Ch.16: A Chemistry Primer

The appendix of the book "On Food and Cooking: The Science and Lore of the Kitchen" by Harold McGee (1984) Atoms, Molecules, and Chemical Bonds Energy The Phases of Matter

68 flashcards · Other · by lakesare

Part of Public › Books › On Food and Cooking (Harold McGee) · 16 courses

1. [Atoms, Molecules, and Chemical Bonds] What does the Greek word atom mean?

   "Uncuttable", that is, indivisible.

2. [Atoms, Molecules, and Chemical Bonds] What is a chemical element?

   One of the roughly 100 pure substances of which all matter on earth is a mixture. Example: hydrogen, oxygen, nitrogen, carbon.

3. [Atoms, Molecules, and Chemical Bonds] What is an atom?

   The smallest particle of an element that still has the element's characteristic properties. Example: a row of about 2 million atoms, each about 0.25 nm across, spans a printed full stop (0.5 mm).

4. [Atoms, Molecules, and Chemical Bonds] Which smaller particles are atoms made of?

   Electrons, protons and neutrons. A carbon atom: 6 protons and 6 neutrons in the nucleus, 6 electrons around it.

5. [Atoms, Molecules, and Chemical Bonds] What mainly makes the atoms of one element different from those of another?

   Their numbers of protons and electrons. Example: hydrogen has 1 proton and 1 electron, oxygen 8 of each, iron 26 of each.

6. [Atoms, Molecules, and Chemical Bonds] What is a molecule?

   Two or more atoms held together by shared electrons. It is the smallest unit of a chemical compound that still has the compound's properties.A molecule is to a compound what an atom is to an element. Most matter, food included, is a mixture of many compounds.

7. [Atoms, Molecules, and Chemical Bonds] What electrical charges do protons, electrons and neutrons carry?

   Proton: positive. Electron: an equal negative charge. Neutron: none. Opposite charges attract, like charges repel.

8. [Atoms, Molecules, and Chemical Bonds] What one force drives all the chemistry of life and cooking?

   The electrical attraction between protons and electrons.

9. [Atoms, Molecules, and Chemical Bonds] In an atom of the stable form of an element, how does the number of electrons compare with the number of protons?

   The two numbers are equal, so the atom carries no net electrical charge.

10. [Atoms, Molecules, and Chemical Bonds] Electrons are attracted to the protons in the nucleus. Why don't they fall into it?

    Electrons are by nature always in motion, so they move around the nucleus without ever reaching it.

11. [Atoms, Molecules, and Chemical Bonds] Protons repel each other electrically. Why does the nucleus of an atom not fly apart?

    The strong nuclear force, which binds protons and neutrons together. It is far stronger than the electrical repulsion between the protons.

12. [Atoms, Molecules, and Chemical Bonds] Which electrons of an atom largely determine how the element behaves chemically?

    The outermost electrons, which are held most weakly by the nucleus.

13. [Atoms, Molecules, and Chemical Bonds] How do metals behave towards their outermost electrons?

    They hold them weakly and readily give them up to elements that attract electrons more strongly. Example: copper, aluminum and iron give electrons to oxygen or chlorine.

14. [Atoms, Molecules, and Chemical Bonds] What is a chemical reaction, in terms of electrons?

    Atoms or molecules meeting and losing, gaining or sharing electrons. This changes the properties of the atoms or molecules taking part.Reactions are driven by the differences between elements in how strongly they attract electrons.

15. [Atoms, Molecules, and Chemical Bonds] What is oxidation, in the general chemical sense?

    The taking of electrons from other atoms, whatever atom takes them. Named after oxygen, the most important electron-taking element.Example: a chlorine atom taking an electron is also oxidation.

16. [Atoms, Molecules, and Chemical Bonds] Why does oxidation make fats, oils and aromas deteriorate in the kitchen?

    Oxygen takes electrons from their carbon chains, which starts a chain of reactions. The cascade breaks the large molecules into small, strong-smelling fragments.

17. [Atoms, Molecules, and Chemical Bonds] How do antioxidants protect fats from oxidation?

