AP Environmental Science

## Learning Outcomes

- Understand what ethanol is and why it is considered a renewable biofuel
- Distinguish between ethanol, gasohol, and common fuel blends such as E10, E15, and E85
- Explain how ethanol is produced from biomass using fermentation and distillation
- Compare the advantages and disadvantages of ethanol as a substitute for gasoline
- Calculate why higher ethanol blends usually reduce fuel economy
- Evaluate ethanol using lifecycle greenhouse gas emissions, land use, and food-versus-fuel reasoning

## AP Environmental Science Syllabus

For the AP Environmental Science, you are expected to understand ethanol as a biomass-based transportation fuel, with a focus on the following syllabus points:

- Know that ethanol is an alcohol biofuel produced from biomass such as corn, sugarcane, or other plant material
- Identify gasohol as gasoline mixed with ethanol, and distinguish among E10, E15, and E85 fuel blends
- Explain why ethanol is considered renewable but is not impact-free or carbon-free
- Understand that ethanol has lower energy content per gallon than gasoline, so fuel economy usually falls as ethanol percentage rises
- Explain how ethanol combustion and production affect air pollution, greenhouse gases, and resource use
- Evaluate the food-versus-fuel trade-off, including land use, fertilizer runoff, irrigation demand, and monoculture impacts
- Distinguish tailpipe emissions from lifecycle greenhouse gas emissions
- Recognise that vehicle compatibility matters, especially that E85 is for flex-fuel vehicles

## Test Your Knowledge

_Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, look more closely at that area during your revision._

1. What is the difference between ethanol, gasohol, and E85?
2. Why do vehicles usually get fewer miles per gallon when ethanol content in fuel increases?
3. How can ethanol reduce greenhouse gas emissions compared with gasoline, yet still create environmental problems?
4. What is meant by the food-versus-fuel trade-off?
5. Why is lifecycle analysis more useful than looking only at tailpipe emissions?

## Introduction

Ethanol is one of the main biofuels students need to know for AP Environmental Science. It appears at the meeting point of Unit 6 energy resources, Unit 5 agriculture, Unit 7 air pollution, and Unit 9 climate change. That makes it a classic exam topic because it is not enough to define it. You also need to explain how it is produced, why it is considered renewable, and why that does not automatically make it sustainable.

On the exam, ethanol is often tested through trade-offs. It can reduce dependence on petroleum and may lower some emissions compared with conventional gasoline. At the same time, it can require large amounts of cropland, fertilizer, water, and energy to produce. Strong answers show both sides and then explain what conditions affect the final judgement.

> **Key Term: ethanol**
> An alcohol biofuel produced from biomass, usually plant material such as corn or sugarcane, and commonly blended with gasoline for transport use.

## Introduction

> **Key Term: biofuel**
> A liquid fuel produced from living or recently living biological material.

### What ethanol fuel is and how the blends work

Ethanol is usually not used by itself in ordinary transport systems. Instead, it is blended with gasoline. This matters because exam questions may use several similar terms that students mix up.

**Gasohol** is gasoline extended with ethanol. In older textbook language, this often means about 90% gasoline and 10% ethanol. In modern blend labels, the number after the E shows the approximate percentage of ethanol by volume.

> **Key Term: gasohol**
> Gasoline blended with ethanol, commonly around 10% ethanol and 90% gasoline.

The main blends you should know are:

- **E10** = 10% ethanol, 90% gasoline
- **E15** = 15% ethanol, 85% gasoline
- **E85** = a high-ethanol blend used in flex-fuel vehicles

E10 is the most common U.S. blend and is used because it is an easy way to mix a renewable fuel into the existing gasoline supply. E15 contains more ethanol, so it can displace a little more petroleum, but it is not approved for every engine. E85 contains much more ethanol, but it is not for ordinary vehicles.

> **Key Term: flex-fuel vehicle (FFV)**
> A vehicle designed to run on gasoline or on high-ethanol blends such as E85.

This vehicle issue matters because AP questions often ask students to evaluate feasibility, not just chemistry. A fuel can sound environmentally useful, but if most vehicles cannot use it, expansion is limited.

