Understanding Pure Substances and Mixtures

What is a pure substance?

• A mixture of different elements and compounds

• A material with distinct measurable properties

• A substance that cannot be broken down into simpler substances

• A combination of substances with variable composition

Answer explanation

A pure substance is defined as a material with distinct measurable properties, such as boiling point and melting point, which are consistent throughout. This distinguishes it from mixtures, which have variable compositions.

Which of the following is an example of a compound?

• Gold (Au)

Answer explanation

Water (H2O) is a compound because it consists of two different elements, hydrogen and oxygen, chemically bonded together. In contrast, O2 and N2 are diatomic molecules of single elements, and gold (Au) is a pure element.

What property is NOT typically used to identify a pure substance?

• Melting point

• Colour

• Density

• Volume

Answer explanation

Volume is not a characteristic property of a pure substance, as it can vary with the amount of substance present. In contrast, melting point, colour, and density are intrinsic properties that help identify a pure substance.

Which of the following is a characteristic of a homogeneous mixture?

• The components are visibly distinct

• The composition is uniform throughout

• The components can be easily separated by physical means

• The properties vary in different parts of the mixture

Answer explanation

A homogeneous mixture has a uniform composition throughout, meaning its components are evenly distributed and not visibly distinct. This distinguishes it from heterogeneous mixtures, where components can be seen and separated.

Which of the following is a heterogeneous mixture?

• Saltwater

• Air

• Sand and water

• Vinegar

Answer explanation

A heterogeneous mixture consists of visibly different substances. Sand and water do not mix uniformly, allowing you to see the separate components, unlike saltwater, air, or vinegar, which are homogeneous mixtures.

What is the main difference between a pure substance and a mixture?

• Pure substances have variable composition, mixtures do not

• Mixtures have distinct measurable properties, pure substances do not

• Pure substances have fixed composition, mixtures have variable composition

• Mixtures are always homogeneous, pure substances are not

Answer explanation

The main difference is that pure substances have a fixed composition, meaning their chemical makeup is consistent, while mixtures can vary in composition, containing different proportions of their components.

Which of the following is NOT a property of a pure substance?

• Reactivity

• Strength

• Boiling point

• Texture

Answer explanation

Texture is not a fundamental property of a pure substance, as it can vary based on the form or state of the substance. In contrast, reactivity, strength, and boiling point are intrinsic properties that define a pure substance.

Which of the following statements is true about elements?

• Elements can be broken down into simpler substances

• Elements are always found in mixtures

• Elements consist of only one type of atom

• Elements have variable composition

Answer explanation

The correct statement is that elements consist of only one type of atom. This means they cannot be broken down into simpler substances, are not always found in mixtures, and have a fixed composition.

What is the boiling point of water at standard atmospheric pressure?

• 90°C

• 100°C

• 110°C

• 120°C

Answer explanation

The boiling point of water at standard atmospheric pressure (1 atm) is 100°C. This is a well-established scientific fact, making 100°C the correct answer among the given choices.

Which of the following is a property that can be used to distinguish between a pure substance and a mixture?

• Colour

• Volume

• Melting point

• Shape

Answer explanation

The melting point is a characteristic property of pure substances, remaining constant, while mixtures have varying melting points due to their different components. Thus, it effectively distinguishes between pure substances and mixtures.