An isotope of carbon has a mass number of 14 and an atomic number of 6. How many neutrons does each atom of this isotope contain?
8
✓
6
14
20
Correct Answer
8
The number of neutrons equals the mass number minus the atomic number, so 14 minus 6 gives 8. The value 6 is the number of protons, not neutrons. The value 14 is the total of protons and neutrons. The value 20 comes from adding the two numbers instead of subtracting.
Category: Atoms and Ions
Question 2
A nucleotide is made of a nitrogenous base, a phosphate group and which other component?
Six-carbon sugar
Five-carbon sugar
✓
Fatty acid chain
Amino group
Correct Answer
Five-carbon sugar
Each nucleotide contains a nitrogenous base, a phosphate group and a five-carbon, or pentose, sugar. A six-carbon sugar such as glucose is a monosaccharide used for energy, not a nucleotide part. A fatty acid chain is a component of lipids. An amino group is a part of every amino acid, not a nucleotide component.
Category: Proteins and Nucleic Acids
Question 3
When a neutral sodium atom loses one electron, it becomes
A negatively charged ion
A new isotope of sodium
A positively charged ion
✓
An atom of another element
Correct Answer
A positively charged ion
Losing a negatively charged electron leaves sodium with 11 protons and 10 electrons, giving a net charge of +1, which makes it a cation. A negatively charged ion, or anion, forms when an atom gains electrons. A new isotope would require a change in neutrons, not electrons. An atom of another element would require a change in the number of protons.
Category: Atoms and Ions
Question 4
What is the maximum number of electrons the first electron shell of an atom can hold?
8
4
18
2
✓
Correct Answer
2
The innermost shell can hold only 2 electrons, which is why hydrogen and helium are the only elements in the first row of the periodic table. The value 8 is the number of electrons the second shell can hold, and the octet rule says atoms are most stable with eight electrons in their outer shell. The value 4 is the number of outer-shell electrons in carbon. The value 18 is the full capacity of the third shell, not the first.
Category: Atoms and Ions
Question 5
Table salt forms when sodium gives an electron to chlorine and the resulting oppositely charged particles attract each other. Which type of bond is this?
Ionic bond
✓
Nonpolar covalent bond
Polar covalent bond
Hydrogen bond
Correct Answer
Ionic bond
An ionic bond is the attraction between oppositely charged ions formed by the transfer of electrons, as in sodium chloride. A nonpolar covalent bond involves equal sharing of electrons. A polar covalent bond involves unequal sharing of electrons. A hydrogen bond is a weak attraction between a slightly positive hydrogen and a slightly negative atom.
Category: Chemical Bonds
Question 6
In a molecule of oxygen gas (O2), two identical atoms share electrons equally. What type of bond holds them together?
Polar covalent bond
Nonpolar covalent bond
✓
Ionic bond
Hydrogen bond
Correct Answer
Nonpolar covalent bond
When two atoms of the same element share electrons, neither pulls harder, so the sharing is equal and the bond is nonpolar covalent. A polar covalent bond forms when one atom pulls shared electrons more strongly, as in water. An ionic bond involves a transfer of electrons, not sharing. A hydrogen bond is a weak attraction between separate molecules, not the bond within O2.
Category: Chemical Bonds
Question 7
The weak attraction between the slightly positive hydrogen of one water molecule and the slightly negative oxygen of a neighboring water molecule is a(n)
Ionic bond
Nonpolar covalent bond
Hydrogen bond
✓
Peptide bond
Correct Answer
Hydrogen bond
A hydrogen bond is a weak attraction between a slightly positive hydrogen atom and a slightly negative atom such as oxygen in another molecule, and it explains many of water's properties. An ionic bond is between fully charged ions. A nonpolar covalent bond is equal sharing of electrons within a molecule. A peptide bond is a covalent bond that links amino acids.
Category: Chemical Bonds
Question 8
Why does ice float on liquid water?
It is more dense than liquid water
It contains many trapped air bubbles
It has no hydrogen bonds at all
It is less dense than liquid water
✓
Correct Answer
It is less dense than liquid water
As water freezes, hydrogen bonds lock the molecules into a lattice that holds them farther apart, so ice is less dense than liquid water and floats. If it were more dense than liquid water, it would sink. Trapped air bubbles are not the reason; even clear ice floats. Ice is held together by hydrogen bonds, so it is not true that it has none.
Category: Properties of Water
Question 9
Substances such as table sugar that form hydrogen bonds with water and dissolve readily in it are described as
Hydrophilic
✓
Hydrophobic
Nonpolar
Saturated
Correct Answer
Hydrophilic
Hydrophilic means water-loving, describing polar or charged substances that interact with water and dissolve. Hydrophobic means water-fearing and describes substances such as oils that do not dissolve. Nonpolar molecules lack partial charges, which is why they do not mix with water. Saturated describes fatty acids with only single bonds between carbons, not solubility.
Category: Properties of Water
Question 10
Water climbs higher along the inside wall of a narrow straw than in its center because water molecules are attracted to the straw. This attraction is called
Cohesion
Adhesion
✓
Surface tension
Evaporation
Correct Answer
Adhesion
Adhesion is the attraction between water molecules and molecules of a different substance, such as the material of a straw. Cohesion is the attraction of water molecules to each other. Surface tension is the resistance of a water surface to breaking, which results from cohesion. Evaporation is the change from liquid to gas.
