Core Concepts

What is an acid? In this tutorial, you will learn about the distinctive properties between acids and bases, what defines an acid vs base, and also what is an amphoteric compound. If you enjoy this article be sure to check out our other acid-base tutorials linked below.

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What are acids and bases?

There are currently three definitions for acids and bases that entail how they behave when placed in solutions. These are the Lewis, Bronsted-lowry, and Arrhenius definitions. So what is an acid? Essentially, acids accept electron pairs and donate hydrogen protons. In contrast, bases donate electrons and accept hydrogen protons. In addition to these definitions, acids and bases can be classified by their properties. These properties include pH, taste, texture, reactivity, and conductivity.

The name “acid” comes from the Latin acidus, which means “sour,” and refers to the sour taste and sharp odor of many acids. 

pH of Acids and Bases


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The pH scale is a measure of how acidic or basic a solution is and it ranges from 0 to 14. As you can see in the image above, acidic solutions have a pH of less than 7, whereas bases have a pH higher than 7. Solutions with a pH of 7, such as water (H2O), are considered neutral. There are many different ways to measure the pH of a solution but two common methods are the use of indicators and probes. Probes tend to be more accurate because the device is placed into the solution and digitally read. Meanwhile, with indicators, you must use your eyes to observe any color change and compare it to the pH scale. One type of indicator is litmus paper. Acids will turn blue litmus paper red and bases will turn red litmus paper blue. Notice how this correlates to the colors in the image above.

So, what does it mean for a solution to be acidic or basic? Acidic solutions have a high concentration of hydrogen protons, while basic solutions have a high concentration of hydroxide ions. When these two solutions are combined, they cancel each other out in what is called a neutralization reaction. In this reaction, the products are salts and water–which is formed when the hydrogen protons and hydroxide ions combine.

Taste and Texture of Acids and Bases


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Another defining feature between acids and bases is taste and texture. It must be noted, you should always follow lab safety guidelines and never consume or directly touch chemicals. Before these regulations were set in stone to protect us, chemists used to taste chemicals. In fact, famous physicist and mathematician sir Isaac Newton tasted over a hundred different chemicals when he practiced alchemy. Among these was the element mercury–which is highly toxic and has been attributed to his death. As previously mentioned, acidic solutions have a high concentration of hydrogen ions. This makes acids taste tart or sour. Though you should never consume any acids in the lab, you can relate this to the taste of a lemon. Lemons are acidic as they contain high amounts of citric acid. Moreover, lemons have a pH of around 2-3 which falls well under the acidic category on a pH scale. As for bases, they tend to taste bitter. An example of this would be if you’ve ever tasted a bubble of soap.

Again, you should never deliberately touch chemicals on your skin as they can be harmful and abrasive. However, if it were to happen in a lab, you could be able to identify whether it’s an acid or base based on touch. If the solution feels soapy or slippery, it is a base. This is because bases dissolve the fatty-acid oils from our skin and essentially turn our skin into soap. Meanwhile, acids are typically rough to the touch.

Reactivity of Acids and Bases


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Saponification Reaction

Reactions with Metals

When acids react with metals, the outcome is very similar to a neutralization reaction. The only difference is instead of producing water, you will get hydrogen gas. Regardless of what acid or metal is used, it will always produce salt and H2 gas. One example of this is the reaction between magnesium and hydrochloric acid. When combined, the resulting products are magnesium chloride and hydrogen gas. As for bases, they do not typically react with metals, but there are a few metals that make exceptions such as zinc and aluminum. These reactions also result in salts and hydrogen gas.

Reactions with Carbonates

Acids can also react with carbonates, in which case produce salt, water, and carbon dioxide. Carbonates are formed when combined with metals or organic compounds. The unique feature here is the production of carbon dioxide. This can be manipulated in the lab to determine whether an unknown solution is basic or acidic. Simply add the carbonate solution and if carbon dioxide is produced, the solution is likely acidic.

Reactions with Fats/Oils

Earlier in the article, it was mentioned that bases dissolve the oils on our skin and essentially turn our skin into soap. This process of dissolving the fats and oils is called hydrolysis and when in presence of a base, is known as saponification. Essentially this reaction means bases combine with fats to form glycerol or soap.

Conductivity of Acids and Bases

Throughout this article, we have talked about how acids and bases can ionize into hydrogen protons and hydroxide ions when placed in a solution. Electrolytes are salts/molecules that ionize completely in polar substances such as water. For this reason, strong acids and bases are also strong electrolytes. These electrolytes are capable of conducting electricity in their aqueous states because their ions are mobile.

Summary Comparison Chart

PropertiesAcidsBases
Tastetart/ sourbitter
TextureRoughSoapy and slippery
pHLess than 7Greater than 7
Turns litmus paper..Blue—->RedRed—->Blue
Reactivity– Reacts with metals to produce H2 gas– Reacts with carbonated compounds to produce CO2– Does not typically react with metals or with carbonated compounds– Does react with oils and fats
Conducts electricity in wateryesyes
Examplesvinegar (ethanoic acid) , lemon juice (citric acid)Baking soda (sodium bicarbonate), ammonia water (ammonia hydroxide)

What is an Amphoteric Compound?

If a compound is amphoteric, it can react as either an acid or a base. Amphoteric compounds are usually metal oxides or hydroxides. They react with acids to form a metal salt, and with a strong base to form a polyatomic metallic ion. To react with a base, the amphoteric hydroxide often needs to have been freshly produced, and the base must be hot and concentrated. In the following example, zinc oxide becomes the zincate ion Zn(OH)4– as part of soluble sodium zincate, when added to concentrated base.

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In acid: ZnO + H2SO4→ ZnSO4+ H2OIn base: ZnO + 2 NaOH + H2O → Na2

Examples of amphoteric compounds

Chromium hydroxide, tin hydroxide, lead hydroxide, cobalt hydroxide, zinc hydroxide, zinc oxide, aluminum hydroxide, and aluminum oxide.