Answer:
The liquid collected during distillation when the evaporated substance condenses. A separation technique that uses evaporation to separate substances. The mixture is heated so that one substance evaporates. The vapour is collected and condenses into a liquid.
Fossils are fundamental to the geologic time scale. The names of most of the eons and eras end in zoic, because these time intervals are often recognized on the basis of animal life. Rocks formed during the Proterozoic Eon may have fossils of relative simple organisms, such as bacteria, algae, and wormlike animals
The difference in the concentration of a substance between two areas is called the concentration gradient. When a region has a higher particle concentration than another, this is known as a concentration gradient. A concentration gradient will cause particles in passive transport to diffuse down it from higher concentration to lower concentration until they are evenly spaced.
The gradual separation of a region of high density from a region of low density in a solution in terms of the concentration of a dissolved material. Understanding how ions and particles flow randomly in a solution or gas depends on the concentration gradient.
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Answer: 17) d. 
18. c. The empirical formula of a compound can be twice the molecular formula.
Explanation:
Molecular formula is the chemical formula which depicts the actual number of atoms of each element present in the compound.
Empirical formula is the simplest chemical formula which depicts the whole number of atoms of each element present in the compound.
To calculate the molecular formula, we need to find the valency which is multiplied by each element to get the molecular formula.
The equation used to calculate the valency is:
The empirical mass can be calculated from empirical formula and molar mass must be known.
17. Thus the empirical formula of should be 
18. The molecular formula will either be same as empirical formula or is a whole number multiple of empirical formula. Thus the empirical formula of a compound can never be twice the molecular formula.
