Exam1 misconceptions

What is the best description of a sample of H₂O(l)?

• a mixture that has constant composition and properties throughout

• a mixture that varies in both composition and properties

• a substance that has constant composition and properties throughout

• a substance that varies in both composition and properties

Answer explanation

H2O(l) is a pure substance, meaning it has a constant composition (H2 and O) and uniform properties throughout. This distinguishes it from mixtures, which can vary in composition and properties.

Which phrase describes a compound that consists of two elements?

• a mixture in which the elements are in a variable proportion

• a mixture in which the elements are in a fixed proportion

• a substance in which the elements are chemically combined in a variable proportion

• a substance in which the elements are chemically combined in a fixed proportion

  
  

Answer explanation

A compound is defined as a substance formed when two or more elements are chemically combined in a fixed proportion, distinguishing it from mixtures where proportions can vary.

Which particle diagram represents a mixture?

• 1

• 2

• 3

• 4

Answer explanation

Choice 3 represents a mixture as it shows different types of particles that are not chemically bonded, indicating a combination of substances. In contrast, the other diagrams likely depict pure substances or compounds.

Which formula represents a homogeneous mixture?

• NaCl (aq)

• CO₂(g)

• Fe(s)

• CH₄(g)

Answer explanation

NaCl (aq) represents a homogeneous mixture because it is a solution where sodium chloride is dissolved in water, resulting in a uniform composition. The other options are either pure substances or not uniformly mixed.

Oxygen can exist as diatomic oxygen gas, O₂(g),or ozone, O₃(g). These two forms of oxygen have

• the same structure and the same properties

• different structures and different properties

• the same structure and different properties

• different structures and the same properties

Answer explanation

Diatomic oxygen (O2) and ozone (O3) have different molecular structures: O2 consists of two oxygen atoms, while O3 consists of three. This structural difference leads to distinct properties, making the correct choice 'different structures and different properties'.

A sample of copper has a mass of 15.75 grams. How many significant figures are present in this measurement?

• 1

• 2

• 3

• 4

Answer explanation

The measurement 15.75 grams has four significant figures: the digits 1, 5, 7, and 5 are all non-zero and counted. Therefore, the correct answer is 4.

Which substance can not be broken down by a chemical change?

• gold

• carbon dioxide

• propene

• water

Answer explanation

Gold is a pure element and cannot be broken down into simpler substances by chemical means, unlike carbon dioxide, propene, and water, which can be decomposed into their constituent elements.

A mixture of crystals of salt and sugar is added to water and stirred until all solids have dissolved. Which statement best describes the resulting mixture?

• The mixture is homogeneous and can be separated by filtration.

• The mixture is homogeneous and cannot be separated by filtration.

• The mixture is heterogeneous and can be separated by filtration.

• The mixture is heterogeneous and cannot be separated by filtration.

Answer explanation

The mixture is homogeneous because the salt and sugar dissolve completely in water, creating a uniform solution. It cannot be separated by filtration since both solutes are dissolved, making filtration ineffective.

Convert the units 15.0 km to m.

• 0.15

• 15,000

• 15,000,000

• 0.015

Answer explanation

To convert kilometers to meters, multiply by 1,000. Thus, 15.0 km x 1,000 = 15,000 m. Therefore, the correct answer is 15,000.

A sample of neon gas has a volume of 1.5 L. Neon has a density of 0.9 g/L. What is the mass

in grams?

• 1.35 g

• 0.45 g

• 1.5 g

• 2.7 g

Answer explanation

To find the mass of neon gas, use the formula: mass = density × volume. Here, mass = 0.9 g/L × 1.5 L = 1.35 g. Thus, the correct answer is 1.35 g.