOMOGENEOUS MIXTURES |
+-------------------+---------------------------+-----------------------------+
| Classification | Microscopic Distribution | Everyday Real-World Examples|
+-------------------+---------------------------+-----------------------------+
| Heterogeneous | Unevenly mixed; different | Garden salad, chocolate chip|
| Mixture | components and phases are | cookies, granite rock, oily |
| | visibly distinct | salad dressing, muddy water |
+-------------------+---------------------------+-----------------------------+
| Homogeneous | Uniformly distributed at | Saltwater, dissolved sugar, |
| Mixture | the molecular level; looks| clean air (gas mixture), |
| (Solution) | identical throughout | brass (copper-zinc alloy) |
+-------------------+---------------------------+-----------------------------+
Exercises and Solutions
Exercise 1: Classifying Matter in the Kitchen. Classify each of the following household kitchen substances into one of the four categories of matter (element, compound, heterogeneous mixture, or homogeneous mixture): (a) Pure aluminum foil; (b) Baking soda (sodium bicarbonate, NaHCO3); (c) A bowl of chicken noodle soup; (d) A cup of clear, completely dissolved apple juice.
Solution 1: (a) Pure aluminum foil is an element because it consists solely of aluminum atoms (Al) that cannot be broken down by chemical reactions. (b) Baking soda is a compound because it consists of sodium, hydrogen, carbon, and oxygen atoms chemically bonded in a fixed ratio of NaHCO3. (c) Chicken noodle soup is a heterogeneous mixture because the noodles, broth, chicken chunks, and vegetables are unevenly mixed, visually distinct, and can be physically separated. (d) Clear apple juice is a homogeneous mixture (a solution) because water, dissolved sugars, malic acid, and flavor compounds are evenly dispersed at the molecular scale, presenting a uniform appearance throughout.
Exercise 2: Contrasting Compounds and Mixtures. A student mixes 10 grams of powdered yellow sulfur and 10 grams of gray iron filings in a glass petri dish. The student runs a magnet over the dish, and all the iron filings stick to the magnet, leaving the sulfur behind. Next, the student remixes the powders, places the mixture into a test tube, and heats it strongly over a flame until it glows red hot. After cooling, the student runs the magnet over the resulting black solid, but the magnet fails to attract anything. Explain the fundamental chemical difference between what existed in the petri dish and what existed in the test tube after heating.
Solution 2: In the petri dish, the iron and sulfur formed a physical mixture. Because no chemical bonds formed between the atoms, both elements retained their individual physical properties: the sulfur retained its yellow color, and the iron retained its magnetic attraction, allowing physical separation with a magnet. When heated in the test tube, a chemical reaction occurred: the iron and sulfur atoms bonded to synthesize an entirely new chemical compound called iron(II) sulfide (FeS). In this compound, the magnetic property of elemental iron was lost because the iron atoms are chemically bonded into a rigid crystal lattice with sulfur, creating a new substance that no longer responds to a magnetic field.