A solution is a mixture spread so evenly that it is uniform from end to end. Its parts are named by amount: the solvent is whichever part is present in the greater amount, the solutes are the smaller parts, and nearly any gas, liquid or solid is able to serve as the solvent. Sugar stirred into water spreads its molecules evenly between the water molecules, and gravity does not draw them back out over time. Dissolving sugar in coffee is a physical change, so it makes no new substance, while a chemical change does, and in neither case is any matter lost. Some pairs of liquids can be mixed in any proportion to make a solution, and chemists call such pairs miscible. Ethanol and ethylene glycol antifreeze are completely miscible with water. In plain terms, a solution looks and acts the same in every part, and the dissolved parts stay spread out. You can think of it as a drink or a mix where you cannot pick out the separate pieces, because they are all spread out together and look the same.
Solubility is the greatest quantity of solute a solvent can take up in given conditions. A solution holding that much is saturated, while one holding more is supersaturated. Gases usually dissolve less well as water warms, so bubbles of air collect in a chilled glass as it warms up. An open fizzy drink is supersaturated with carbon dioxide, and the extra gas leaves over time until the drink goes flat. Baking soda and vinegar also fizz, with carbon dioxide as the gas: an acid and baking soda react to make carbonic acid, and when that compound breaks down it gives off the gas. The same warming effect lies behind a major result of thermal pollution: warm water released into a river or lake can hold less dissolved oxygen, which can harm the water's ecosystems and even cause fish kills. So a drink left open slowly loses its fizz. A warm lake holds less oxygen for the fish that live in it. This is why a can of soda goes flat if you leave it open for a long time, and why a river that gets too warm can be bad for the fish that live in it.
The shape of the water molecule explains why salt dissolves: it carries a slightly positive charge on one side and a slightly negative charge on the other. The positive side of each water molecule is attracted to chloride ions and the negative side to sodium ions, so water pulls the ions apart, surrounds them and leaves a uniform solution. The same logic covers polar molecules in general: when they meet a polar solvent such as water, the dipole-dipole attractions between them are energetically favorable, so they dissolve. Oil is the contrast, and it floats on top of water, so shaken oil and water make two layers. Water molecules cling to one another, and oil molecules lack the polarity to pull them apart, so oil and vinegar settle into two separate layers. In short, water can pull salt apart, but it cannot pull oil apart. That is why salt dissolves and oil stays separate. You can see the difference when you shake oil and water, because the oil rises to the top and the water stays below it, while salt dissolves in the water.
Three attractions matter whenever something dissolves: solute to solute, solvent to solvent, and solute to solvent. Breaking the first two takes energy, and forming the third gives energy back. Some solutions form even though they take in heat. The solute needs more energy to pull apart than comes back when solvent surrounds it. The process still happens, because the mixture becomes more disordered. An instant cold pack shows the effect: when ammonium nitrate goes into water it takes in heat, so the pack gets cold, and chemists call such a step endothermic. An endothermic reaction is assigned a positive enthalpy change, and an exothermic one a negative change. Heating or cooling can therefore serve as a clue about what a mixture is doing, since reactions may release energy or absorb it. Put simply, a mixture can dissolve even when it gets colder, as long as it also gets messier. The reason a cold pack feels cold is that it takes in heat as the salt dissolves, and the same idea shows up whenever something dissolves and takes heat with it.
An emulsion forms when liquids that will not dissolve in each other are blended, leaving tiny drops of one spread through the other. Milk, mayonnaise and many cosmetics belong to this group. Left alone, an emulsion tends to separate as the dispersed drops merge, which is why an emulsifying agent is usually added. Surfactants play that part: each molecule has a polar part and a nonpolar part, and the molecule bridges the water-based and oil-based liquids. In mayonnaise the agent is lecithin from egg yolk, with one end attracted to water and one to oil. In an emulsion, a phospholipid such as lecithin surrounds the droplets and forms a barrier between water and oil. Soap molecules are built with one end that loves water and one that loves oil. So an emulsifier is a go-between. It sits between the oil and the water and helps them stay together. Think of the go-between as something that has a friendly end for the water and a friendly end for the oil, so that both of them are happy to stay in the same mix.
For two substances to react, their particles must collide in the right orientation and with enough energy. The contact has to line up the atoms that will bond, and the energy must be enough for the electrons to rearrange into new bonds. The smallest energy a collision needs in order to make products is called the activation energy. If it is much bigger than the average energy of the moving molecules, the reaction is slow, because only a few fast molecules have enough. Colliding molecules that qualify briefly form an unstable in-between arrangement called a transition state, which lasts for a very short time, and most instruments cannot detect it. Temperature is a measure of the average kinetic energy of a sample's atoms or molecules, and those energies are spread out among the molecules. Warming a sample puts more molecules above the activation energy. That is why most reactions go faster at higher temperature. Put plainly, a collision has to hit the right way and hit hard enough. If it does not, no new substance forms.
Chemists express speed as a rate: how much reactant is used up, or how much product is made, in a given amount of time, so a faster reaction uses up reactant, or makes product, in less time. Rates usually go up when a reactant becomes more concentrated, and a solid in fine pieces outpaces one large lump, because more of its surface meets the other substance. The nature of the substances matters too, so reactions that look alike can have different speeds. Some reactions are reversible, meaning they can run both forwards and backwards. A system at equilibrium is in a dynamic balance, not a standstill: the forward and reverse reactions keep going at equal rates, and each substance's amount holds steady. If more reactant is added, the forward rate briefly beats the reverse rate, and the system makes a temporary net shift toward more product until the balance returns. This is Le Chatelier's principle. Put simply, a rate is how much changes in a set time. Several things can make it bigger or smaller.