    They give oxygen electrons in place of the fat, without setting off further reactions. So the fat molecules are spared.Example: phenolic compounds in many plant foods.

18. [Atoms, Molecules, and Chemical Bonds] What is a chemical bond?

    An interaction that holds atoms or molecules together, loosely or tightly, briefly or indefinitely. Bonds arise from the differences between atoms in how strongly they attract electrons.

19. [Atoms, Molecules, and Chemical Bonds] What are the four kinds of chemical bond important in the kitchen?

    Ionic bond One atom completely takes one or more electrons from another; the two oppositely charged ions attract. Example: sodium and chloride in salt. Covalent bond Two atoms share electrons; this makes stable molecules. Example: the bonds inside water and sugar molecules. Hydrogen bond Attraction between oppositely charged ends of polar molecules; about 110\frac{1}{10}101​ as strong as a covalent bond. Example: the bonds between water molecules. Van der Waals bond Brief attraction that even n…

20. [Atoms, Molecules, and Chemical Bonds] What is an ionic bond?

    One atom takes electrons from another, and the two charged atoms attract. One atom ends up negative, the other positive.It occurs when the two atoms differ greatly in how strongly they attract electrons. Left: ionic bonds (one atom has taken the other's electron). Right: covalent bonds (the electron clouds overlap and electrons are shared).

21. [Atoms, Molecules, and Chemical Bonds] What is an ion?

    An atom that is electrically charged because it has gained extra electrons or lost some of its own. Michael Faraday coined the word, from the Greek for "going": ions move when an electric field is applied to a water solution.

22. [Atoms, Molecules, and Chemical Bonds] What happens to a compound held together by ionic bonds when it dissolves in water?

    It comes apart into separate ions. Example: table salt dissolves into positive sodium ions Na+\mathrm{Na^+}Na+ and negative chloride ions Cl−\mathrm{Cl^-}Cl− .

23. [Atoms, Molecules, and Chemical Bonds] Why can't one speak of individual molecules of salt in a salt crystal?

    Each sodium ion is bonded equally to several chloride ions, not to one partner. And each chloride to several sodiums, so there are no separate sodium-chlorine pairs.In the crystal, positive sodium ions and negative chloride ions alternate; the sodiums have given their electrons to the chlorines.

24. [Atoms, Molecules, and Chemical Bonds] What is a covalent bond?

    A bond in which two atoms share electrons. The word is from Latin for "of equal power".

25. [Atoms, Molecules, and Chemical Bonds] When do two atoms form a covalent bond rather than an ionic one?

    When they attract electrons about equally strongly. Then neither atom can take the other's electrons completely.

26. [Atoms, Molecules, and Chemical Bonds] Why do covalent bonds give a molecule a definite shape?

    Sharing electrons requires the atoms' electron clouds to overlap, which fixes their positions.

27. [Atoms, Molecules, and Chemical Bonds] Why does the shape of a molecule matter chemically?

    It determines how the molecule can react with other molecules. The shape follows from the fixed geometry of its covalent bonds.

28. [Atoms, Molecules, and Chemical Bonds] Which six elements most important to life tend to form covalent bonds?

    Hydrogen, oxygen, carbon, nitrogen, phosphorus, sulfur. Their covalent bonds make possible the large, stable structures of bodies and foods.Example: water is two hydrogens and one oxygen; table sugar (sucrose) is carbon, hydrogen and oxygen.

29. [Atoms, Molecules, and Chemical Bonds] Covalent bonds are strong and stable at room temperature. What breaks them in significant numbers?

    Heat, or reactive chemicals, including enzymes.

30. [Atoms, Molecules, and Chemical Bonds] How does a covalently bonded substance that dissolves in water (e.g. sugar) dissolve, compared with salt?

    As whole, electrically neutral molecules, whereas salt separates into charged ions.

31. [Atoms, Molecules, and Chemical Bonds] What is a polar molecule?

    A molecule with a positively charged end and a negatively charged end. The two poles are two separate centers of charge.

32. [Atoms, Molecules, and Chemical Bonds] Why is the water molecule H2O\mathrm{H_2O}H2​O polar?

