Notes, abstracts, papers, exams and problems of Chemistry

States of Matter and Their Changes

State changes are processes experienced by the body according to the environmental conditions of pressure and temperature.

• Solid-Liquid (Fusion)
• Liquid-Gas (Vaporization)
• Gas-Solid (Deposition)
• Liquid-Solid (Solidification)
• Solid-Gas (Sublimation)
• Gas-Liquid (Condensation)

When heating a body, the energy of the particles increases. There comes a moment when this energy is sufficient to overcome the forces of cohesion, and then fusion occurs when we consider that part of a solid body. The same applies to all states of aggregation. The form of fusion of a body depends on its nature. We will distinguish between crystalline and amorphous bodies.

• In a crystalline body, such as ice, melting is produced at a constant temperature

Atomic Theory: From Democritus to Quantum Models

Democritus' Atomic Theory

Democritus, in ancient Greece, founded the atomist school. This school, taking a pluralistic approach, proposed that atoms were the fundamental particles. The core principles of atomism are:

1. Atoms move in a vacuum.
2. The shape and size of atoms determine the substance.

Aristotle, conversely, believed that matter was a single, continuous entity.

Dalton's Atomic Theory

John Dalton proposed his atomic theory in the early 19th century. Its key postulates are:

1. Elements are composed of indivisible and indestructible particles called atoms.
2. Atoms of the same element have the same mass and properties.
3. Atoms of different elements have different masses and properties.
4. Chemical compounds are

Understanding Material Systems and the Atmosphere

Background: A material system is a set of interrelated elements considered as a unit for study.

Homogeneous and Heterogeneous Material Systems

Material systems can be homogeneous or heterogeneous.

In a homogeneous system, different substances are not readily apparent.

A mixture is a material system (either homogeneous or heterogeneous) from which different substances can be separated by physical processes.

Solutions and Their Components

A solution is a homogeneous mixture formed by two or more substances in variable amounts, which can be separated by physical means.

In a solution, the substance present in the greater proportion is called the solvent; the others are called solutes.

Separation Techniques

Classification of Matter

Matter is classified as homogeneous or heterogeneous according to its appearance.

• Homogeneous: It looks the same everywhere.
• Heterogeneous: It does not have the same aspect throughout.

Classification of Substances

Substances can be classified according to their material composition into pure substances or mixtures.

• Pure substances: Are formed by a single substance.

Classification of Pure Substances

Pure substances are classified into chemical elements and compounds

• Chemical Elements: Are pure substances that cannot be decomposed into simpler ones.
• Compounds: Are pure substances that can be decomposed into simpler ones.

• Mixtures: A mixture is formed by several pure substances. Mixtures have no fixed composition.

Separation of Mixing

Drinking Water Purification

Raw Water Treatment

1. Screening: Raw water passes through gratings, filters, and screens to remove coarse solids.
2. Aeration: Oxidation of undesirable organic species improves taste and odor.
3. Coagulation: Colloidal particles are coagulated and flocculated.
4. Sedimentation: Solids are allowed to settle and are removed.
5. Filtration: Removal of any remaining coagulated solids.
6. Disinfection: Control of bacteria.
7. Softening: Using lime and soda methods.
8. Demineralization: By distillation or ion exchange resins.

Wastewater Treatment

Primary Treatment

1. Screening/Roughing: Removal of large objects and solids through sieves.
2. Grit Removal
3. Skimming
4. Sedimentation: Takes place in tanks or chambers where suspended solids settle. This removes approximately

Arrhenius Theory of Acids and Bases

According to the Arrhenius theory, acids and bases exist only in aqueous solution:

• Acid: Any substance that in aqueous solution dissociates with the formation of hydrogen ions, H⁺.
• Base: Any substance that in aqueous solution dissociates with the formation of hydroxyl ions, OH^-.

Neutralization Reaction

Neutralization is the complete reaction of an acid and a base with the formation of a salt and water. HA + BOH → AB + H₂O.

Brønsted-Lowry Theory

According to the Brønsted-Lowry theory, the concepts of acid and base were expanded:

• Acid: Any chemical species, molecular or ionic, capable of yielding an H⁺ ion to another substance.
• Base: Any chemical species, molecule or ion, capable of receiving an H⁺ ion from another

Mixtures

A mixture is defined as a material system formed by combining two or more substances whose proportions can vary.

• Homogeneous mixtures: These consist of two or more components that cannot be distinguished visually.
• Heterogeneous mixtures: These are formed by two or more components that can be visually distinguished. They have different properties depending on the proportion of the sample taken.

Pure Substances

A pure substance is one in which, regardless of its state of aggregation, its properties are characteristic and allow differentiation from other substances.

• Chemical elements: These are pure substances that cannot be decomposed into simpler substances through normal physical and chemical processes.
• Compounds: These are pure substances

Chemical Bonds and Atomic Structure

Most atoms are present together with other peers, forming elements or different ones to form compounds. The physical states evolve toward states of minimum potential energy, which correspond with maximum stability.

Chemical Bonding

Two or more atoms are held together if the energy of the system is less when they are together than when separated.

The Octet Rule

The noble gases have very stable atoms that do not combine with other atoms and do not alter their electronic structure. All of them have 8 electrons except helium. It follows that the electronic provision of a noble gas property is a particularly stable arrangement called the octet.

Octet Rule

In forming compounds, atoms exchange electrons to acquire 8 electrons... Continue reading "Chemical Bonds and Atomic Structure: Understanding Stability" »

States of Matter and Their Properties

Solids: Maintain a constant volume and shape. They possess a high density, are rigid, and are difficult to compress. Liquids: Are not rigid and are difficult to compress, maintaining a constant volume but with a variable shape. They usually have a high density. Gases: Easily expand and compress, having a variable shape and very low density.

Kinetic Molecular Theory

The discontinuous kinetic molecular model (composed of many particles) explains the behavior of matter. Molecules are in continuous motion, with attractive forces between particles. Solids: The attraction between particles is very intense, keeping them very close together and occupying a fixed position. Their movement is limited to vibration. Liquids: