between / among

between -- used when two (and only two) things are being compared.
The mass difference between the two samples was 10%.

among -- used when more than two things are being compared.
The differences among all the populations were insignificant.

its / it's

its --possessive. The possessive forms of pronouns do not use an apostrophe (my, our, your, his, her, their, its). For obvious reasons, only this one tends to cause confusion. If the word cannot be replaced with "it is" (see following), use "its."
Its temperature was much higher after several minutes.

it's -- a contraction of the two words, "it is."
It's obvious why the temperature is higher.

since / because

since -- refers to some event taking place after some other event in the past. Implies that the situation was different before.
Since the water has been boiling, its temperature has been constant.

because -- used when one thing depends on another in a causal fashion. This word is commonly replaced with "since" in speech, but in formal writing, it is considered incorrect.
Because chlorophyll is green, it will absorb red light. (Chlorophyll was always green, therefore "since" would be incorrect.)

less / fewer

less -- used to describe a comparison between two things that differ in quantity, that cannot be counted, but are measured in bulk. The noun it modifies will usually be collective.
There was less sugar in the second sample than the first.

fewer -- used to describe a comparison between two things that differ in number. The objects can be counted, rather than measured in bulk. The noun it modifies will usually be plural.
There were fewer molecules being formed in the reaction.

their / there / they're

their -- possessive. Used when something belongs to something or someone else.
Their color was brighter after the polishing.

there -- denotes location or being.
There were fewer molecules there than before.

they're -- contraction of "they are."
They're better off for having studied all night.

bacterium / bacteria / bacterial

bacterium -- singular noun.
A bacterium is most sensitive to mutation while it is dividing.

bacteria -- plural noun.
There were fewer bacteria in the sample than expected.

bacterial -- adjective. Used to modify nouns.
The bacterial cell walls were compromised by the drug.

dilutions / dilution's

dilutions -- plural noun.
The dilutions were made according to the protocol.

dilution's -- possessive, but awkward. This form often appears as a computer-generated error. It seems that spell-checkers do not recognize the plural, but offer the possessive as a choice. Students accept the computer's offering.
awkward - no example.

data / datum

data -- plural noun.
These data are in support of the hypothesis.

datum -- singular noun. Rarely used.

affect / effect

affect -- verb. If the word is acting on another word, or can take "-ed" as an ending, use this word.
The incubation temperature affected the entire experiment.

effect -- noun. If the word is preceded by the word "an" or "the," then use this word.
The reagent had a dramatic effect on the reaction rate.

accept / except

accept -- used to show agreement with or adoption of a point of view.
Despite our inclinations to the contrary, their data forced us to accept their view.

except -- indicates difference from the group.
Every animal cooperated except the Great White.

medium / media

medium -- singluar noun
The bacteria were inoculated into minimal medium.

media -- plural noun
The yeast were grown in three different media to assess the effect of ionic strength.

site / cite / sight

site -- noun. a location
The restriction enzyme cuts a site very close to the gene.

cite -- verb. to give reference to some source in the literature.
We were able to cite seven papers to support our view.

sight -- noun. vision or view.
The end of the ordeal was in sight.

vial / vile

vial -- noun. a small tube or bottle for holding a sample. (spelled "phial" in the UK)
The vial contained some black gelatinous material.

vile -- adjective. disgusting, revolting.
The black gelatinous material had a vile odor.

number / amount

number -- used when a material or group of objects can be counted.
The number of samples made the experiment unwieldy.

amount -- used to describe bulk materials that cannot be counted.
The small amount of sample in the tube made it difficult to remove everything.

assume / conclude // assumption / conclusion

assume -- to take certain information as "given," not subject to testing in an experiment.
It was assumed that all bottles provided by the staff were one millimolar sodium chloride.
In our initial analysis, we made the assumption that we had pipetted all volumes correctly.

conclude -- to determine information by experiment and logic.
From the conductivity, it was concluded that the samples were all one millimolar sodium chloride.
From our results, we were forced to conclude that our pipetting technique was sloppy.

imply / infer

imply -- to communicate an idea without saying it directly.
The teacher tried to imply that studying would be important for the final exam.

infer -- to obtain information indirectly.
We inferred from her comments that we would need to study hard for the final.

strain / strand

strain -- A variety of bacteria or other microorganism that has a distinct, identifiable genotype.
The two strains of bacteria differed by only a single gene.

strand -- A single molecule of nucleic acid bound covalently by a single sugar-phosphate backbone. One half of the DNA double helix.
The sequence along one strand is complementary to that on the other strand.

sufficient / enough

These two words do not fall into the same either/or category as the words above. In fact, quite the opposite. There is no clear place where using one or the other sounds right or wrong. These two words have the same meaning, so using them together is redundant. In student papers they are commonly, but incorrectly, found together:

The amount of nitrogen in the medium was sufficient enough to support growth of the bacteria.

Dropping either word would be an improvement.

then / than

then -- Refers to something that takes place later in time.
The samples were then taken out of the incubator and stored in the refrigerator.

than -- Refers to a comparison. Used with words such as "more" or "less".
The samples were incubated longer than they should have been.