Structure 1.1.1 – Elements and Compounds & Mixtures

Elements:

Elements are the simplest pure substances that consist of only one type of atom. They cannot be broken down into simpler substances by chemical means. Each element is defined by its atomic number, which is the number of protons in its atoms.

• There are over 100 known elements (listed in the Periodic Table).
• Examples: Hydrogen (H), Oxygen (O), Iron (Fe), Gold (Au).
• Some elements exist as molecules (e.g., O₂, N₂), but they still consist of the same type of atom.

Compounds:

Compounds are pure substances formed when two or more different elements are chemically bonded together in a fixed ratio. The atoms in a compound are held together by chemical bonds (either ionic, covalent, or metallic).

• Compounds have different properties from the elements they are made from.
• They can only be separated into their elements by chemical reactions.
• Examples: Water (H₂O), Carbon dioxide (CO₂), Sodium chloride (NaCl).

 Mixtures

 A mixture is a physical combination of two or more substances (elements and/or compounds) that are not chemically bonded and are present in any proportion. The components retain their individual properties and can be separated using physical methods.

• No fixed composition or formula
• No chemical bonding between components
• Properties are the sum of the individual components

Types of Mixtures:

Homogeneous mixtures:

Uniform composition throughout the mixture. The different components are not visibly distinguishable.

• • Examples: Salt solution, air, alloys (e.g., brass)

Heterogeneous mixtures:

Non-uniform composition. Different components are visibly distinct.

• • Examples: Oil and water, sand and iron filings, salad

Comparison of Elements, Compounds, and Mixtures:

Separation Techniques for Mixtures:

Solvation:

This is the process by which solute particles are surrounded by solvent molecules upon dissolution. When solvation occurs in water, it is called hydration. Solvation involves interactions such as ion-dipole attractions, dipole-dipole forces, or hydrogen bonding depending on the nature of the solute and solvent.

 In ionic compounds like sodium chloride, water molecules (polar) surround the dissociated ions.
 The partial negative oxygen of water attracts \( \text{Na}^+ \), and the partial positive hydrogen attracts \( \text{Cl}^- \).

Example: Dissolving salt in water

\( \text{NaCl (s)} \rightarrow \text{Na}^+ (aq) + \text{Cl}^- (aq) \)

This process increases entropy (disorder) and is typically exothermic.

Filtration:

A mechanical or physical process used to separate an insoluble solid from a liquid. It uses a filter medium (e.g., filter paper) that allows only the liquid (filtrate) to pass through while the solid (residue) is retained.

•  Works only when the solid does not dissolve in the liquid.
•  It is commonly used in both laboratory and industrial settings.

Example:

Sand and water mixture

Sand remains on the filter paper, while water passes through as filtrate.

Recrystallization:

A purification technique used for solid compounds, especially when they contain impurities.

Process:

• Dissolve the impure solid in a minimum amount of hot solvent (solubility increases with temperature).
• Filter to remove any insoluble impurities.
• Cool the solution slowly – pure compound crystallizes out due to reduced solubility at low temperature.
• Filter again to collect purified crystals.

Example:

Separation of pure copper sulfate crystals from impure copper sulfate by crystallization.

This method relies on the differences in solubility of the compound and impurities at different temperatures.

Evaporation:

A method used to separate a solute that is dissolved in a solvent by heating the solution until the solvent evaporates, leaving behind the solute.

• The solvent (often water) escapes into the air as vapor.
• Only works if the solute is heat-stable and the solvent is not needed.

Example:Recovering salt from saltwater:

\( \text{NaCl (aq)} \xrightarrow{\text{heat}} \text{NaCl (s)} + \text{H}_2\text{O (g)} \)

Note: This is not suitable if the solute decomposes upon heating (e.g., sugar).

Distillation:

A technique used to separate mixtures based on differences in boiling points of the components. It involves vaporizing the more volatile component and then condensing it back into liquid form.

Simple Distillation:

• Used when separating a solvent from a solution or when the boiling point difference between two liquids is large (typically > 25°C).
• Common example: Separating water from a salt solution.

Fractional Distillation:

• Used when separating a mixture of two or more miscible liquids with closer boiling points.
• A fractionating column improves separation by allowing repeated condensation and evaporation.
• Common example: Separating ethanol (bp 78°C) from water (bp 100°C).

Key Principle: The substance with the lower boiling point evaporates first and is collected after condensation.

Paper Chromatography:

An analytical method used to separate mixtures of dissolved substances based on their relative solubilities and interactions with the stationary phase (paper) and mobile phase (solvent).

• The mixture is spotted near the base of a strip of chromatography paper.
• The paper is then placed in a solvent. The solvent rises by capillary action, carrying the substances with it.

Different substances move at different rates depending on:

• Solubility in the mobile phase
• Affinity to the stationary phase
• More soluble substances travel further.
• Used in separating dyes, food coloring, amino acids, or inks.

Retention Factor (Rf):

\( R_f = \frac{\text{distance moved by solute}}{\text{distance moved by solvent}} \)

Rf values are unitless and always between 0 and 1.
Helps identify substances when compared with reference values.