    Oxygen pulls the shared electrons harder than hydrogen does. So the oxygen side is negative and the hydrogen side positive.The angle between the two covalent bonds keeps these charges on opposite sides.

33. [Atoms, Molecules, and Chemical Bonds] What is a hydrogen bond?

    The pull between the positive end of one polar molecule and the negative end of another. It also forms between polar regions of large molecules.About a tenth as strong as a covalent bond. It forms no molecules; it links molecules, or parts of one large molecule, temporarily.

34. [Atoms, Molecules, and Chemical Bonds] Why are hydrogen bonds so important in foods and living cells?

    They form wherever there is water, and break and re-form quickly at room temperature. They also hold different kinds of molecules close together.In both plant and animal cells, much of the chemistry takes place via hydrogen bonds.

35. [Atoms, Molecules, and Chemical Bonds] What are van der Waals bonds?

    Very weak attractions that even nonpolar molecules feel. They come from brief shifts in the positions of the molecules' electrons. 1100\frac{1}{100}1001​ to 110 000\frac{1}{10\,000}100001​ as strong as a covalent bond. Named after a Dutch chemist. Two carbon-hydrogen chains of fat molecules attracting each other (dotted lines).

36. [Atoms, Molecules, and Chemical Bonds] Which kind of bond holds the molecules of liquid water together?

    Hydrogen bonds, because water molecules are polar.

37. [Atoms, Molecules, and Chemical Bonds] Which kind of bond holds the molecules of a liquid oil together?

    Van der Waals bonds, because fat molecules are nonpolar. These bonds also give oils their thick consistency.

38. [Atoms, Molecules, and Chemical Bonds] Van der Waals bonds are very weak. Why do they still give oils a noticeably thick consistency?

    Each long fat molecule forms many such bonds with its neighbors, and they add up. Fat molecules are chains of dozens of carbon atoms.A small water molecule can interact with far fewer neighbors.

39. [Energy] In chemical terms, what does most cooking consist of?

    Breaking certain chemical bonds and forming others. McGee: "Cooking is applied chemistry."

40. [Energy] What is energy?

    The capacity for doing work: the property of a physical system that makes change possible. A system with little energy changes little; the more energy, the more likely it is to change or to change its surroundings.

41. [Energy] How do the stove and the refrigerator each use the link between energy and change?

    The stove adds heat to change food; the fridge removes heat to slow spoilage. Spoilage is a set of chemical reactions, and less energy means slower reactions.

42. [Energy] What are the two forms of energy that matter most in the kitchen?

    Kinetic energy The energy of motion of atoms and molecules; heat is this energy. Example: the molecules of a hot pan move faster than those of a cold one. Bond energy The energy associated with the chemical bonds that hold molecules together. Example: the covalent bonds of proteins, carbohydrates and fats.

43. [Energy] What is the relation between heat, temperature and molecular motion?

    Heat is the kinetic energy of a material's molecules; temperature is a measure of it. The higher the temperature, the faster the molecules move (from place to place, spinning, vibrating) and collide.

44. [Energy] Why does heating encourage chemical reactions?

    Faster motion and harder collisions overcome the electrical forces holding atoms together. Atoms freed this way can rearrange into new molecules.

45. [Energy] What is the strength of a chemical bond, by definition?

    The energy released when the atoms form the bond, which equals the energy needed to break it.

46. [Energy] Why do strong bonds "contain" less energy than weak bonds?

    A strong bond gives off more energy when it forms, so less is left in it. The released energy leaves the molecule as motion.So strong bonds are more stable, less easily changed, than weak bonds.

47. [Energy] Bond strength is the energy released when two atoms bond.When does heating start to break a bond?

    When the atoms move with as much kinetic energy as they released in forming the bond. Then the molecule begins to react and change.

48. [Energy] About how much energy is needed to break the covalent bonds of proteins, carbohydrates and fats, compared with the average kinetic energy of molecules at room temperature?

    About 100 times as much. So these bonds rarely break at room temperature, and the molecules change significantly only when heated.

49. [Energy] What happens to hydrogen bonds and van der Waals bonds at room temperature?

