Lesson 8 – Acids and Bases

Objectives:

  • to predict formulas for common acids and bases
  • to classify acids and bases based on their  properties

Timeline:

  • 1 class

Prelab:

In this lab, we’ll be testing substances with pH paper, bromothymol blue and phenolphthalien in order to determine what the properties of acidic and basic substances are.

Bromothymol blue is often used to test the pH of fish tanks. This is because it turns yellow in acidic substances and stays blue in neutral or basic substances. Fish inhale oxygen from water, through their gills, and exhale carbon dioxide. The carbon dioxide dissolves in the water and forms carbonic acid (this is the same stuff that makes pop fizzy). As more carbon dioxide enters the water, the more acidic it gets, potentially harming the fish. By testing a sample of water with bromothymol blue, you can get a quick visual as to the health of your fish tank.

Phenolphthalein is the same stuff they use on CSI to test for blood. It stays colourless in acidic or neutral solutions, but turns bright pink in basic ones. A forensic investigator will take a blood swab, drop some phenolphthalein on it, and after some hydrogen peroxide. Due to a chemical reaction, the mixture will immediately turn pink. This is due to the oxygen that blood carries, so it wont work if the substance isn’t blood.

We will be testing a known strong acid, a known strong base, and then various unknown substances to determine their properties. You will test it first with bromothymol blue, second with phenolphthalein, and lastly with pH paper. Your pre-lab is to write up a procedure, and create a data table in order to record your results.

More Information:

The definition of acids and bases has varied over time – like most scientific theories. As our knowledge gets better, the chaff gets knocked off, and the theory gets refined. We’re using a theory developed by Svante Arrhenius. He did experiments in 1884 with water, acids, bases and salts. One of the things he theorized, is that ionic substances will break apart (dissociate) in water. That means that a solution would contain water molecules, positive ions, and negative ions all mixed together.

He found that pure water does not conduct electricity, however when an acid or a base is dissolved in water, it will start to conduct – they are electrolytes. Arrhenius predicted (and was correct) that acids break apart in water to form hydrogen ions (H+), and that bases break apart in water to form hydroxide ions (OH).

Properties of Acids

  • Sour flavour (lemons, limes, citrus fruits)
  • Can remove rust
  • React with water to produce hydrogen ions

Ex: HCl + H20 H+ + Cl– H20

Properties of Bases
  • Bitter flavour (cabbage, fish)
  • Slippery feel (liquid fabric softener)
  • React with water to produce hydroxide ions

Ex. NaOH + H20 Na+ + OH+ H20

Neutralization

Acids and bases can be combined together. We commonly do this if one has been spilled. When you mix and acid and a base, they cancel each other out, and become neutral.

Ex: NaOH + HCl –> NaCl + H20

You can see that the products of the reaction are water, and a salt.

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