Sure, you’ve heard of acids and bases, but do you know all of the places where people need to monitor the level of acids and bases? From swimming pools to gardens, this lesson will discuss acid-base indicators and their uses.

What Is an Acid-Base Indicator?

Did you know that when you put blueberries in an acidic solution, they turn red? Or that beets go from red to purple in a base? Did you know if you place red cabbage in different acidic and basic solutions, it will change into a variety of colors depending on the pH? And, that you can’t smell vanilla in the vanilla extract when it is placed in a basic solution?

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You may have some questions… like what is an acid, a base, and a pH? Let’s answer some of these questions.

An acid has a pH less than 7. It releases hydrogen ions when dissolved in water and tastes sour. Examples of acids include lemon and vinegar. A base, sometimes referred to as alkaline, has a pH greater than 7. It releases hydroxide ions when dissolved in water, and it tastes bitter. Examples include bleach and Drano.

A pH scale is a tool used to measure acids and bases. A pH of 7 is neutral, meaning it is neither acidic nor basic. The blueberries, beets, red cabbage, and vanilla extract were all acid-base indicators, or substances that change in some form, like color or odor, in the presence of an acid or base. In other words, the indicator ‘indicates’ that a substance is an acid or a base.

Most of the time people don’t use household food items as indicators. Oftentimes they rely on pH strips. When a pH strip is placed in an acid or base, it changes color, and then this color is matched to a color key that shows which color is associated with which pH. For example, if the pH strip turns yellow, that may indicate a pH of 4. There are also pH meters, where a probe is placed into the solution and a screen indicates the pH of the material.

Now that you know what an acid-base indicator is, when are they used? They are often used in science labs, but they are also used in everyday applications. Let’s delve into the areas you might see acid and base indicators.

Soil Testing

Before people plant gardens, they usually take a closer look at the soil to see if its acidic or basic. There are soil-testing kits that utilize pH strips or pH meters and can give the gardener a good idea of his or her soil’s pH.

Some plants grow better in acidic soil, whereas others prefer basic soil. For example, if your soil pH falls between 5.1 and 6.0, it is ideal for rhododendrons. Most plants prefer a pH that falls between 6.1 and 7.0. If a gardener’s soil is too basic or too acidic, there are steps that can be taken, such as adding lime (a base) to acidic soil.

Swimming Pools

Another important place where the pH should be monitored is in swimming pools. If the water is too acidic or too basic, problems can arise. For example, the swimmer may experience eye or mouth burning, parts of the pool might corrode, the water may get cloudy, and the chlorine (which is used to disinfect the water) will not be as effective. Like the soil testing kits, there are pool-testing kits that utilize the pH strips or pH meters. Ideally, the pool should be kept at a pH between 7.0 to 7.6.


Acids and bases can find their way into the water cycle, where they can wreak havoc on plants and animals. Of course, acids and bases can naturally be found in many environments, but sometimes (due to humans) more is inadvertently added. Here are just a few examples:

  • Acid rain, which is rain with a pH around 4, is often caused by coal and fossil fuel burning and can decrease the pH of bodies of water and soil, which impacts aquatic and plant life.
  • Acid runoff from mines can find its way into bodies of water.
  • Industrial plants also can release acids and bases into the environment. For example, wastewater from paper mills and leather making plants can release bases into wastewater.

Wastewater treatment facilities monitor the pH of liquid wastes and attempt to keep the pH as close to neutral as possible. Obviously, this doesn’t impact acid rain, but it can help regulate industrial and residential wastewater. As a general rule, wastewater with a low pH (2 or below) and wastewater with a high pH (12.5 or above) are considered corrosive, meaning they can damage solid material like steel (not to mention severely damage plant and animal tissues).

Household Substances

From cooking to cleaning, classifying acids and bases is important in the home as well. For example, did you know that acids (like lemon) can soften up proteins, like fish? Or that when you’re baking, the baking soda (a base) reacts with buttermilk (an acid) to cause bubbles, thus causing your baked goods to rise?

Many cleaning products are acids and bases. It’s good to note which cleaners are acidic and which are basic and avoid mixing the two. For example, mixing bleach (a base) with vinegar (an acid), creates chlorine gas. Chlorine gas is nasty stuff and can cause irritation of the eyes and throat and, at high enough levels, can kill you.

Lesson Summary

From beets to vanilla to pH strips, there are many acid-base indicators, or substances that show an acid or base is present. Most times, people use pH strips or pH meters to test the pH of substances.

  • An acid is a substance that generates hydrogen ions when dissolved in water and has a pH less than 7.
  • A base is a substance that generates hydroxide ions when dissolved in water and has a pH greater than 7.
  • A pH scale is a tool used to determine if something is an acid or a base. Oftentimes, people use pH strips to test the pH of materials.

While we often associate pH with chemistry labs, there are other important places where knowing the pH of substances is beneficial. Gardeners test the pH of their soil. A high or low pH impacts the growth of plants in gardens, with optimal growing occurring between a pH of 6.1 and 7.0. Owners of swimming pools must test the pH of the water. A high or low pH can cause health problems, can corrode pool equipment, and can make the chlorine ineffective.

Pollution control is another area in which pH monitoring plays an important role. Acids and bases can find their way into the water supply through industrial waste, acid rain, or runoff from mines. Wastewater treatment facilities work to keep the pH near neutral before water is returned to the environment.

It’s also important to know about pH of common household products. Acids and bases can be found throughout the house. It’s important to know what you are doing when mixing acids and bases. While some can be delicious, such as in cooking, others can be hazardous, such as in cleaning products.