Why Do Onions Make You Cry? The Chemistry Behind Kitchen Science

Why do onions make you cry when vegetables such as potatoes and carrots do not?

The answer lies in onion chemistry. Cutting an onion breaks open its cells and starts a series of chemical reactions. These reactions produce a volatile substance that travels through the air, irritates your eyes and causes tears.

Understanding the chemistry behind onion tears is an excellent way for students to see how concepts such as plant cells, enzymes, chemical reactions and the human nervous system apply to everyday life.

Why Do Onions Make You Cry When You Cut Them?

An onion is made of millions of plant cells. Inside these cells are enzymes and sulfur-containing compounds. While the onion remains whole, these substances are stored separately and cannot react with each other.

When you slice, chop or crush an onion, the knife breaks open its cells. This allows the enzymes and sulfur-containing compounds to mix.

The process happens in several stages:

• Cutting damages the onion’s plant cells.
• An enzyme called alliinase comes into contact with sulfur-containing compounds.
• The enzyme converts these compounds into unstable sulfenic acids.
• Another enzyme, called lachrymatory factor synthase, transforms one of the intermediate compounds.
• A volatile, tear-producing chemical is released into the air.

This chemical reaction explains why a whole onion does not usually irritate your eyes, but a freshly cut onion does.

What Chemical in Onions Makes You Cry?

The main tear-producing chemical is called propanethial S-oxide.

It is also known as the onion’s lachrymatory factor. The word “lachrymatory” refers to something that causes tears.

Propanethial S-oxide is volatile, which means it can quickly turn into vapour and move through the surrounding air. Once released from the damaged onion cells, the vapour can travel upwards towards your face.

This is why standing directly over a chopping board may cause your eyes to sting more quickly.

Why Onions Irritate Your Eyes

To understand why onions irritate your eyes, we need to examine the body’s protective response.

When propanethial S-oxide reaches the surface of your eyes, it stimulates sensitive nerves in the cornea. Your nervous system recognises the substance as an irritant and sends a signal to your tear glands.

The tear glands then produce additional tears to dilute and remove the irritating substance.

The process can be summarised as:

Damaged onion cells → enzyme reaction → volatile chemical → eye irritation → tears

Crying while cutting onions is therefore not an emotional reaction. It is your body’s natural attempt to protect your eyes.

Is Sulfuric Acid Formed in Your Eyes?

A common online explanation claims that onions make you cry because they produce sulfuric acid in your eyes. This is an oversimplification.

The more accurate explanation is that propanethial S-oxide irritates sensory nerves on the surface of the eyes. This irritation causes the tear glands to produce tears.

The temporary burning sensation does not mean that a dangerous amount of strong acid has formed in your eyes. For most people, the discomfort stops once they move away from the onion or improve the ventilation around them.

Why Do Onions Contain Tear-Producing Chemicals?

Scientists believe that the sulfur compounds in onions may form part of the plant’s natural defence system.

Unlike animals, plants cannot move away from insects or other threats. Instead, many plants produce chemicals that make them unpleasant or difficult to eat.

When an onion is damaged, its chemical reactions create strong-smelling and irritating substances. These substances may discourage insects and animals from continuing to attack the plant.

This is an example of chemical defence.

The same sulfur chemistry also contributes to the strong smell and distinctive flavour that make onions useful in cooking.

Why Do Some Onions Make You Cry More Than Others?

Different onions can produce different levels of eye irritation. Several factors may affect the intensity of the reaction.

Onion variety

Different onion varieties contain different quantities of enzymes and sulfur-containing compounds. Mild or sweet onions may cause less irritation than stronger varieties.

Growing conditions

The soil, climate and amount of available sulfur can influence the chemical composition of an onion.

Storage conditions

An onion’s chemical activity may change during storage. Its freshness, age and storage environment can affect the intensity of the reaction.

Cutting technique

A blunt knife may crush and damage more cells. This can release more of the substances needed to produce the lachrymatory factor.

Distance from the onion

The closer your face is to the cutting surface, the easier it is for the volatile substance to reach your eyes.

