PRAXIS II: Chemistry Exam Prep

Explanation
Hydrogen is absent in a hydrocarbon because hydrocarbons are compounds composed only of carbon and hydrogen atoms. They are organic compounds that form the basis of many important substances such as gasoline, natural gas, and plastics. Nitrogen, oxygen, and carbon can be present in other compounds, but in a hydrocarbon, only carbon and hydrogen atoms are present.

Explanation
Allotropy is the correct answer because it refers to the phenomenon where an element can exist in different physical forms or allotropes. These allotropes have different structural arrangements and properties, such as different crystal structures or bonding arrangements. Examples of elements that exhibit allotropy include carbon (which can exist as graphite, diamond, or fullerenes) and oxygen (which can exist as O2 or O3).

Explanation
Radioactivity is the odd one out in chemistry because it is the only option that is not directly related to chemical properties or reactions. Electronegativity, bonding, and entropy are all concepts that are fundamental to the study of chemistry and are directly related to chemical interactions and reactions. Radioactivity, on the other hand, is a phenomenon that involves the spontaneous decay of atomic nuclei and is more closely associated with nuclear physics rather than traditional chemistry.

Explanation
Chromatography is a separation technique that involves the separation of different components of a mixture based on their different affinities for a stationary phase and a mobile phase. In chromatography, a mixture is dissolved in a solvent and then passed through a stationary phase, which can be a solid or a liquid. The different components of the mixture interact differently with the stationary phase and the mobile phase, causing them to move at different rates. This results in the separation of the components based on their different colors or pigments.

Explanation
Sieving is the best separation method for a mixture of solids of different sizes because it involves passing the mixture through a sieve or mesh to separate the particles based on their size. This method is effective in separating larger particles from smaller ones, as the smaller particles will pass through the sieve while the larger ones will be retained. Filtration and distillation methods are more suitable for separating solids from liquids or for separating liquids with different boiling points. Fractional distillation is specifically used for separating liquids with similar boiling points.

Explanation
Sodium is not electronegative because it has a low electronegativity value compared to the other elements listed. Electronegativity is a measure of an atom's ability to attract electrons towards itself in a chemical bond. Sodium has a relatively low electronegativity value compared to bromine, oxygen, and fluorine, which are all highly electronegative elements. This means that sodium is less likely to attract electrons towards itself in a chemical bond compared to the other elements listed.

Explanation
The valency of an element refers to its ability to combine with other elements to form compounds. In the case of sodium, its valency is 1. This means that sodium has a tendency to lose one electron in chemical reactions, resulting in a stable electron configuration. This valency allows sodium to easily form compounds with elements that have a valency of -1, such as chlorine, forming sodium chloride.

Explanation
Oxygen is not an inert, noble gas element. It is a highly reactive gas and is essential for supporting combustion. In contrast, Argon, Krypton, and Xenon are all inert, noble gases that are known for their low reactivity and stability.

Explanation
The temperature at which a liquid turns to gas is called the boiling point. This is the point at which the vapor pressure of the liquid equals the atmospheric pressure, causing the liquid to change into a gas.