MCAS Chemistry Exam Practice

NaCl is a common salt because it is the chemical formula for table salt, sodium chloride. It is widely used in cooking and food preparation as a seasoning and preservative. NaOH, KCl, and KOH are not common salts. NaOH is sodium hydroxide, a strong base. KCl is potassium chloride, which is often used as a substitute for table salt but is not as commonly used. KOH is potassium hydroxide, another strong base commonly used in industry and laboratory settings.

The symbol for the element named Iron is Fe.

Isotopes are elements that have the same atomic number, which means they have the same number of protons in their nucleus, but they have different atomic masses. This is because isotopes have a different number of neutrons in their nucleus, which affects their atomic mass. Therefore, isotopes are the elements that have the same atomic number but different atomic mass.

Halogens is the correct answer because it refers to group 7 elements on the periodic table. The elements in this group include fluorine, chlorine, bromine, iodine, and astatine. These elements are known for their high reactivity and ability to form salts. They are also characterized by having seven valence electrons in their outermost energy level.

Hydrogen is the correct answer because it is the first element of the periodic table. The periodic table is a tabular arrangement of chemical elements, organized based on their atomic number, which represents the number of protons in the nucleus of an atom. Hydrogen has an atomic number of 1, making it the first element in the periodic table.

The periodic table consists of 8 groups of elements. Each group represents a column in the table and contains elements with similar properties and electron configurations. These groups are numbered from 1 to 18, and elements within the same group tend to have similar chemical behaviors and react in similar ways. Therefore, the correct answer is 8.

Allotropy is the phenomenon where an element exists in multiple forms with different physical and chemical properties. Zinc, calcium, and boron do not exhibit allotropy as they exist in only one form. Phosphorus, on the other hand, exhibits allotropy as it can exist in multiple forms such as white phosphorus, red phosphorus, and black phosphorus, each with different properties.

Oxygen has a valency of -2 because it has six valence electrons in its outermost shell. In order to achieve a stable electron configuration, oxygen tends to gain two electrons to complete its octet. By gaining two electrons, oxygen achieves a full outer shell and a stable configuration, resulting in a valency of -2.

NaOH is a base because it is a hydroxide compound. Bases are substances that release hydroxide ions (OH-) when dissolved in water. NaOH dissociates in water to form Na+ ions and OH- ions, making it a strong base. In contrast, Zn(OH)Cl is not a base because it does not release hydroxide ions when dissolved in water. KCl and NaCl are both salts and do not have the ability to release hydroxide ions, so they are not bases.

Calcium is not a metallic element because it does not exhibit the typical properties of metals. Metallic elements are generally shiny, malleable, and good conductors of heat and electricity. However, calcium is a dull, brittle, and poor conductor of electricity. Therefore, it does not fall under the category of metallic elements.