How Can You Cut an Onion With Fewer Tears?

You may not be able to prevent every tear, but you can reduce your exposure to the irritating vapour.

Use a sharp knife

A sharp knife makes cleaner cuts and crushes fewer onion cells. This may reduce the amount of tear-producing chemical released into the air.

Improve ventilation

Use a kitchen exhaust fan or open a nearby window. Moving air can carry the volatile chemical away from your face.

Avoid leaning over the onion

Keep your face away from the chopping board so that less vapour reaches your eyes.

Wear eye protection

Kitchen goggles create a physical barrier between your eyes and the vapour. This is one of the most direct ways to prevent irritation.

Avoid unsafe methods

Some online suggestions involve cutting onions under running water. This can make the knife or onion more difficult to control. Kitchen safety should always come first.

Is Cutting an Onion a Physical or Chemical Change?

Cutting an onion involves both physical and chemical changes.

The act of cutting changes the onion’s shape and size. This is a physical change.

However, cutting also breaks open the cells and allows previously separated substances to react. New chemical substances are then produced. This is a chemical change.

An onion therefore provides an interesting example of how a physical action can trigger a chemical reaction.

What Can Students Learn From Onion Chemistry?

Exploring onion chemistry helps students connect several classroom topics to an experience they can observe at home.

Plant cells

The reaction begins when the organised structure of the onion’s plant cells is damaged.

Enzymes

Alliinase and lachrymatory factor synthase are biological catalysts. They help chemical reactions occur more quickly without being permanently used up.

Chemical reactions

The substances inside the onion are converted into new products with different properties.

Diffusion

The volatile chemical spreads through the air from an area of higher concentration near the onion to the surrounding environment.

The nervous system

Sensory nerves detect the irritant, while the tear glands respond by producing tears.

Plant defence

The onion demonstrates how plants can use chemical substances to protect themselves from damage.

These connections make the onion an excellent topic for classroom discussions and simple kitchen science experiments.

Simple Kitchen Science Observation for Students

Students can conduct a safe observation with adult supervision.

Prepare two onion halves using a sharp knife. Place one half near an operating kitchen exhaust fan and the other in an area without strong airflow. Observe whether the airflow changes how quickly the onion vapour reaches the eyes.

Students can then consider the following questions:

• What is the independent variable?
• What is the dependent variable?
• Which conditions should remain constant?
• How does air movement affect the result?
• How could the investigation be made more reliable?

Students should not place onions directly against their eyes or deliberately expose themselves to excessive irritation. The purpose is to understand experimental variables, not to cause discomfort.

Frequently Asked Questions

Why do onions make you cry?

Cutting an onion damages its cells and starts enzyme-controlled chemical reactions. These reactions produce propanethial S-oxide, a volatile substance that irritates the eyes and triggers tear production.

What is the lachrymatory factor in onions?

The lachrymatory factor is the tear-producing chemical released when onion cells are damaged. Its chemical name is propanethial S-oxide.

Why do my eyes burn when cutting onions?

The volatile irritant reaches the cornea and activates sensitive nerves. Your tear glands produce tears to wash the substance away.

Does using a sharp knife help?

A sharp knife may reduce tears because it makes cleaner cuts and crushes fewer onion cells.

Why do cooked onions not make you cry?

Heat changes the onion’s chemical environment and reduces enzyme activity. Without active enzymes, the tear-producing reaction cannot occur as effectively.

Is onion cutting a chemical reaction?

Cutting is a physical change, but it breaks open cells and triggers chemical reactions that produce new substances.

Make Science Relevant to Everyday Life

The chemistry behind onion tears shows that science is not limited to laboratories and textbooks. Chemical reactions occur throughout daily life, from onions releasing irritants to cakes rising and fruits turning brown.

At Miracle Learning Centre, we use clear explanations and relevant examples to help students understand difficult scientific concepts. Parents searching for science tuition Singapore can explore our programmes for Primary, Secondary, IP and JC students.

We also provide subject-focused support for families seeking chemistry tuition Bukit Timah, with lessons designed to strengthen conceptual understanding, problem-solving skills and exam preparation.

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