SAT2生物考试知识点分析之atoms and Buffers.

　　SAT2生物考试是科学考试科目中中国考生选择比较少的一项，因为在词汇和知识点上，物理和化学都相对会轻松一些。对于参加这门考试的考生来说，下面澳际小编为大家整理的是关于知识点的介绍就弥足珍贵了。

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　　SAT2是SAT subject test的通常说法（学能倾向测验），SAT2考试有别于SAT1，它是一种专项考试，考察学生某一科目的能力，故又被称为SAT SUBJECT TESTS。

　　Sometimes atoms give their electrons up altogether instead of sharing them in a chemical bond. This process is known as disassociation. Water, for instance, dissociates by the following formula:

　　H₂OH⁺ + OH^–

　　The hydrogen atom gives up a negatively charged electron, gaining a positive charge, and the OH compound gains a negatively charged electron, taking on a negative charge. The H⁺ is known as a hydrogen ion and OH^– ion is known as a hydroxide ion.

　　The disassociation of water produces equal amounts of hydrogen and hydroxide ions. However, the disassociation of some compounds produces solutions with high proportions of either hydrogen or hydroxide ions. Solutions high in hydrogen ions are known as acids, while solutions high in hydroxide ions are known as bases. Both types of solution are extremely reactive—likely to form bonds—because they contain so many charged particles.

　　The technical dinition of an acid is that it is a hydrogen ion donor, or a proton donor, as hydrogen ions are consist of only a single proton. Acids put H⁺ ions into solution. The dinition of a base is a little more complicated: they are H⁺ ion or proton acceptors, which means that they remove H⁺ ions from solution. Some bases can directly produce OH^– ions that will take H⁺ out of solution. NaOH is an example of this type of base:

　　A second type of base can directly take H⁺ out of an H₂O solution. Ammonia (NH₃) is a common example of this sort of base:

　　NH₃ + H₂ONH₄⁺ + OH^–

　　From time to time, the SAT II Biology has been known to ask whether ammonia is a base.

　　The pH Scale

　　The pH scale, which ranges from 0 to 14, measures the degree to which a solution is acidic or basic. If the proportion of hydrogen ions in a solution is the same as the proportion of hydroxide ions or equivalent, the solution has a pH of 7, which is neutral. The most acidic solutions (those with a high proportion of H⁺) have pHs approaching 0, while the most basic solutions (those with a high proportion of OH^– or equivalent) have pHs closer to 14.

　　Water has a pH of 7 because it has equal proportions of H⁺ and OH^– ions. In contrast, when a compound called hydrogen fluoride (HF) disassociates, it forms only hydroxide ions. HF is therore quite acidic and has a pH well below 7. Some acids are more acidic than others because they put more H⁺ ions into solution. Stomach fluid, for example, is more acidic than saliva.

　　When sodium hydroxide (NaOH) disassociates, it forms only hydroxide ions, making it a base and giving it a pH above 7. Like acids, bases can be strong or weak depending on how many hydroxide ions they put in solution or how many hydrogen ions they take out of solution.

　　Buffers

　　Some substances resist changes in pH even when acids or bases are added to them. These substances are known as buffers. The cell contains many buffers because wide swings in pH can negatively impact the chemical reactions of cell processes.

　　以上就是关于SAT2物理考试知识点的介绍，主要把重点放在了两个问题上。大家在备考自己的SAT2生物考试的时候，可以根据自己的实际情况进行更加详细的备考，这样才能在备考中有更多更好的收获。