chem
Chemical vs. Physical Changes
Chemical Changes
Definition: Changes that result in the formation of new substances.
Example: Rust Formation
Iron reacts with oxygen to form rust (iron oxide). This process changes the identity of the substance.
Diatomic Molecules: Certain gases exist as pairs of atoms, such as:
Oxygen:
Nitrogen:
Hydrogen:
Physical Changes
Definition: Changes that do not alter the identity of a substance.
Example: Melting Ice
Ice melting into water does not change the identity of the compound.
Other Changes:
Melting, freezing, evaporation, and condensation are all examples of physical changes due to changes in state, such as solid to liquid to gas.
Types of Compounds
Covalent Compounds
Definition: Compounds formed by the sharing of electrons between nonmetals.
Example: Water (H2O) consists of nonmetals bonded together by covalent bonds.
Ionic Compounds
Definition: Compounds formed from the electrostatic attraction between positively charged ions (cations) and negatively charged ions (anions), typically between metals and nonmetals.
Example: Sodium chloride (NaCl) forms when sodium (Na) donates an electron to chlorine (Cl) creating oppositely charged ions that attract each other.
Mixtures vs. Pure Substances
Pure Substances
Definition: Materials that have a definite and constant composition, which can be either elements or compounds.
Mixtures
Definition: Combinations of two or more substances that retain their individual properties.
Subtypes:
Homogeneous Mixtures (solutions): Uniform in composition; components are not distinguishable.
Example: Salt dissolved in water, where the salt is not visible.
Heterogeneous Mixtures: Components are not uniformly distributed; individual substances can be distinguished.
Example: Sand mixed with water, where sand particles remain visible.
Separation of Mixtures
Mixtures can be separated by physical means.
Methods of Separation:
Evaporation: Useful for separating a soluble solid (salt) from a liquid (water).
Decantation: Separating liquid from solid, such as pouring off water from sand without disturbing the solid.
Filtration: Using filter paper to separate solids from liquids.
Distillation: Used to separate two liquids with different boiling points, e.g., separating ethanol from water.
Extraction: Using a solvent to dissolve a soluble component from a mixture.
Definitions of Components
Elements: Pure substances consisting of only one type of atom.
Example: Gold (Au) is an element composed entirely of gold atoms.
Compounds: Substances formed when two or more elements chemically combine in a specific ratio.
Example: Water (H2O) always consists of two hydrogen atoms and one oxygen atom.
Mixtures: Combinations of different substances that do not combine chemically and can have variable compositions.
Composition of Thus Substances
Definite Composition for Pure Substances:
Elements: Consist of only one element.
Compounds: Have a constant ratio of elements.
Variable Composition for Mixtures: Mixtures do not have a fixed composition; amounts of components can vary.
Unit Conversion and Dimensional Analysis
Conversions: Understanding how to convert between different units is vital, particularly for applications in science and medicine.
Equality Relation:
Example: 1 foot = 12 inches
Conversion Factor Examples:
For converting inches to feet:
If given 9.1 inches, use:
Mass Conversions:
Example: Convert between milligrams and grams.
Dimensional Analysis: Process of converting between units and ensuring units cancel out appropriately when performing calculations. Units should always cancel to verify the conversion’s accuracy.