Which statement is true about the elements present in a pure substance? a they maintain their chemical properties. b they are composed of many compounds. c they can be separated by physical methods. d they can only be separated by chemical methods.

Option (D) is the correct answer.

Explanation:

An element is defined as a substance that contains atoms of one type.

For example, a piece of sodium metal will contain only atoms of sodium.

Therefore, it is a pure substance. Whereas a compound is a substance that consists of two or more different atoms in similar composition.

For example, is a compound.

When elements change into compounds then they tend to change their chemical properties.

Hence, we can conclude that the statement they can only be separated by chemical methods, is true about the elements present in a pure substance.

1. the question asks the most appropriate, which seems to rule out "all of the above." your choice appears to be the correct one.

2. the protocol described is good for identifying correlation, but not cause and effect. for cause/effect, you need to devise a protocol that will assign the treatment (or not) to groups that have essentially the same "demographics" with respect to all the other variables. your choice regarding control group seems appropriate. random assignment of treatment (choice d) may be appropriate also. (i think you can argue that b and d are just different descriptions of the same method.)

3. d will randomize the birds, but may not guarantee there are 10 in each group. e will do both, so seems the better choice.

comment on question 2

based on the problem statement, it seems clear that confident, attentive drivers are more likely to use daytime running lights. so, the protocol might need to be devised in such a way that the lights would be turned on or off without the driver's knowledge. (an attentive driver will figure it out.) assigning a confident, attentive driver to a group required to drive without running lights might mess with his/her confidence and attentiveness. you don't want your experiment to do that. (nor do you want the experiment to subject a driver to greater risk of accident as a consequence of driving without running lights.) experiments involving human safety are always problematic.