Questions: Classification of Matter: Elements, Compounds, and Mixtures

Question 1

Saltwater is classified as a homogeneous mixture rather than a compound. Which explanation best captures the key distinction?

• A. Saltwater contains two elements, while compounds must contain at least three
• B. Saltwater looks uniform, but compounds must have different visible phases
• C. In saltwater the components are physically combined in variable ratios and retain their individual identities; they are not chemically bonded and can be separated by physical means
• D. Saltwater is a compound because it has a definite boiling point

Answer: (C)

The defining feature of a compound is a chemical bond in a fixed ratio — NaCl is always one sodium per one chloride, and breaking it requires a chemical reaction. Saltwater has no chemical bonds between salt and water, can be made at any concentration, and the components are recoverable by evaporation. The uniform appearance is what makes it homogeneous, but uniformity alone doesn't make something a compound. Many students confuse 'homogeneous' with 'compound' — this is the key distinction to avoid.

Question 2

A student claims that burning wood is a physical change because no new material appears to arrive — the wood just shrinks and blackens. The correct response is:

• A. The student is correct — burning rearranges the wood but does not create new substances
• B. Burning is a chemical change: new substances with different compositions (CO₂, water vapor, ash) are produced, and the process cannot be reversed by physical means
• C. Whether burning is physical or chemical depends on how completely the wood combusts
• D. Burning is a physical change because the carbon atoms in wood are the same as those in the ash

Answer: (B)

A chemical change produces new substances with different compositions and properties — burning wood produces carbon dioxide, water vapor, and ash, none of which have the composition of wood. The fact that the products are gaseous and invisible doesn't mean they haven't formed; it just means they've dispersed. The key test: can the process be reversed by physical means? No — you cannot 'unburn' wood by cooling it. Chemical changes alter molecular identity; physical changes alter form while preserving identity.

Question 3

A solution of sugar in water is a compound because it has a uniform composition throughout its volume.

• T. True
• F. False

Answer: False

Uniform composition is what makes a mixture homogeneous, not what makes it a compound. A compound requires chemical bonding in a fixed ratio — sugar and water are not chemically bonded in solution; they are physically mixed. You can dissolve more or less sugar to get different concentrations (variable composition), and you can recover the original sugar by evaporating the water. Both of these facts confirm it is a mixture. The confusion arises from conflating 'homogeneous' (uniform throughout) with 'compound' (chemically bonded, fixed ratio).

Question 4

The components of a compound cannot be separated by physical means such as filtering, evaporating, or boiling.

• T. True
• F. False

Answer: True

This is one of the defining features of a compound. Water (H₂O) cannot be separated into hydrogen and oxygen by boiling — boiling just converts liquid water to gaseous water, preserving the chemical bond. Separating a compound into its elements requires a chemical reaction (electrolysis splits water; intense heat decomposes some compounds). By contrast, the components of a mixture retain their identities and can be recovered by physical means appropriate to their different properties.

Question 5

Explain the key distinction between a compound and a homogeneous mixture, using table salt (NaCl) and saltwater as examples.

Model answer: A compound has elements chemically bonded in a fixed ratio, producing a new substance with properties different from its components — NaCl is always 1:1 sodium to chloride, and it behaves as a single substance. A homogeneous mixture like saltwater has components physically combined in variable amounts, retaining their individual identities and separable by physical means. The test: changing the ratio doesn't change a compound's identity (you'd get a different substance), but you can make saltwater at any concentration. Separation requires chemistry for compounds; physics suffices for mixtures.

The compound/mixture distinction is deeper than appearance. Both NaCl and saltwater are white or clear and uniformly composed, but one involved bond formation and has fixed stoichiometry while the other is purely mechanical mixing. This is why 'looking homogeneous' is not the test — the test is whether a chemical reaction occurred.