Which of these is a pure substance: sodium metal, solid sodium chloride, sodium chloride dissolved in water?

Pure substance is

a) sodium metal : it an element and it is pure substance

b) NaCl : it is an ionic compound and it is also a pure substance

c) NaCl dissolved in water : it is a homogeneous mixture of NaCl in water and hence it is not a pure substance

answer: option (4) is the correct answer.

explanation:

areas which are at low altitudes and near the equator tend to receive direct sunlight from the sun. hence, heat energy or radiations from the sun make these areas warmer and they have warmest water.

whereas areas which are at high altitudes and away from the equator do no receive direct sunlight. hence, no direct contact from the sun makes these regions colder and thus, they have colder water.

thus, we can conclude that the statement areas at low latitudes near the equator have the warmest water because they receive the most direct sunlight best explains the relationship between an area's geography and the temperature of its surface water.