A catalyst makes reactions faster and is not used up by them. A homogeneous catalyst is in the same phase as the reactants. It reacts with a reactant to form an in-between substance, which goes on to make the product and gives back the original catalyst. A heterogeneous catalyst is in a different phase, usually a solid, and works by giving the reaction an active surface to happen on. Car catalytic converters use a blend of catalytically active metals to push the carbon-containing compounds in exhaust toward complete burning, and they were developed to cut the toxic emissions made by burning gasoline in engines. Ozone high in the atmosphere can break down on its own, but a number of substances catalyze that break-down. Nitric oxide is used up in one step and made again in the last, which is the mark of a catalyst. In everyday terms, a catalyst is a helper that is not used up. The same catalyst can help again and again. Because the helper is not used up, it can take part in the reaction again and again, and that is the mark of a catalyst.
Living things use enzymes as catalysts, and an enzyme can multiply a reaction's speed by a factor ranging from 100 million to 10 billion. The spot where a substrate binds is the enzyme's active site, and because enzymes are proteins, that site has its own unique mix of amino acid side chains. Scientists once pictured enzyme and substrate fitting like a lock and key in one step, but current research supports a more flexible picture called induced fit, in which the enzyme's shape shifts a little into the best binding arrangement as they come together. Enzymes work best in a certain range of temperature and acidity, and the active site is where the action happens. A little warmth usually speeds reactions up. Too much heat or an extreme pH changes the enzyme's shape so it no longer works well, which is called denaturing. Because enzymes are proteins, shape matters to them, and that is why denaturing stops them working well. In short, enzymes are picky about their conditions. Too much heat can ruin them.
The word oxidation first meant combining with oxygen, but chemists widened it to mean any reaction in which atoms lose electrons. Combustion is a kind of oxidation, though the widened meaning no longer needs oxygen. Combustion, the reaction behind a flame, usually involves oxygen and gives off heat and light, and it speeds itself up, because it makes more heat than can escape, so the temperature of the burning material rises. A flame needs heat to keep going, so a burning match goes out if wind blows the heat away, which means a flame depends both on making heat and on keeping it. Iron rusts when it is exposed to oxygen and water, through a galvanic cell at the iron surface, and a more readily oxidized metal such as zinc can protect iron by corroding first. Put simply, a flame is a reaction that makes its own heat, and it needs that heat to go on. Rust is also a reaction with oxygen and water. A match needs heat to keep burning, and the burning itself makes more heat, which is why the flame can keep going on its own.
The pH of a solution is the negative base-10 logarithm of its hydronium ion concentration, so one step of pH is a tenfold change in concentration, and it is hydronium ions that the definition counts. A neutral solution holds equal amounts of hydronium and hydroxide ions, an acidic one holds more hydronium, and a basic one holds fewer. An acid-base indicator is a dye that changes color when the acidity of a solution reaches a certain level. Phenolphthalein, for example, is colorless in solutions below about pH 8.3 and pink above it, a change that marks the point where acidity crosses that level, and indicators are themselves weak organic acids or bases. These ideas reach from the laboratory bench to the sea. Ocean acidification is a slow change in seawater chemistry: the ocean takes up carbon dioxide from the air, a series of reactions raises the concentration of hydrogen ions, and the seawater becomes more acidic with fewer carbonate ions, the building blocks of shells and coral. Put simply, pH tells how acidic or basic a water-based mixture is. A dye can show where that point is.
Fermentation today means the enzyme-catalyzed way cells break down molecules such as glucose without oxygen while capturing energy. So the word now covers more than its older yeast meaning. It is not limited to yeast: the same early steps happen in muscle, some bacteria and plants. Louis Pasteur used the word in the 19th century for changes made by yeasts and other microorganisms growing without air, and he saw that alcohol and carbon dioxide are not its only products. The foam on fermenting wine and beer comes from carbon dioxide gas. People have made these drinks for at least 10,000 years, but nobody recognized that the foaming was a gas until the 17th century. Bread dough rises as its yeast ferments and the gas it releases swells the dough's tiny air bubbles. Put simply, fermentation is a way for cells to get energy without oxygen. Yeast is only one of the living things that do it. The same gas that is made this way makes bread dough rise and makes wine and beer foam.
No matter is lost when things mix or react, and the overall total stays constant. In a chemical change, atoms are rearranged into new substances, and none are created or destroyed. Careful weighing by Antoine Lavoisier (1743-1794) showed that the mass leaving a reaction equals the mass that entered it. Metals heated in closed vessels, all weighed with everything else, gained mass equal to the air's loss, so the gain and the loss were equal and the totals matched. In brewing, grains, malt, hops, sugar, yeast and water become beer with no actual loss of substance, since conservation of matter is a law that holds the total amount of matter constant. Balanced chemical equations show matching atom counts for each element before and after, which agrees with conservation of matter. Put simply, nothing disappears in a reaction. The atoms only change partners. Weighing is how Lavoisier showed it. So when you add up everything that goes into a reaction and everything that comes out, the two totals are the same, and that is what the balance showed.
Words to know
- solvent: dissolving liquid
- miscible: fully mixable
- emulsion: droplets in liquid
- equilibrium: steady balance
- denaturing: shape loss
- catalyst: speeds up reactions