    They constantly break and re-form, and more so as the temperature rises.

50. [Energy] Why do fats melt and get thinner as they are heated?

    Faster-moving molecules increasingly break the weak bonds between them. These are van der Waals bonds.

51. [The Phases of Matter] What are the three phases of matter met in everyday life?

    Solid, liquid and gas. Crystalline solid (regular array), amorphous solid (random arrangement), liquid (loosely bonded, fluid), gas (dispersed, independent particles).

52. [The Phases of Matter] What determines the temperatures at which a material melts and boils?

    How strongly its molecules bond to each other: stronger bonds, higher temperatures. Stronger bonds need more energy to overcome.

53. [The Phases of Matter] What happens to the temperature of a material while it is changing phase?

    It stays roughly constant: all added heat goes into the phase change until it is complete. Example: a pot of boiling water on a high flame stays at the boiling point, 212°F/100°C at sea level, until all the water has turned to steam.

54. [The Phases of Matter] What characterizes the solid phase at the molecular level?

    The molecules are tightly bonded in fixed places, and can only rotate and vibrate. Ionic, hydrogen and van der Waals bonds can all hold a solid together.

55. [The Phases of Matter] What is the difference between a crystalline and an amorphous solid?

    Crystalline: the particles form a regular, repeating array. Amorphous: they are arranged randomly. Example: salt, sugar and tempered chocolate are crystalline; hard candies and glass are amorphous.

56. [The Phases of Matter] How are the molecules arranged in solid proteins and starch?

    In both crystalline (ordered) regions and amorphous (disordered) regions within the same piece. These molecules are large and irregular.

57. [The Phases of Matter] What happens at the molecular level when a solid melts into a liquid?

    The molecules break free of their fixed places, but stay loosely together. Rotation and vibration overcome the forces holding them in place.The molecules still move slowly enough to attract each other.McGee: "They're free to move, but move together."

58. [The Phases of Matter] What distinguishes a gas from a liquid at the molecular level?

    The molecules move fast enough to escape each other and move independently.

59. [The Phases of Matter] Why does water evaporate at room temperature, far below its boiling point?

    A few molecules are always moving fast enough to escape from the surface. The molecules of a liquid move at a wide range of speeds.

60. [The Phases of Matter] The direct change of a solid into a gas, without melting, is called sublimation.

    Example: ice in food slowly turning to vapor in the cold, dry air of a freezer causes freezer burn; freeze-drying uses the same process under control.

61. [The Phases of Matter] Why can't most food molecules simply melt and then boil when heated?

    Separating them takes as much energy as breaking them, so they react into other molecules instead. They are large and form very many weak bonds with each other, which add up to a strong hold.

62. [The Phases of Matter] What happens when each of these is heated: sugar, fats and oils, starch and proteins?

    Sugar Melts, then breaks apart into hundreds of new compounds instead of boiling. Example: this is caramelization. Fats and oils Melt, then break down and smoke before they boil. Starch and proteins Do not melt at all; they break down while still solid. Example: starch is a long chain of sugar molecules.

63. [The Phases of Matter] What is a solution?

    A liquid in which individual ions or molecules of another substance are dispersed. Example: salt brine, sugar syrup.

64. [The Phases of Matter] What is a suspension?

    A liquid in which a substance is dispersed as particles or clusters of many molecules. Example: nonfat milk, with particles of milk protein dispersed in water.

65. [The Phases of Matter] Why are suspensions usually cloudy while solutions are clear?

    Clusters of molecules are big enough to scatter light; single molecules are not.

66. [The Phases of Matter] What is an emulsion?

    A suspension of droplets of a liquid in another liquid with which it cannot mix evenly. Example: cream is butterfat dispersed in water; an oil-and-vinegar dressing is usually vinegar dispersed in oil.

67. [The Phases of Matter] What is a gel?

    Water trapped in a solid: a network of molecules holds pockets of water. Example: gelatin jellies (savory or sweet), fruit jellies set with pectin.

68. [The Phases of Matter] What is a foam?

    Gas bubbles dispersed in a liquid or a solid. Example: soufflés, bread, the head on a glass of beer.