Parts-per notation facts for kids

This picture shows how a chemical called Fluorescein looks when it's mixed with water at different strengths. The weakest mix (1 part-per-million) is a very light yellow. As you add more chemical, the color gets brighter yellow, then orange, and the strongest mix (10,000 parts-per-million) is a deep red.

In science and engineering, we often need to talk about very tiny amounts of something mixed into a much larger amount. This is where parts-per notation comes in handy. It's a simple way to describe how much of one thing is in another.

Imagine you have a huge container of water, and you add just a tiny drop of dye. Parts-per notation helps you say exactly how much dye is in that water.

Common terms you'll hear are:

• Parts-per-million (ppm), which means 1 part in a million parts (like 1 drop in 50 liters of water).
• Parts-per-billion (ppb), which means 1 part in a billion parts.
• Parts-per-trillion (ppt), which means 1 part in a trillion parts.
• Parts-per-quadrillion (ppq), meaning 1 part in a quadrillion parts.

These terms are like saying "a fraction" or "a percentage," but for much smaller amounts. For example, 1% means 1 part per hundred.

How We Use Parts-Per Notation

Parts-per notation is used in many areas to describe small amounts.

In Chemistry: Water and Pollution

One common use is in chemistry, especially when talking about things dissolved in water. For example, if you hear that there's 1 ppm of a pollutant in water, it means there's one part of that pollutant for every million parts of water.

Think of it this way:

• 1 ppm in water is like having 1 milligram (mg) of something in 1 liter (L) of water.
• 1 ppb in water is like having 1 microgram (μg) of something in 1 L of water.

This helps scientists measure how clean our drinking water is or how much pollution is in a lake.

In Physics and Engineering: Measurements and Materials

Parts-per notation is also used in physics and engineering.

• Material Expansion: Imagine a metal rod that gets longer when it heats up. If it expands 1.2 micrometers for every meter of its length for each degree Celsius it heats up, we can say its expansion is 1.2 ppm/°C. This tells engineers how much a material will change size with temperature.
• Measurement Accuracy: When measuring distances with special tools like a laser rangefinder, there's always a tiny bit of error. If the tool is accurate to 1 millimeter for every kilometer it measures, we can say its accuracy is 1 ppm. This means for every million parts of distance measured, the error is only one part.

In NMR: Chemical Shift

In a special type of science called nuclear magnetic resonance spectroscopy (NMR), parts-per notation helps describe something called "chemical shift." This tells scientists about the structure of molecules. It shows how much a measured frequency is different from a standard frequency, expressed in ppm. This way, the numbers stay the same no matter what kind of NMR machine is used.

Understanding Parts-Per Expressions

Let's look at what each "parts-per" term really means:

• Percent (%)

* Means "parts per hundred." * It's 1 part in 100 parts. * Example: 1% is like 14 minutes out of a whole day.

• Per mille (‰)

* Means "parts per thousand." * It's 1 part in 1,000 parts. * Example: 1‰ is like 90 seconds out of a whole day. * Note: Sometimes "ppt" is used for "parts per thousand," but it usually means "parts per trillion," so it's better to write "per thousand" fully.

• Per myriad (‱)

* Means "parts per ten thousand." * It's 1 part in 10,000 parts. * Example: 1‱ is like 9 seconds out of a whole day.

• Per cent mille (pcm)

* Means "parts per hundred thousand." * It's 1 part in 100,000 parts. * Example: 1 pcm is like 5 minutes out of a whole year. * It's used in areas like studying diseases or in nuclear reactors.

• Parts-per-million (ppm)

* Means "parts per million." * It's 1 part in 1,000,000 parts. * Example: 1 ppm is like 32 seconds out of a whole year. * In mining, 1 ppm can mean 1 gram of a valuable metal in 1 metric ton of rock.

• Parts-per-billion (ppb)

* Means "parts per billion." * It's 1 part in 1,000,000,000 parts. * Example: 1 ppb is like 3 seconds out of a whole century (100 years)!

• Parts-per-trillion (ppt)

* Means "parts per trillion." * It's 1 part in 1,000,000,000,000 parts. * Example: 1 ppt is like 30 seconds out of every million years.

• Parts-per-quadrillion (ppq)

* Means "parts per quadrillion." * It's 1 part in 1,000,000,000,000,000 parts. * Example: 1 ppq is like two and a half minutes out of the entire age of the Earth (about 4.5 billion years)! Even though these amounts are incredibly small, scientists can sometimes measure them.

Why Parts-Per Notation Can Be Tricky

Even though parts-per notation is useful, it can sometimes cause confusion.

Different Meanings for "Billion" and "Trillion"

One problem is that the words "billion" and "trillion" mean different numbers in different countries. For example, in the United States, a billion is 1,000,000,000 (a thousand million). But in some other countries, a billion used to mean a million million (1,000,000,000,000). To avoid misunderstandings, international groups like the BIPM (International Bureau of Weights and Measures) suggest not using "ppb" and "ppt."

Also, "ppt" can sometimes mean "parts per thousand" instead of "parts per trillion." This means you always need to be careful and check what the writer means!

What Is Being Measured?

Another issue is that "parts-per" can refer to different types of measurements:

• Mass fraction: How much mass of one thing is in the total mass (like grams per gram).
• Mole fraction: How many molecules of one thing are in the total number of molecules.
• Volume fraction: How much volume of one thing is in the total volume (like liters per liter).

Usually, the type of measurement isn't stated, which can lead to mistakes. For example, if you're talking about gases, the difference between a mass fraction and a volume fraction can be huge! It's always best to be clear and say "mg/kg" or "μL/L" instead of just "ppm." Sometimes, people add a letter like "V" for volume (e.g., ppmV) or "w" for weight/mass (e.g., ppmw) to make it clearer.

Scientists in different fields might use parts-per notation in their own specific ways. This can make it hard for someone from one field to understand a report from another field if the details aren't clearly explained.

Better Ways to Express Small Amounts

Because of these confusions, many scientists prefer to use standard International System of Units (SI) measurements. Instead of saying "2 ppm," they might say "2 μV/V" (2 microvolts per volt) or "2 mg/kg" (2 milligrams per kilogram). These units are clearer because they show exactly what is being measured.

For example:

• Instead of "2 parts per million," you could say "2 μV/V."
• Instead of "2 parts per billion," you could say "2 nV/V."
• Instead of "2 parts per trillion," you could say "2 pV/V."

These expressions are still about tiny amounts, but they use units that make it clear what is being compared.

Images for kids

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  This image helps you visualize how small parts-per amounts are compared to a whole.

See also

In Spanish: Partes por notación para niños