Another reason ethanol is blended into gasoline is that it has a high octane rating. That means it helps fuel resist engine knocking. This is useful for engine performance, but it does not mean ethanol is automatically the better environmental option. Students often stop at “higher octane” and forget to discuss energy yield and agricultural impacts.

> **Test Tip:** If a question asks for a distinction, do not just list E10, E15, and E85. State what the numbers mean and mention that E85 requires flex-fuel vehicles.

### How ethanol is produced from biomass

Ethanol is made by converting plant material into alcohol. The exact steps depend on the feedstock, but the general AP-level process is straightforward.

First, the feedstock is prepared. If the source is **corn**, the starch in the kernels must be broken into simple sugars. If the source is **sugarcane**, the process is shorter because the plant already contains fermentable sugars. If the source is cellulosic biomass such as grasses or crop residues, the material must be treated more heavily because cellulose is hard to break down.

Next comes **fermentation**. Microorganisms such as yeast convert sugars into ethanol and carbon dioxide.

> **Key Term: fermentation**
> A biological process in which microorganisms convert sugars into ethanol and carbon dioxide.

After fermentation, the mixture still contains water and other substances, so the ethanol must be separated. This is done by **distillation**, which uses different boiling points to concentrate the ethanol. Further drying produces fuel-grade ethanol, which is then blended with gasoline.

> **Key Term: distillation**
> A separation process that heats a liquid mixture so one component vaporizes and can be collected and condensed.

This process shows why ethanol is not impact-free. Students sometimes hear “plants absorb carbon dioxide while growing” and jump to “ethanol is carbon neutral.” That is too simple. Ethanol production also involves tractors, fertilizers, processing plants, transport, and blending infrastructure. All of those can use energy and create pollution.

The feedstock source matters a lot:

- **Corn ethanol** is common in the United States
- **Sugarcane ethanol** is important in Brazil
- **Cellulosic ethanol** has strong long-term potential because it can use nonfood biomass, but it is harder and often more expensive to produce

From an AP perspective, corn ethanol is the main example because it shows the strongest trade-offs between renewable energy and agriculture.

### Why ethanol can help and why it also causes problems

Ethanol has several real advantages. First, it is **renewable on human timescales** if the feedstock crops are replanted. Gasoline comes from fossil carbon stored for millions of years, while ethanol comes from recently grown biomass.

Second, ethanol can reduce dependence on petroleum. That matters for energy security as well as for resource conservation. A country using more ethanol may need less gasoline from crude oil.

Third, ethanol can offer lifecycle greenhouse gas benefits compared with gasoline. When ethanol burns, it releases carbon dioxide, but much of that carbon was recently taken from the atmosphere by the growing crop. This does not make ethanol zero-emission, but it can reduce total emissions compared with fossil fuels.

> **Key Term: lifecycle greenhouse gas emissions**
> Total greenhouse gas emissions from all stages of a fuel’s production and use, including feedstock growth, processing, transport, distribution, and combustion.

Some analyses under U.S. policy frameworks use a threshold of about **20% lifecycle greenhouse gas reduction** for conventional renewable fuels such as many corn-ethanol pathways, relative to baseline gasoline. The exact outcome depends on farming methods, refinery energy sources, and land-use assumptions.

Ethanol can also lower some pollutants from combustion because it is an oxygenated fuel and can burn more completely. But this must be handled carefully in exam answers, because not all air pollution effects improve.

The disadvantages are just as important.

The first major drawback is **lower energy content per gallon**. Ethanol contains less energy than gasoline, so vehicles burn through it faster to go the same distance.

A useful comparison is:

- Gasoline ≈ **114,000 BTU per gallon**
- Ethanol ≈ **76,000 BTU per gallon**

That means ethanol has only about two-thirds of the energy of gasoline.

Worked example:

Suppose a car gets **30 miles per gallon** on gasoline. If the fuel used had about **27% less energy per gallon**, like some E85 blends, fuel economy would also drop by about 27% under similar conditions.

Step 1: Find 27% of 30 mpg  
27% of 30 = 0.27 × 30 = **8.1 mpg**

Step 2: Subtract from original fuel economy  
30 − 8.1 = **21.9 mpg**

So the car would get about **21.9 miles per gallon**.

This is why “renewable” does not always mean “efficient.”

