Overview
This lecture explains the definition, classification, and properties of matter, focusing on the differences between pure substances and mixtures, how chemists classify matter, and methods for separating mixtures. It also provides examples and practice for classifying different materials.
What is Matter?
- Matter is anything that has both mass and volume.
- Chemists are especially interested in very small forms of matter, such as atoms and molecules.
- In chemistry, experiments are usually done with pure samples of matter to ensure consistent and predictable results.
- In the real world, perfectly pure samples are rare; chemists often need to purify materials before studying their chemical behavior.
Classification of Matter
- All matter can be classified as either pure substances or mixtures.
- Pure substances have a definite, unchanging composition.
- Mixtures have variable compositions that can change from sample to sample.
- Chemists use this classification to better predict and understand the behavior of materials.
Pure Substances: Elements and Compounds
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Pure substances are either elements or compounds.
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Elements:
- Made up of only one kind of atom (e.g., a gold bar contains only gold atoms).
- Cannot be separated into simpler substances.
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Compounds:
- Consist of two or more elements chemically combined in a fixed ratio, represented by a specific chemical formula (e.g., H₂O for water).
- The Law of Constant Composition (Law of Definite Proportions), proposed by Joseph Louis Proust, states that a compound always contains the same elements in the same ratio.
- Different compounds can be made from the same types of atoms if they are combined in different ratios or arrangements (e.g., H₂O vs. H₂O₂).
- Compounds have unique physical and chemical properties that distinguish them from each other, even if they appear similar.
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A pure sample of a compound is chemically identical regardless of its source (e.g., a vitamin from a vegetable is the same as one synthesized in a lab).
Mixtures: Homogeneous and Heterogeneous
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A mixture is a physical blend of two or more substances, and its composition can vary.
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The components of a mixture retain their original chemical properties; they do not chemically react with each other unless a chemical reaction occurs.
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Homogeneous mixtures:
- Have a uniform composition throughout; the mixture looks the same everywhere (e.g., homogenized milk, Tang drink mix, well-mixed paint).
- The term "homogeneous" comes from the Greek "homos," meaning "the same."
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Heterogeneous mixtures:
- Do not have a uniform composition; different parts of the mixture can be identified (e.g., trail mix, fresh squeezed orange juice with pulp, beach sand, cereal in milk).
- The term "heterogeneous" comes from the Greek "heteros," meaning "different."
- In heterogeneous mixtures, you can see or separate the different phases or components.
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Real-world mixtures, like air and lemonade, do not have fixed compositions and can vary depending on location or recipe.
Separation of Mixtures
- Components of mixtures can be separated using their physical properties.
- Example: To separate salt from sand in beach sand:
- Add water to dissolve the salt, leaving the sand undissolved.
- Filter the mixture to separate the sand from the salty water.
- Evaporate the water to recover the salt.
- This process uses differences in solubility and physical state to separate mixture components.
Classification Practice
- Helium: Pure substance, element (He).
- Milk (homogenized): Mixture, homogeneous (evenly mixed due to mechanical processing).
- Milk (non-homogenized, straight from a cow): Mixture, heterogeneous (cream separates and rises to the top).
- Glucose: Pure substance, compound (C₆H₁₂O₆).
- Gravity: Not matter; it is a force.
- Carbon dioxide: Pure substance, compound (CO₂).
- Sunbeam: Not matter; photons have no mass.
- Trail mix: Mixture, heterogeneous (different components like nuts and raisins are visible).
- Table salt: Pure substance, compound (NaCl).
- Beach sand: Mixture, heterogeneous (contains silica, salt, shell fragments, and other materials; composition varies by location).
Key Terms & Definitions
- Matter: Anything with mass and volume.
- Pure substance: Material with a definite, unchanging composition.
- Element: Substance made of one kind of atom; cannot be separated into simpler substances.
- Compound: Substance made of two or more elements chemically combined in a fixed ratio.
- Law of Constant Composition (Law of Definite Proportions): Each compound always contains the same elements in the same ratio.
- Mixture: Physical blend of two or more substances with variable composition.
- Homogeneous mixture: Mixture with uniform composition throughout.
- Heterogeneous mixture: Mixture with non-uniform composition and visible phases.
Action Items / Next Steps
- Take a practice test to identify areas that need more study.
- Review the video on physical and chemical properties for further understanding.
- Visit Socratica.com for additional materials and practice tests.