Some Basic Concepts of Chemistry

Physical and Chemical Properties

Understanding the properties of substances is fundamental in chemistry. These properties are broadly classified as physical or chemical, based on whether the substance's composition is altered during measurement.

• Physical Property: A property that can be measured or observed without changing the chemical composition of a substance. Examples include mass, volume, and density.

• Chemical Property: A property that can only be observed by changing the chemical identity of a substance. This involves evaluating the substance at the cost of its matter, such as flammability or reactivity.

• Example: Measuring the boiling point of water is a physical property, while observing iron rusting (reacting with oxygen) is a chemical property.

Classification of Matter

Matter is anything that has mass and occupies space. It can be classified based on its composition and uniformity.

• Pure Substance: Matter with a fixed composition and distinct properties. Pure substances are further divided into:

  • Element: A substance that cannot be broken down into simpler substances by chemical means (e.g., O2, Fe).

  • Compound: A substance composed of two or more elements chemically combined in a fixed ratio (e.g., H2O, NaCl).

• Mixture: A combination of two or more substances in which each retains its own properties. Mixtures are classified as:

  • Homogeneous Mixture: Has a uniform composition throughout (e.g., salt water, air).

  • Heterogeneous Mixture: Does not have a uniform composition (e.g., sand in water, salad).

Law of Chemical Combination

Chemical reactions are governed by fundamental laws that describe how substances interact and combine.

Law of Conservation of Mass

This law states that in any chemical reaction, the total mass of the reactants is always equal to the total mass of the products formed. Matter is neither created nor destroyed during a chemical change.

• Mathematical Expression:

• Example: When hydrogen reacts with oxygen to form water, the combined mass of hydrogen and oxygen before the reaction equals the mass of water produced.

Additional info: The Law of Conservation of Mass was first formulated by Antoine Lavoisier in the 18th century and is foundational to all chemical equations and stoichiometric calculations.