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pH, acids, and bases review

Key terms

TermMeaning
pHThe measure of the hydrogen ion (H+) concentration in a solution
AcidSolution with a higher concentration of hydrogen ions than pure water
BaseSolution with a lower concentration of hydrogen ions than pure water

Reading the pH scale

The pH scale is used to rank solutions in terms of how acidic or how basic they are. It indicates the concentration of hydrogen ions (H+) and hydroxide ions (OH-) in a solution. These ion concentrations are equal in pure water, which has a pH of 7.
pH scale showing bleach (12.6), baking soda (9), blood (7.3), and lemon juice (2)
This pH value of 7 is important because it indicates a neutral solution. All other substances are compared to this neutral point. Any solution that has a pH of less than 7 is considered acidic, and anything above a pH of 7 is basic.

Common mistakes and misconceptions

  • Acids are not always dangerous. Acids sometimes have a bad reputation for being dangerous, but there are plenty of acidic substances that are useful (and even tasty!). For example, orange juice and coffee are acidic substances. In addition, though there are a few acids that you want to avoid handling (ie: hydrofluoric acid), don’t be fooled: strong bases can be damaging as well.
  • Lower pH number means stronger acid, higher pH number means stronger base. It can be a little confusing, but the lower the pH of a substance is, the stronger of an acid it is. Sometimes it is more helpful to think that the further away from neutral a substance is, the stronger it is in either the acidic or basic direction.
  • Acids and bases are not only important in chemistry. Although you may go into more detail learning about them in your chemistry class, acids and bases are extremely important to living things! In fact, most enzymes work best at a specific pH. For example, the enzyme pepsin is necessary for the digestion of proteins in the stomach, but it only functions at a very low pH. Therefore, it is important for the body to maintain an acidic environment in the stomach to make sure food gets digested well.

Want to join the conversation?

  • leafers seedling style avatar for user Nikolas Dahmen
    If the pH scale is 0-14, how can you have acids that are ranked at -6, -10, -18, -32, etc. pH?
    (25 votes)
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    • piceratops tree style avatar for user A.T
      Hi, the answer lies in the way the pH is calculated. Although this may seem a little advanced. The pH is calculated by taking the -log of the concentration of a certain ion in a solution. In scientific notation, for example, if the concentration of this ion is 1x10^-10 mol/L, then the pH would be 10! Therefore, you can work backwards to see that if an acid had a concentration of 1x10^2 mol/L, then its pH would be -2! so basically negative pH indicates that the acid is very strong. (Just copy and pasted Aeshons comment so everyone could see. This is not my original comment. Aeshon gets all the credit)
      (6 votes)
  • male robot donald style avatar for user Michael Spivey
    I thought that bleach was acidic because of the way it turns whatever it touches white?
    (2 votes)
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  • blobby green style avatar for user r.praveengowda
    is lemon juice more acidic than bleach
    (5 votes)
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  • mr pants purple style avatar for user Thomas
    Is there a reason alkaline batteries have the word "alkaline" in them? Because I saw on the scale battery acid is very acidic, but in the video it said alkaline was a term for basic solutions ()
    (6 votes)
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  • leaf blue style avatar for user Sarah
    I have an interesting question: Do medicine pill or tablets even have a pH of acidic or basic?
    (5 votes)
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  • duskpin seedling style avatar for user Leden521
    What is considered a weak acid pm.
    (3 votes)
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  • leaf green style avatar for user jegededave42
    it was said that an Acidic solution is one with a higher concentration of hydrogen ion than pure water.
    what does this mean?
    does it mean that if the concentration of H ion in a solution is higher than the concentration of H ion in pure water then that substance is acidic?
    (1 vote)
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    • old spice man green style avatar for user Elijah Daniels
      Yes! The Hydrogen (or hydronium) ion concentration of pure water at 25 degrees Celsius is 10^-7 (if you're not sure where this came from, Khan Academy has some videos on it). If the Hydrogen concentration of a solution is higher than 10^-7, the solution is said to be acidic.
      (2 votes)
  • blobby green style avatar for user twymakir
    I have two questions
    1. is it possible for there to be a base with the properties of an acid, or an acid with the properties of a base?
    2. Can a substance be both an acid and a base? And if so where would it be on the pH scale?
    (1 vote)
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    • male robot donald style avatar for user Tybalt
      1. On a molecular scale, acids will always donate their protons and bases will always accept protons and form hydroxide ions. However, on a macroscopic scale, each can be corrosive and dangerous, if this is what you mean.

      2. In fact, yes! An example of such a substance is water, which will act like a base if it reacts with an acid, and will act like an acid should it react with a base. From my knowledge, they do not have specific charges.

      Does this help?
      (3 votes)
  • blobby green style avatar for user Parsa Payandeh
    It is said above that " Most enzymes work best at a specific pH" Why is it so?
    (2 votes)
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    • leaf green style avatar for user Anil Mathew
      It's because enzymes are sensitive to pH. For an enzyme to work and for a product, its active site must combine with the substrate to for an enzyme-substrate complex as in the induced fit model. However, in conditions that are not optimal, such as when an enzyme is placed in a solution with a pH value that's outside an enzyme's optimum, it can become denatured. In this, the enzyme loses its 3D shape and its ability to react with the substrate to form a product. This is why enzymes work best at a specific pH. Hope this helps.
      (0 votes)
  • blobby green style avatar for user adani510723
    How can someone keep there PH balanced? Is it through things we eat but could it also be things that we are around? or use on a daily basis? Hope this question makes sense. Does lifestyle matter?
    (1 vote)
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