It is difficult to evaluate the toxicity of substances what factors can vary how harmful a chemical is dose of exposure age and genetic makeup will affect response to a toxin.

The explanation is given below

Explanation:

Toxicity is a measure of how harmful a substance is on a living organism (causing illness, injury or even death). There are several factors that must be considered when evaluating toxicity:

- Dose of exposure: the amount of substance to which the organism has been exposed to;

- Frequency of exposure: how many times (and for how long) the individual has been exposed to the substance;

- Age and health condition of the individual exposed;

- Genetic makeup: the genetic backgroun of an organism will determine its response and degree of sensitivity to a given substance.

Moreover, there are five main characteristics of substances that determine its toxicity: Solubility (hidrophilic or lipophilic substances), persistence (for how long the substance remains the same and cause the same damage), bioaccumulation (when the substance is progresivelly incorporated in tissues), biomagnification (when the amount of substance rise along trophic levels) and other chemical interactions (different interactions with other chemicals can increase the degree of damage) .  

1. they vary with a higher number because boiling is way hotter than melting