1.B: Chemistry of Life (Exercises)
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1.4: The Building Blocks of Molecules
At its most fundamental level, life is made up of matter. Matter occupies space and has mass. All matter is composed of elements, substances that cannot be broken down or transformed chemically into other substances. Each element is made of atoms, each with a constant number of protons and unique properties. Each element is designated by its chemical symbol and possesses unique properties. These unique properties allow elements to combine and to bond with each other in specific ways.
Multiple Choice
Magnesium has an atomic number of 12. Which of the following statements is true of a neutral magnesium atom?
A. It has 12 protons, 12 electrons, and 12 neutrons.
B. It has 12 protons, 12 electrons, and six neutrons.
C. It has six protons, six electrons, and no neutrons.
D. It has six protons, six electrons, and six neutrons.
- Answer
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A
Which type of bond represents a weak chemical bond?
A. hydrogen bond
B. ionic bond
C. covalent bond
D. polar covalent bond
- Answer
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A
Free Response
Why are hydrogen bonds and van der Waals interactions necessary for cells?
- Answer
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Hydrogen bonds and van der Waals interactions form weak associations between different molecules. They provide the structure and shape necessary for proteins and DNA within cells so that they function properly. Hydrogen bonds also give water its unique properties, which are necessary for life.
1.5: Water
Do you ever wonder why scientists spend time looking for water on other planets? It is because water is essential to life; even minute traces of it on another planet can indicate that life could or did exist on that planet. Water is one of the more abundant molecules in living cells and the one most critical to life as we know it. Approximately 60–70 percent of your body is made up of water. Without it, life simply would not exist.
Multiple Choice
Which of the following statements is not true?
A. Water is polar.
B. Water stabilizes temperature.
C. Water is essential for life.
D. Water is the most abundant atom in Earth’s atmosphere.
- Answer
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D
Using a pH meter, you find the pH of an unknown solution to be 8.0. How would you describe this solution?
A. weakly acidic
B. strongly acidic
C. weakly basic
D. strongly basic
- Answer
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C
The pH of lemon juice is about 2.0, whereas tomato juice's pH is about 4.0. Approximately how much of an increase in hydrogen ion concentration is there between tomato juice and lemon juice?
A. 2 times
B. 10 times
C. 100 times
D. 1000 times
- Answer
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C
Free Response
Describe the structure of a single water molecule, including all bonds that occur within and between a water molecule and dissolved substance such as NaCl.
Answer
One oxygen atom is covalently bonded to two hydrogen atoms resulting in polarity in the water molecule. The oxygen has a a partially negative charge due to it electromagnetic pull on the electrons and the two hydrogen atoms are partially negative. Due to this polarity, both hydrogen and ionic bonds are readily formed between Na and Cl when salt is dissolved in water.
Why can some insects walk on water?
- Answer
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Some insects can walk on water, although they are heavier (denser) than water, because of the surface tension of water. Surface tension results from cohesion, or the attraction between water molecules at the surface of the body of water [the liquid-air (gas) interface].
Explain why water is an excellent solvent.
- Answer
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Water molecules are polar, meaning they have separated partial positive and negative charges. Because of these charges, water molecules are able to surround charged particles created when a substance dissociates. The surrounding layer of water molecules stabilizes the ion and keeps differently charged ions from reassociating, so the substance stays dissolved.