Which of these is an indication that a chemical reaction has occurred?

a. freezing of a substance
b. melting of a substance
c. boiling of a substance
d. formation of a gas

The correct option is D.

A chemical change is a type of change in which a new substance is formed and which is always irreversible. Chemical changes involves the breaking and forming of bonds and the original composition of the substances involved are usually altered. The indicators that show that a chemical change has occur include: change in temperature, change in colour, formation of precipitate and formation of gas bubbles.

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.