Which statement about a chemical reaction is true? it occurs only at low temperatures. it occurs only at high temperatures. it occurs whenever a physical change takes place. it occurs whenever a chemical change takes place.

Option (d) is the correct answer.

Explanation:

A reaction which causes a change in the rearrangement of atoms or chemical composition of a substance is known as a chemical change.

A chemical change can take place rapidly, slowly or too slowly.

For example, rusting of iron is a slow chemical change.

Therefore, the statement which is true about a chemical reaction is that it occurs whenever a chemical change takes place.

The correct statement is it occurs whenever a chemical change takes place.

Explanation:

A physical reaction is defined as the reaction in which a change in shape, size takes place. No new substance gets formed in these reactions.

A chemical reaction is defined as the reaction in which a change in chemical composition takes place. A new substance is formed in these reactions.

These reactions can happen at any temperature. Like in cold conditions, Azo dye is prepared and at high temperatures, dissociation of water takes place.

Thus, a chemical reaction occurs only when a chemical change takes place in a substance.

Hence, the correct statement is it occurs whenever a chemical change takes place.

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.