Properties and Changes of Matter

Which term represents a physical property of matter?

• density

• flammability

• reactivity

• toxicity

Answer explanation

Density is a physical property that describes how much mass is contained in a given volume. In contrast, flammability, reactivity, and toxicity are chemical properties that describe how a substance interacts with other substances.

When a scientist performs an experiment, there are many changes that can occur. Which observation should she list as a chemical change?

• The substance was heated.

• The shape of the substance changed.

• The volume of the substance changed.

• The substance changed into a new substance.

Answer explanation

The correct observation indicating a chemical change is that the substance changed into a new substance. This signifies a transformation at the molecular level, unlike heating, shape, or volume changes, which are physical.

Which observation indicates that a chemical reaction has taken place?

• A. A substance becomes warmer.

• B. A material changes from a solid to liquid.

• C. Bubbles form when a liquid is heated.

• D. A new chemical substance is formed.

Answer explanation

The formation of a new chemical substance is a definitive indicator of a chemical reaction. While other options may suggest physical changes or reactions, only option D confirms a chemical change has occurred.

Which substance is comprised of more than one element?

• aluminum

• helium

• silver

• steel

Answer explanation

Steel is an alloy made from iron and carbon, thus it is comprised of more than one element. In contrast, aluminum, helium, and silver are all pure elements.

A substance is examined under a high-power electron microscope. The substance is found to contain only one type of atom. What is the substance?

• compound

• element

• mixture

• solution

Answer explanation

The substance contains only one type of atom, which defines it as an element. Compounds consist of two or more types of atoms, while mixtures and solutions involve combinations of different substances.

Since sodium chloride is classified as a compound, a sample of it contains

• atoms that are gases, liquids, and solids.

• more than one kind of atom.

• matter that cannot be reduced to atoms.

• a solution of atoms and molecules.

Answer explanation

Sodium chloride (NaCl) is a compound made of sodium and chlorine atoms. Therefore, a sample of it contains more than one kind of atom, which is the correct choice.

What distinguishes compounds from mixtures?

• Compounds are composed of elements that have been chemically combined.

• Compounds must be physically separated back into their original substances.

• Mixtures are composed of elements that have been chemically combined.

• Mixtures must be chemically separated back into their original substances.

Answer explanation

Compounds are distinct because they consist of elements that are chemically combined, forming a new substance with unique properties. In contrast, mixtures retain the individual properties of their components and do not require chemical changes to separate.

The light and dark circles in the pictures below represent different types of atoms. Which picture best represents a pure compound?

• 
• 
• 
• 

Answer explanation

The correct choice is the image of only one type of molecule, each made of one light and one dark circle. This represents a pure compound, as it consists of identical molecules, unlike the other options which show mixtures.

Which situation is an example of a physical change?

• Dough rises with yeast.

• Brass corrodes over time.

• Charcoal burns in a grill.

• Water boils in a pot on the stove.

Answer explanation

Water boiling is a physical change as it involves a change in state from liquid to gas without altering its chemical composition. In contrast, the other options involve chemical changes.

Which statement describes a chemical property?

• Steel is shaped into automobile fenders.

• Water begins to boil when it reaches 100ºC.

• Oil floats on water because it is less dense.

• Iron slowly forms rust when exposed to oxygen gas.

Answer explanation

The correct choice, 'Iron slowly forms rust when exposed to oxygen gas,' describes a chemical property because it involves a change in the substance's chemical composition, unlike the other options which describe physical properties.