Salt Water and Fresh Water on Earth - 5th Grade Science

SECTION 1

The Phenomenon: A Planet of Water — But Not Enough to Drink

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Anchoring Phenomenon

How is it possible that a planet practically covered in water could have a water-shortage problem? The answer lies in the difference between salt water and fresh water — and how much of each actually exists on Earth.

Earth from space — mostly covered in water

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Thinking Questions

If so much of Earth is covered in water, why do some places run short of drinking water?
What is different about the water in the ocean compared to the water in a lake or river?
How could you use data to figure out exactly how much of Earth's water people can use?

SECTION 2

What Scientists Know About Earth's Water

Earth has a lot of water, but not all water is the same. The two main types are salt water and fresh water. Understanding the difference — and the amounts of each — is a key part of studying Earth's systems.

Salt Water Dominates

About 97% of all water on Earth is salt water found in the oceans. Salt water contains dissolved minerals — mostly sodium chloride, the same substance in table salt. Humans cannot drink salt water because the high salt concentration would actually dehydrate our bodies instead of quenching our thirst.

Fresh Water Is Rare

Only about 3% of Earth's water is fresh water. Fresh water has a very low amount of dissolved salt, making it safe for humans, animals, and plants. It is found in glaciers, ice caps, groundwater, lakes, rivers, and the atmosphere.

Most Fresh Water Is Frozen

Here's the tricky part: about two-thirds of that 3% of fresh water is locked up in glaciers and polar ice caps. That water is frozen solid and not readily available for people to use. Only a small fraction exists as liquid fresh water in lakes, rivers, and underground.

Usable Water Is Tiny

When you add it all up, less than 1% of all of Earth's water is both fresh and liquid and accessible on or near the surface. That is the water available for drinking, farming, and everything else humans, animals, and plants need.

SECTION 3

Let's Investigate: Analyzing Earth's Water Data

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Investigation Spotlight

Your investigation: Below is a data table showing the distribution of Earth's water. Your task is to analyze the numbers, compare amounts, and use the data as evidence to explain why freshwater shortages can happen even though Earth has so much water overall.

What you need: The data table below, a calculator (optional), and your reasoning skills.

What to look for: Notice the scale differences between categories. Which numbers are enormous, and which are surprisingly small? How many times larger is one category compared to another?

Earth's Water Distribution Data

Water Source	Type	Percentage of Total Water	Volume (km³)
Oceans	Salt water	`96.5%`	`1,338,000,000`
Saline groundwater & salt lakes	Salt water	`0.9%`	`12,870,000`
Glaciers & ice caps	Fresh water (frozen)	`1.74%`	`24,064,000`
Fresh groundwater	Fresh water (liquid)	`0.76%`	`10,530,000`
Lakes (freshwater)	Fresh water (liquid)	`0.007%`	`91,000`
Rivers	Fresh water (liquid)	`0.0002%`	`2,120`
Atmosphere (water vapor)	Fresh water (gas)	`0.001%`	`12,900`

Source: USGS (U.S. Geological Survey), estimated global water distribution data.

SECTION 4

What the Data Tells Us

When you look at the data table and bar chart from Section 3, several important patterns jump out. The most striking discovery is just how uneven Earth's water distribution really is. Salt water in the oceans makes up 96.5% of all the water on the planet — and when you add saline (salty) groundwater and salt lakes, the total salt water comes to about 97.4%. That means almost all of Earth's water has too much dissolved salt for humans, most animals, or crops to use directly.

Fresh water accounts for only about 2.6% of all water. But even that small fraction is misleading, because most fresh water is not available in liquid form. Ice caps and glaciers — mainly in Antarctica and Greenland — store about 1.74% of total water as frozen ice. Fresh groundwater makes up 0.76%, and it is often deep underground, requiring wells and pumps to access. The water sources we are most familiar with — lakes and rivers — hold a remarkably tiny share: only about 0.007% (lakes) and 0.0002% (rivers).

To put these numbers in perspective, if you placed all of Earth's water in a large swimming pool, the amount of easily accessible fresh water (lakes, rivers, and shallow groundwater) would be roughly equivalent to a single cup scooped from that pool. The data makes clear why managing fresh water carefully is so important — there simply isn't very much of it compared to the vast oceans.

