Advanced Biology Matter and Energy Chemistry Molecules of Life

(1) A known amount of acid is pipetted into a conical flask and universal indicator added. (2-3) The acid is titrated with the alkali in the burette until the indicator turns green and the volume of alkali noted. (1-3) are repeated with both known volumes mixed together BUT without the contaminating indicator. (4) The solution is transferred to an evaporating dish and heated to partially evaporate the water. (5) The solution is left to cool to complete the crystallization. (6) The residual liquid can be decanted away and the crystals can be carefully collected and dried by 'dabbing' with a filter paper OR the crystals can be collected by filtration and dried.

(b) Reacting an acid with a metal or with an insoluble base (eg a metal oxide, hydroxide or carbonate, often of a transition metal like iron, zinc or copper). (1)The metal, oxide, hydroxide or carbonate is stirred with the acid and maybe heated to speed up the reaction (2). When no more of the solid dissolves the solution can be partly evaporated with further heating(2). The hot solution (with care!) is filtered (3), to remove the excess solid, into an evaporating dish and left to cool and crystallise (4). Then collect and dry.

pH scale

pH pH represents the effective concentration (activity) of hydrogen ions (H + ) in water. This concentration could be expressed in the same kind of units as other dissolved species, but H + concentrations are much smaller than other species in most waters.

pH and logarithmic units The activity of hydrogen ions can be expressed most conveniently in logarithmic units. pH is defined as the negative logarithm of the activity of H + ions: –pH = -log [H + ] –where [H + ] is the concentration of H + ions in moles per liter (a mole is a unit of measurement, equal to x atoms). Because H + ions associate with water molecules to form hydronium (H 3 O + ) ions, pH is often expressed in terms of the concentration of hydronium ions.

pH –Since pH is a log scale based on 10, the pH changes by 1 for every power of 10 change in [H + ]. –A solution of pH 3 has an H + concentration 10 times that of a solution of pH 4. The pH scale ranges from 0 to 14. –However, pH values less than 0 and greater than 14 have been observed in very rare concentrated solutions.

Pure water –In pure water at 22° C (72° F), H 3 O + and hydroxyl (OH - ) ions exist in equal quantities; the concentration of each is 1.0 x moles per liter (mol/L). –Therefore, pH of pure water = -log (1.0 x ) = -(-7.00) = Because pH is defined as –log [H + ], pH decreases as [H + ] increases (which will happen if acid is added to the water).

Dehydration synthesis Sucrose is common disaccharide which functions as a transport sugar in plants. The production of sucrose by means of a dehydration synthesis is shown here. Each sucrose molecule is made by chemically combining a glucose and a fructose molecule.

Enzyme action All proteins associated with the human body, or with any living system, do something. Some of them form structures, like fingernails; others, like hormones, participate in chemical reactions.

More on enzymes… Enzymes are proteins which stimulate chemical reactions between other proteins. They are present in every living thing, and may continue to function long after the organism is no longer technically alive.

tenderness of meat Beef, like fine wine, improves with age. The term aging simply means the length of time beef cuts are stored under controlled conditions of temperature and humidity before they are packaged for the meat counter. Aging allows naturally occurring enzymes within the meat to slowly break down some of the connective tissues that contribute to toughness. Beef aging significantly increases tenderness. Aging times vary considerably from 3 to 21 days. For the best eating quality, look for beef aged at least 10 to 14 days.