The most common state of matter in the universe is _______.

• gas

• solid

• plasma

• liquid

Answer explanation

The most common state of matter in the universe is plasma, which makes up about 99% of the visible universe. It consists of ionized gases found in stars, including our sun, and is distinct from solids, liquids, and gases.

Which of the following is evidence that a physical change has occurred?

• Formation of a solid precipitate

• Rust

• Broken glass

• Bubbles

Answer explanation

The formation of a solid precipitate indicates a physical change, as it involves a change in state without altering the chemical composition. In contrast, rust and broken glass involve chemical changes or physical damage, respectively.

Matter in which the particles are free to move in all directions until they have spread evenly throughout their container is a _______.

• gas

• solid

• liquid

• plasma

Answer explanation

Matter in which particles move freely in all directions and spread evenly throughout their container is classified as a gas. Unlike solids and liquids, gases have no fixed shape or volume.

Which of the following is the reason why atoms of sodium and chlorine attract each other in a salt crystal?

• Sodium has a positive charge and chlorine has a negative charge

• Both have a positive charge

• Both have a negative charge

• Sodium has a negative charge and chlorine has a positive charge

Answer explanation

Sodium has a positive charge due to losing an electron, while chlorine has a negative charge from gaining an electron. This opposite charge creates an electrostatic attraction, leading to the formation of salt crystals.

How many electrons are needed in the outer energy levels of most atoms for the atom to be chemically stable?

• 2

• 6

• 4

• 8

Answer explanation

Most atoms achieve chemical stability by having 8 electrons in their outer energy levels, known as the octet rule. This configuration allows them to be less reactive and more stable.

Which part of the periodic table indicates the number of energy levels in the electron cloud?

• Period

• Family

• Valence electrons

• Group

Answer explanation

The period of the periodic table indicates the number of energy levels in the electron cloud. Each period corresponds to a principal energy level, with elements in the same period having the same number of energy levels.

Which of the following elements would be the least reactive?

• Iodine

• Silicon

• Argon

• Silver

Answer explanation

Argon is a noble gas, known for its lack of reactivity due to a full valence shell of electrons. In contrast, iodine, silicon, and silver are more reactive, making argon the least reactive element among the choices.

Which of the following elements would be the most reactive?

• Argon

• Iodine

• Silicon

• Potassium

Answer explanation

Potassium is an alkali metal, known for its high reactivity, especially with water. In contrast, argon is a noble gas and inert, iodine is less reactive, and silicon is a metalloid. Therefore, potassium is the most reactive element listed.

Mrs. James blends lemons, sugar, and water in a pitcher. She has made a _______.

• compound

• heterogeneous mixture

• substance

• homogeneous mixture

Answer explanation

Mrs. James blends lemons, sugar, and water, creating a uniform mixture where the components are evenly distributed. This is a homogeneous mixture, as opposed to a heterogeneous mixture where the components remain distinct.

A flask containing molecules of gas A and a separate flask containing molecules of gas B are both at the same temperature. Gases A and B must have equal _______.

• average kinetic energies

• volumes

• pressures

• masses

Answer explanation

At the same temperature, gases A and B have equal average kinetic energies. However, since they are in separate flasks, the pressures can differ. The correct answer is that gases A and B must have equal pressures due to the ideal gas law.