Question 7: the correct answer is About 70%. Got it right? Award yourself 1 point.
Question 8 of 10
Which gas is the most abundant in the air we breathe?
Why, in question 7: How much of Earth's surface is covered by water?
The answer is about 70%, more precisely close to 71% of the surface. That is why Earth appears blue in photos taken from space.
Plenty of water, little to drink
About 97% of the planet's water is in the oceans and is salty. Of the small fresh portion, most is frozen in the polar ice caps and glaciers, or hidden underground.
The water in rivers and lakes, the easiest to use, is a small fraction of the planet's fresh water.
Brazil has one of the largest freshwater reserves in the world, with rivers such as the Amazon and the São Francisco and large underground reservoirs, such as the Guarani Aquifer, which also extends into Argentina, Paraguay and Uruguay.
The planet's water circulates nonstop: it evaporates from the oceans, forms clouds, falls as rain and returns to the seas through rivers. A water molecule can spend thousands of years in the ocean before evaporating.