Category: Properties of Water
Question 11
Why does sweating help cool the body?
Freezing water releases heat onto the skin
Sweat slows the body's rate of metabolism
Evaporating water carries heat away from the skin
✓
Sweat blocks outside heat from reaching the skin
Correct Answer
Evaporating water carries heat away from the skin
Turning liquid water into vapor requires energy to break hydrogen bonds, so evaporating sweat absorbs heat from the skin and carries it away. The idea that freezing water releases heat onto the skin does not apply, because sweat does not freeze. The claim that sweat slows the body's rate of metabolism is wrong, since sweating does not change metabolism. The idea that sweat blocks outside heat from reaching the skin is also wrong, because sweat cools by evaporating, not by acting as a barrier.
Category: Properties of Water
Question 12
A solution with a pH of 3 has how many times the hydrogen ion concentration of a solution with a pH of 5?
2
10
1,000
100
✓
Correct Answer
100
Each step of one pH unit is a tenfold change in hydrogen ion concentration, so two units is 10 times 10, or 100. The value 2 is the difference in pH units, not the concentration ratio. The value 10 is the change for a single pH unit. The value 1,000 would be the change over three pH units.
Category: pH and Buffers
Question 13
In the blood's buffer system, bicarbonate ions combine directly with excess hydrogen ions to form
Carbonic acid
✓
Sodium chloride
Calcium carbonate
Hydrochloric acid
Correct Answer
Carbonic acid
Bicarbonate (HCO3-) binds an excess hydrogen ion to form carbonic acid (H2CO3), which keeps the hydrogen ion from lowering blood pH. Sodium chloride is table salt and is not produced in this reaction. Calcium carbonate is a mineral found in shells and bone, not a product of the blood buffer. Hydrochloric acid is the strong acid made in the stomach.
Category: pH and Buffers
Question 14
Normal human blood has a pH of about
6.8
7.4
✓
2.0
8.6
Correct Answer
7.4
Blood is slightly basic, with a pH close to 7.4, and buffers keep it within a narrow range. A pH of 6.8 would be slightly acidic and dangerous for body cells. A pH of 2.0 is typical of stomach acid. A pH of 8.6 would be far too basic for blood.
Category: pH and Buffers
Question 15
Starch and cellulose are both polymers built from which monomer?
Amino acids
Fatty acids
Glucose
✓
Nucleotides
Correct Answer
Glucose
Both of these polysaccharides are long chains of glucose units; they differ in how the units are linked and branched. Amino acids are the monomers of proteins. Fatty acids are components of lipids such as triglycerides. Nucleotides are the monomers of nucleic acids.
Category: Carbohydrates
Question 16
Which disaccharide is the sugar naturally found in milk?
Sucrose
Maltose
Fructose
Lactose
✓
Correct Answer
Lactose
Lactose, the sugar of milk, is made of one glucose and one galactose unit. Sucrose is table sugar, made of glucose and fructose. Maltose is made of two glucose units and comes from the breakdown of starch. Fructose is a monosaccharide found in fruit, not a disaccharide.
Category: Carbohydrates
Question 17
Animals store excess carbohydrate as a highly branched polysaccharide kept mainly in liver and muscle cells. What is it called?
Glycogen
✓
Starch
Cellulose
Chitin
Correct Answer
Glycogen
Glycogen is the storage polysaccharide of animals and is held in liver and muscle cells. Starch is the storage polysaccharide of plants. Cellulose is a structural polysaccharide in plant cell walls. Chitin is a structural polysaccharide found in arthropod exoskeletons and fungal cell walls.
Category: Carbohydrates
Question 18
A fatty acid that has only single bonds between its carbon atoms, so it holds the most hydrogen possible, is called
Monounsaturated
Saturated
✓
Polyunsaturated
Omega-3
Correct Answer
Saturated
A saturated fatty acid has no double bonds between carbons, so its chain is straight and filled with hydrogen; fats rich in it tend to be solid at room temperature. Monounsaturated fatty acids have one double bond. Polyunsaturated fatty acids have more than one double bond. Omega-3 fatty acids are a type of unsaturated fatty acid with double bonds.
Category: Lipids
Question 19
Cholesterol and testosterone belong to which class of lipids, built from four linked carbon rings?
Triglycerides
Phospholipids
Steroids
✓
Waxes
Correct Answer
Steroids
Steroids have a structure of four fused carbon rings, and cholesterol is the precursor of steroid hormones such as testosterone. Triglycerides are made of glycerol and three fatty acids. Phospholipids have two fatty acids and a phosphate group on glycerol. Waxes are made of long-chain fatty acids joined to long-chain alcohols.
Category: Lipids
Question 20
Amino acids in a protein are joined to each other by which type of covalent bond?
Glycosidic bond
Hydrogen bond
Ionic bond
Peptide bond
✓
Correct Answer
Peptide bond
A peptide bond forms between the carboxyl group of one amino acid and the amino group of the next, releasing a molecule of water. A glycosidic bond links sugar units in carbohydrates. A hydrogen bond is a weak attraction, not a covalent bond, though it helps shape proteins. An ionic bond is an attraction between charged ions, not a covalent link.
Category: Proteins and Nucleic Acids