Mercedes Paz, age 13, of Lombard, I11., for her question:

HOW MANY COUNTRIES ARE THERE IN THE WORLD?

China, with about 854 million citizens, is the largest country in the world in terms of population. Following in second place with a population of about 624 million is India with Russia and the United States coming up in third and fourth places with 259 million and about 220 million people. Next in line are Indonesia, Brazil and Japan.

There are 158 independent countries in the world, and about 55 other political units, chiefly territories and colonies. The number has grown steadily since the end of World War II as more colonies have become independent countries.

In area, the smallest country in the world is tiny Vatican City, located in the heart of Rome, Italy. It covers only 108.7 acres of land. Largest is Russia which has an area of 8,649,500 square miles.

There are eight huge independent countries in the world, each with more than 1 million square miles of land, and 60 large ones covering from 100,000 to 1 million square miles.

Following Russia in area, in order of size, are Canada, China, the United States, Brazil, Australia, India and Argentina.

The 12 smallest counties, following Vatican City, are Andorra, Bahrain, Barbados, Grenada, Liechtenstein, Maldives, Malta, Monaco, Nauru, San Marino, Seychelles and Singapore. Each of these countries has an area of fewer than 250 square miles.

During all of history, the political map of the world has changed almost constantly. Most of the major changes have resulted from great wars. In ancient times, conquerors such as Julius Caesar and Alexander the Great took over country after country. Then the ancient empires became weak and broke up into many countries again.

Many new countries were established after both World War I and World War II. But the largest change took place recently in Africa among the colonies of various European nations. An extremely strong movement for independence has swept the African continent since the 1950s. About 45 new independent nations have been established during the last 25 years.

After World War II, Russia and other Communist countries became known as the Communist World while the United States and other countries opposed to the spread of Communism became known as the Free World. Most of the new African countries, as well as some not committed to either of the two major Groups, are classed as nonaligned nations and are sometimes called the Third World.