Which of the following is true about melting point determination?
a. at the melting point of a substance, the solid and vapor states exist in equilibrium.
b. an impure compound has a narrow, sharp melting point range.
c. a solid is said to sublime when it first liquefies and then vaporizes to a gas.
d. at the eutectic point, the melting point of a mixture of two compounds is sharp.
e. although a substance may exist in different crystalline forms, it has only one characteristic melting
point.

explanation: it gains negative electrons, therefore becoming negatively charged. as charge cannot be created, nor destroyed, only transferred, this leaves the wool positive. now that they have opposite charges, the two items attract.

acid rain does not usually kill trees directly. instead, it is more likely to weaken the trees by damaging their leaves, limiting the nutrients available to them, or poisoning them with toxic substances slowly released from the soil. the main atmospheric pollutants that affect trees are nitrates and sulphates.