Notes, summaries, assignments, exams, and problems for Chemistry

Material Properties and Characteristics

• Mechanisms: density, hardness, toughness, ductility, and fragility.
• Thermal: melting temperature, thermal conductivity, and thermal expansion.
• Electromagnetic: electrical conductivity.
• Chemistry: resistance to corrosion.
• Acoustics: sound conductivity.
• Optics: color, transparency, reflectivity, and refractive index.

Material Organization and Composites

Materials can be organized in four ways according to their complexity:

1. Chemical elements
2. Chemical compounds
3. Mixtures of materials (alloys)
4. Composites

Composites are materials composed of two or more materials that have very different chemical or physical properties. Together, they form a substance with properties that are, in turn, different from those of its components... Continue reading "Material Properties, Advanced Materials, and Modern Technologies" »

Percentage Composition Calculation

To calculate the percentage composition of a compound, such as H₂O, you first determine the molecular weight of the molecule. Then, for each element, you use a conversion factor based on its contribution to the total molecular weight. This process allows you to find the percentage of each element within the compound, as shown:

Determining Empirical Formula

To calculate the empirical formula from the percentage composition of elements, follow these steps:

1. Convert the percentage of each element to grams (assuming a 100g sample).
2. Divide the mass of each element by its atomic weight to find the number of moles.
3. Divide the number of moles of each element by the smallest number of moles obtained.
4. The resulting whole numbers

Ideal Gas Equation of State

The general equation of state for ideal gases is:

p · V = n · R · T

Kinetic Theory of Gases

Gases consist of molecules that:

• Occupy no volume.
• Move randomly with a velocity whose mean depends on temperature.
• Collide elastically with each other and the vessel walls, causing pressure.

Boyle's Law

P₁ · V₁ = P₂ · V₂

At constant temperature, the volume occupied by a gas is inversely proportional to the pressure it is under.

Charles's and Gay-Lussac's Law

At constant pressure, the volume occupied by a gas is directly proportional to the absolute temperature at which it is located. At constant volume, the pressure of a gas is directly proportional to the absolute temperature at which it is located.

Dalton's Law

The total pressure... Continue reading "Ideal Gas Equation, Kinetic Theory, and Atomic Models" »

Sieving: Separating Solid Particles by Size

Sieving is a physical method used for separating heterogeneous solid mixtures. It involves passing a mixture of solid particles of different sizes through a sieve or colander. The smaller particles pass through the pores, while larger particles are retained by the screen.

This is a very simple method generally used for solid heterogeneous mixtures, such as separating sand (which passes through the sieve) from stones (which are retained). The screen openings are usually of different sizes and are selected according to the size of the particles in the mixture.

Application of Screening

To apply this method, it is necessary that both phases are present in the solid state. Metal or plastic sieves are used,... Continue reading "Fundamental Physical Processes: Sieving, Grinding, Mixing, and Drying" »

Ionic Versus Covalent Chemical Bonds

Ionic Bonds

The ionic bond forms between elements with very different electronegativities. This involves the transfer of electrons from the less electronegative element to the most electronegative one. This process forms respective positive ions (cations, which lose electrons) and negative ions (anions, which gain electrons).

This type of bond typically occurs between elements at opposite ends of the periodic table: specifically, between highly electronegative elements (nonmetals) and less electronegative elements (metals).

Properties of Ionic Compounds

1. High melting and boiling temperatures.
2. Solid at room temperature.
3. The crystal lattice is very stable, making it very difficult to break.
4. They are hard (scratch resistant)

The Pluralistic School

Speculation about the physical world, initiated by the Ionians, was continued in the 5th century BC by Empedocles and Anaxagoras. They developed philosophies which replaced the description of a first Ionian substance only by the assumption of a plurality of substances. Empedocles maintained that all things are composed of four irreducible elements: air, water, earth, and fire, combined or separated by two opposing forces as a process of alternation: love and hate. Through this process, the world evolves from chaos to form and back into chaos again, in a repeated cycle. Empedocles considered the eternal cycle as the true object of religious worship and criticized the popular belief in personal gods, but failed to explain... Continue reading "Pluralism and Atomism in Ancient Greek Philosophy" »

Key Properties of Chemical Substances

Ionic Compounds

• Solid at room temperature
• Form crystal lattices, not individual molecules
• Soluble in water
• Conduct electricity when molten or dissolved
• High hardness and brittle
• High melting and boiling temperatures

Molecular Substances

• Composed of molecules
• Weak intermolecular forces
• Often gaseous (e.g., O₂, N₂, NH₃) or liquid (e.g., H₂O)
• Soft solids
• Low melting and boiling temperatures
• Poor electrical conductors
• Not very soluble in water

Covalent Crystals

• Solid
• Form crystals with atoms joined by covalent bonds in three dimensions
• Insoluble in all solvents
• Generally do not conduct electricity (except graphite)
• Very hard and brittle
• High melting and boiling temperatures (1200 to 3600 °C)

Metals

• Solid at room temperature (except

Understanding Uranium and Nuclear Fuel

Natural uranium is composed of three isotopes: Uranium-238 (U-238), Uranium-235 (U-235), and Uranium-234 (U-234). While U-238 is fertile and can be converted to fissile material, U-235 is the primary fissionable isotope used for power generation. Natural uranium ore cannot be used directly as nuclear fuel because the concentration of U-235 is too low. Therefore, uranium needs to be enriched to achieve a U-235 concentration of more than 3%. To produce one kilogram of enriched uranium, approximately eight kilograms of natural uranium must be purified and refined.

How Nuclear Power Plants Operate

Nuclear power plants utilize uranium as their primary energy source, harnessing nuclear fission to generate electricity.... Continue reading "Nuclear Energy Fundamentals: Fuel Cycle, Power, and Waste" »

Understanding Vaporization and Boiling

The transition from liquid to vapor, known as vaporization, can occur in two primary ways: either only on the surface of the liquid (evaporation) or throughout its entire volume (boiling).

When a liquid in a container is heated, its vapor pressure increases until it equals the atmospheric pressure. At this point, vapor forms throughout the entire mass of the liquid and escapes. Boiling is a specific form of vaporization.

The boiling temperature is a characteristic property of each substance, but it varies with pressure. Vapor pressure itself depends on temperature and increases as temperature rises.

Intermolecular Forces and Boiling Point

The constitution of a liquid significantly influences its boiling behavior.... Continue reading "Fundamentals of Vaporization, Distillation, and Adsorption" »

Copper: Properties and Applications

Copper is reddish in color with a metallic luster. It is characterized as one of the best conductors of electricity and heat. Thanks to its high electrical conductivity, ductility, and malleability, it has become the most widely used material for manufacturing electrical cables and various electrical and electronic components. Copper is a highly durable metal because it can be recycled almost indefinitely without losing its metallic properties.

Refining Copper

Copper refining is the process of removing impurities that make the metal brittle (both cold and hot). This process facilitates rolling and drawing into wire and thread, ensuring optimal application for electrical conductors, as impurities significantly