SECTION 5

Patterns and Connections: Scale, Proportion, and Quantity

One of the most important tools scientists use to make sense of the natural world is the crosscutting concept of Scale, Proportion, and Quantity. This means that understanding the size of things and how they compare to each other can reveal patterns that aren't obvious at first glance. In our water investigation, the raw numbers are so large that they're hard to picture — but when we convert them to percentages and proportions, the pattern becomes crystal clear.

This same crosscutting concept appears across many areas of science. Let's look at a few examples:

Science Example	What Seems True at First	What Proportional Data Reveals
Earth's Water (this lesson)	"Earth has lots of water, so water should be easy to find."	97.4% is salt water. Less than 1% is usable fresh water — proportions show the scarcity.
Earth's Atmosphere	"The atmosphere is a thick blanket around Earth."	If Earth were an apple, the atmosphere would be thinner than the apple's skin. Scale matters.
Solar System	"Planets are spread evenly through the solar system."	The Sun contains 99.8% of all mass in the solar system. The planets together are a tiny proportion.
Living Things in the Ocean	"Large animals like whales make up most ocean life."	Microscopic plankton make up the vast majority of ocean biomass. Size and quantity reveal the real picture.

SECTION 6

Real-World Connections: Why This Matters

Understanding the distribution of salt water and fresh water isn't just an interesting science fact — it directly affects communities around the world and drives real engineering solutions.

💧 Desalination Plants

Engineers have designed desalination plants — facilities that remove salt from ocean water to produce fresh water. Countries like Saudi Arabia, Israel, and Australia use desalination to supply drinking water. However, these plants require a lot of energy and are expensive to operate, so engineers continuously work to make them more efficient.

🌾 Farming and Irrigation

Agriculture uses about 70% of the world's fresh water. Because fresh water is so limited, agricultural engineers design drip irrigation systems, soil moisture sensors, and recycled-water techniques to grow more food with less water. These solutions are critical in dry regions like the American Southwest and sub-Saharan Africa.

🏔️ Glaciers and Climate

As global temperatures rise, glaciers and ice caps are melting faster. This might seem like it would create more fresh water, but much of that meltwater flows into the ocean and becomes salt water. Communities that depend on glacial meltwater for their rivers — such as those near the Himalayas — may face serious water shortages in the future.

🔧 Engineering Challenge

Design thinking: If you were an engineer asked to solve a water-shortage problem for a coastal city, what approach would you take? You might consider desalination, rainwater collection, water recycling, or reducing water waste. Each solution has trade-offs involving cost, energy use, and environmental impact. Engineers compare data about these trade-offs to choose the best option.

SECTION 7

Key Vocabulary Review

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Key Vocabulary

Salt water — Water that contains a high concentration of dissolved salts, primarily found in Earth's oceans. It makes up about 97% of all water on Earth and is not drinkable by humans.
Fresh water — Water with very low amounts of dissolved salt. It is found in glaciers, ice caps, groundwater, lakes, rivers, and the atmosphere. Only about 3% of Earth's water is fresh.
Glacier — A large, slow-moving mass of ice that forms on land over many years. Glaciers store a large portion of Earth's fresh water in frozen form.
Groundwater — Fresh water stored underground in spaces between rocks and soil. It is accessed by digging wells and is an important source of drinking water.
Distribution — How something is spread out or divided among different categories or locations. In this lesson, we studied the distribution of water across salt water and fresh water sources.
Proportion — The amount of something compared to the whole. For example, salt water makes up a large proportion (97%) of Earth's total water.
Desalination — The process of removing salt and other minerals from salt water to produce fresh water that can be used for drinking or agriculture.
Data — Facts, numbers, or measurements collected through observation or investigation. Scientists use data to identify patterns and draw evidence-based conclusions.

SECTION 8

Practice: Test Your Understanding

Question 1: Foundational

A scientist collects data about Earth's water. She finds that out of every 100 liters of water on Earth, about 97 liters are salt water. Based on this data, which statement best describes how Earth's water is distributed?

A. Most of Earth's water is fresh water found in rivers and lakes.
B. Salt water and fresh water exist in roughly equal amounts on Earth.
C. The vast majority of Earth's water is salt water, with only a small fraction being fresh water.
D. Most of Earth's water is frozen in glaciers and ice caps.

