The Handy Chemistry Answer Book. Justin P. Lomont

gives a food its taste?

      Molecules, of course! You probably learned in school that there are four basic tastes: sweet, bitter, sour, and salty. Your science textbook probably also had a diagram like the one on the following page, showing that your bitter tastebuds are located on the back of your tongue, and sweet is tasted on the tip. Wrong. Wrong. Wrong.

      The old notion that different regions of your tongue detect different tastes has been shown to be false. Instead, taste buds all over the tongue’s surface can detect all the basic flavors of sweetness, sourness, saltiness, and bitterness.

      Not only are there at least five basic tastes, but they’re basically spread out evenly across your tongue. The five basic tastes are the four you know from elementary school, and umami. If you grew up eating Western cuisine, umami tastes like MSG, or maybe you just think “Asian” food. There is debate as to whether there is a sixth basic taste bud that senses fat, or maybe another sensor for piquance (spiciness).

      But not even those five (or six, or seven) basic tastes fully account for all the sensations you get during eating. Wine is probably the best example (if you drink wine, of course). What makes wine taste “dry”? The “dry” taste certainly doesn’t fall into one of the five basic groups, but it is known to be related to the presence of tannins. So is there a tannin taste bud? No one knows yet.

      What makes something poisonous?

      There are many ways to seriously disrupt our biological machine. Carbon monoxide binds to hemoglobin and prevents oxygen from getting to our cells. Cyanide shuts down the production of ATP in mitochondria. Hemlock is a weed that contains a mixture of at least eight rather toxic molecules that target the nervous system. Thallium ions (Tl⁺) are particularly toxic because they are highly water soluble and once in the body they bind to ion channels and disrupt other processes that normally function with potassium ions (K⁺).

      Do pesticides make food more dangerous?

      Pesticides certainly don’t help make food safer, but their adverse long-term effects are difficult to measure. Limiting exposure as much as possible is certainly a good idea.

      What gives a substance its smell?

      Substances smell because your nose (or olfactory system) is able to detect the volatile molecules that are being released. Your nose has roughly 350 different receptors that detect molecules and then send a signal up through various parts of your olfactory system, ultimately ending up in your brain. A smell doesn’t come from a single receptor firing off a signal, but rather a whole array of receptors. Your brain takes the combination of signals it receives and translates that into the perception of the odor.

      Receptors in your nose detect molecules in the air, sending signals to your olfactory nerve, which then transmits the signals to your brain, which interprets them as smells.

      Why do some substances smell stronger than others?

      There are a couple of reasons that a substance might smell “stronger” than another. The first is volatility, or vapor pressure, of the molecule you smell. Simply, if there is more of it, it smells stronger. Additionally, some molecules interact more strongly with the receptors in your nose than others, which results in a stronger smell sensation.

      Which elements are liquids at room temperature?

      Actually there are only two elements that are liquids at room temperature: mercury and bromine. Bromine is a diatomic compound, Br₂, while mercury is a liquid metal.

      How dense is mercury compared to other liquids?

      The density of liquid mercury (it is a liquid at room temperature) is about 5.43 g/mL. This is about five and a half times as heavy as water! One of the next most dense liquids is bromine with a density of 3.03 g/mL, but mercury remains almost twice as dense.

      How does the density of snow compare with that of liquid water?

      The density of recently fallen, damp snow is approximately one-tenth that of liquid water. This means that one centimeter of rainfall and ten centimeters of snowfall contain approximately the same quantity of water molecules.

      What is a surfactant?

      Surfactants are molecules that reduce the surface tension of a liquid. These are typically amphiphilic organic molecules, which means that they have one hydrophobic and one hydrophilic section. This will cause them to align in a certain way at the surface of a liquid which interrupts the order of the liquid at the surface, lowering the surface tension.

      Mercury is the only metal that is a liquid at room temperature. It is also toxic and should not be handled with bare hands.

      What is precipitation?

      Precipitation is the formation of a solid in a solution. This happens when a substance has too low of a solubility to remain dissolved in the solution. This process can occur for a variety of reasons, such as when a new product of a chemical reaction has low solubility or when a temperature change takes place. Precipitation begins when a small crystal begins to form, which is part of a process called nucleation.

      What is the Curie point of a material?

      As the temperature of a ferromagnetic material is increased, it can eventually become paramagnetic (see “Atoms and Molecules” for a discussion of ferromagnetism and paramagnetism). The temperature above which it becomes paramagnetic is called the Curie point. This is actually also a phase transition, though it’s not one that’s easy to see with our eyes, such as when water freezes or evaporates.

      What is a physical change?

      A physical change is a change involving the macroscopic properties of a substance without any accompanying change in chemical composition. A few examples of physical changes would be evaporating, melting, cutting, slicing, breaking, and grinding. The key similarity is that none of these processes involve changing the chemical makeup of the substance.

      What is the Mohs scale of hardness?

      Substances, mostly minerals, are ranked by their ability to scratch one another. If one mineral can scratch another mineral it gets a higher ranking on the Mohs hardness scale. When Friedrich Mohs devised the scale, diamond was the hardness substance known, so it was given a value of 10. Talc (also known as talcum powder), being very soft, has a hardest rating of only 1. Mohs devised this scale to help sort out the private rock collection of an Austrian banker, and later the Archduke’s museum collection. There are more quantitatively accurate measurements of hardness available today, but the simplicity of the Mohs scale keeps it relevant and practical.

CHEMICAL REACTIONS

KINETICS AND THERMODYNAMICS

      What is a chemical reaction?

      A chemical reaction is any process that involves the transformation of one or more molecules into one or more molecules—most of the time the reactant(s) and product(s) will be different molecules. A chemical reaction almost always involves the breaking and/or formation of new chemical bonds.

      How do we write an equation for a chemical reaction?

      Chemists