> **Key Term: energy content per gallon**
> The amount of usable energy contained in one gallon of fuel.
>
> **Test Tip:** If asked why higher ethanol blends reduce miles per gallon, do not say “because ethanol burns faster.” The stronger answer is that ethanol has lower energy content per gallon.

The second major drawback is the **food-versus-fuel trade-off**.

> **Key Term: food-versus-fuel trade-off**
> The conflict that occurs when cropland, water, and agricultural inputs are used to grow fuel crops instead of food or other needed crops.

If corn is diverted to ethanol production, that can raise demand for corn and increase pressure on cropland. Even if total food shortages do not result directly, the system can still shift land use and input use in ways that matter environmentally.

Third, ethanol production can intensify agricultural harms:

- more **fertilizer** use can increase nitrogen and phosphorus runoff
- more **pesticide** use can affect natural habitatss
- more **irrigation** can strain freshwater supplies
- more **monoculture corn** can lower biodiversity
- more land conversion can destroy habitat and release stored carbon

These links matter across APES units. Fertilizer runoff can drive **eutrophication**, especially in downstream waters. Corn expansion can raise erosion risk and degrade soils if management is poor. Increased water demand can stress aquifers and rivers.

A fourth drawback is that ethanol is not automatically cleaner for all air pollutants. While it may reduce some emissions, it can also increase evaporative volatile organic compounds under some conditions and can contribute to ozone problems in warm weather. So a complete answer should avoid claiming “ethanol reduces pollution” without qualification.

### How to evaluate ethanol in AP Environmental Science answers

The strongest AP answers do not treat ethanol as simply good or bad. They compare it with gasoline and explain the conditions under which it is more or less sustainable.

A high-quality response usually works through this logic:

1. **Ethanol is renewable** because the feedstock can be regrown.
2. **It can reduce petroleum use** and may lower lifecycle greenhouse gas emissions.
3. **But** it still releases carbon dioxide when burned.
4. **And** the production system may depend on fossil-fuel inputs, fertilizers, irrigation, and transport.
5. Therefore, the net benefit depends on the full lifecycle and the agricultural context.

This is where lifecycle analysis matters more than tailpipe analysis. Tailpipe emissions only tell you what comes out of the vehicle. Lifecycle emissions include growing the corn or sugarcane, making fertilizer, transporting biomass, operating the refinery, distributing the fuel, and then burning it.

Students also need to separate **vehicle compatibility** from **environmental quality**. E85 may displace more gasoline than E10, but only if flex-fuel vehicles are available. E15 may offer some benefits, but not every engine can use it. So scale-up depends on infrastructure, market conditions, and technology, not just chemistry.

A balanced judgement would be:

Ethanol is a useful **partial substitute** for gasoline, especially as a renewable blended fuel. However, its lower energy yield and crop-based production impacts limit how sustainable it is, especially when made from heavily fertilized, irrigated monoculture crops.

That kind of answer scores better than a one-sided claim.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- Ethanol is an alcohol biofuel made from biomass such as corn, sugarcane, or other plant material
- Gasohol is gasoline blended with ethanol, and common blends include E10, E15, and E85
- E85 is a high-ethanol blend intended for flex-fuel vehicles
- Ethanol is produced by preparing biomass, fermenting sugars, and then distilling the ethanol
- Ethanol is renewable on human timescales if feedstock crops are replanted
- Ethanol can reduce petroleum dependence and may lower lifecycle greenhouse gas emissions relative to gasoline
- Ethanol still releases carbon dioxide during combustion, so it is not carbon-free
- Ethanol has lower energy content per gallon than gasoline, so higher ethanol blends usually lower fuel economy
- The food-versus-fuel trade-off means fuel crop production can compete with food production and land use
- Corn ethanol production can increase fertilizer runoff, irrigation demand, monoculture, and habitat loss
- Lifecycle emissions are more useful than tailpipe emissions for evaluating ethanol’s climate impact
- Ethanol is best understood as a partial gasoline substitute with significant environmental trade-offs

## Key Terms and Concepts

- ethanol
- biofuel
- gasohol
- flex-fuel vehicle (FFV)
- fermentation
- distillation
- lifecycle greenhouse gas emissions
- energy content per gallon
- food-versus-fuel trade-off


Energy from Biomass - Ethanol as fuel substitute