Show answer

C is correct. If 97 out of every 100 liters are salt water, then only about 3 liters are fresh water, which means the vast majority of Earth's water is salt water. A is wrong because rivers and lakes hold only a tiny fraction of Earth's water. B is wrong because the data clearly shows salt water far outnumbers fresh water (97 vs. 3). D is wrong because while glaciers do hold a large portion of fresh water, most of Earth's total water is salt water in the oceans, not frozen ice.

Question 2: Intermediate

A class is building a model to show Earth's water distribution. They have 50 blue marbles to represent all the water on Earth. If they want their model to be accurate, approximately how many marbles should represent salt water?

A. About 25 marbles
B. About 35 marbles
C. About 49 marbles
D. About 15 marbles

Show answer

C is correct. About 97% of Earth's water is salt water. 97% of 50 marbles is approximately 48.5, which rounds to about 49 marbles. A is wrong because 25 marbles would represent only 50%, suggesting equal amounts of salt and fresh water. B is wrong because 35 marbles would represent 70%, which is too low. D is wrong because 15 marbles would represent only 30%, greatly underestimating the amount of salt water.

Question 3: Intermediate

A student reads that about 3% of Earth's water is fresh water, but most of that fresh water is locked in glaciers and ice caps. Only a tiny portion is found in rivers, lakes, and underground sources. Which conclusion is best supported by this data?

A. People can easily access most of Earth's fresh water for drinking.
B. The amount of fresh water that people can easily use is a very small part of Earth's total water.
C. Glaciers provide most of the drinking water people use every day.
D. Rivers and lakes hold more water than the oceans do.

Show answer

B is correct. Since fresh water is only 3% of all water and most of that is frozen in glaciers, the water available in rivers, lakes, and underground sources is an extremely small fraction of Earth's total water. A is wrong because most fresh water is locked in ice and not easily accessible. C is wrong because people primarily get drinking water from rivers, lakes, and underground sources, not directly from glaciers. D is wrong because oceans hold about 97% of Earth's water, far more than rivers and lakes.

Question 4: Advanced

Two towns are studying their local water sources. Town A gets its water from a nearby freshwater lake. Town B is located on the coast and wants to use ocean water. A researcher notes that Town B would need to remove the salt from the ocean water before people could drink it. Why does this data about water types matter for the towns?

A. It shows that ocean water is cleaner than lake water.
B. It shows that even though salt water is abundant, it requires extra processing to become usable drinking water.
C. It proves that lakes always have more water available than oceans.
D. It means Town A will never have water supply problems because lakes never dry up.

Show answer

B is correct. Although salt water makes up about 97% of Earth's water, people cannot drink it without first removing the salt, which requires extra effort and resources. A is wrong because the issue is salt content, not cleanliness; ocean water is not necessarily cleaner. C is wrong because oceans hold vastly more water than lakes; the issue is that ocean water is salty. D is wrong because lakes can be affected by drought and overuse, so Town A could still face water supply challenges.

Question 5: Advanced

A group of students made a bar graph comparing where Earth's water is found. Their graph shows these approximate percentages: Oceans 97%, Glaciers and ice caps 2%, and Fresh water in rivers, lakes, and underground less than 1%. A student claims, "Since there is so much water on Earth, we will never run out of water to drink." How does the data from the graph help evaluate this claim?

A. The data supports the claim because 97% of Earth is covered in water we can drink.
B. The data supports the claim because glaciers can easily be melted for drinking water whenever needed.
C. The data challenges the claim because less than 1% of Earth's water is fresh water that people can readily access for drinking.
D. The data challenges the claim because most of Earth's water is found underground where nobody can reach it.

Show answer

C is correct. The graph shows that less than 1% of Earth's water is readily available fresh water in rivers, lakes, and underground sources. Even though Earth has a lot of total water, the drinkable portion is very small, which challenges the student's claim. A is wrong because the 97% is ocean salt water, which people cannot drink without processing. B is wrong because melting glaciers for drinking water is not easy or practical on a large scale, and glaciers are often in remote locations. D is wrong because while some water is underground, the main point is that most of Earth's water is salt water, not that underground water is unreachable—in fact, groundwater is one of the sources people do use.

SECTION 9

What's Next?

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What's Next?