Thursday 3 July 2014

The Acid or Alkali Test

This was a really fun experiment for all my girls (age 4 to 11) however, before I take you through how to do this experiment I think we need to understand more about what acids and bases are... 

Svante Arrhenius was a Swedish scientist who came up with a way to define acids and bases in 1887.
The Arrhenius theory of acids and bases: 
  • Substances that produce hydrogen ions in a solution are acids.
  • Substances that produce hydroxide ions in a solution are bases.
When hydrogen ions and hydroxide ions react to produce water this is known as neutralisation.

Johannes Brønsted and Thomas Martin Lowry were the scientists who came up with the theory of acids and bases that is used in modern day UK 'A' levels today.

Johannes Brønsted was a Danish Chemical Engineer born February 22 1879 who earned his PhD in Chemical Engineering in 1908.

Thomas Martin Lowry was an English chemist born October 26 1874 in West Yorkshire.  Lowry independently came to discover the same idea as Brønsted, he discovered the formulation of the protonic definition of acids and bases in the same year.

The Brønsted-Lowry theory of acids and bases:
  • If it is a proton (hydrogen ion) giver it is an acid
  • If it is a proton (hydrogen ion) taker it is a base.
This theory of Brønsted and Lowry's doesn't disprove Arrhenius' theory but it does add to it.

The pH Scale

Søren Peder Lauritz Sørensen was a Danish Chemist born in Havrebjerg on January 9, 1868.  He is known for the introduction of the idea of pH, a scale scientists use to measure acidity in 1909.

He described two new methods for measuring acidity; the first method was based on electrodes and the second involved a preselected set of indicators and comparing the colours of samples.
pH in the pH scale stands for potential hydrogen.

The pH scale measures how acidic or alkaline a substance is.  If a substance has a pH of 7 they are neutral. A pH any higher and they are alkaline, this is also know as basic and any lower than 7 and they are an acid. 

When hydrogen is released the solution becomes an acid but when a hydroxide ion is released it becomes a base.

Alkalis are bases that are soluble in water.  All alkalis are bases but not all bases are alkalis.

Almost all liquids are either an acid or an alkali!
So now we understand what an acid and an alkali is, let's experiment!

What you will need for this experiment:

You will also need different liquids from around the house, or you can make your own solutions too, for example: salt water.  Here is a list of things that you can use in this experiment
  • liquid detergent, if you use powder add 20ml water to 5ml of washing powder to turn it into a solution.
  • white vinegar
  • brown vinegar
  • white wine vinegar
  • water
  • milk
  • orange juice
  • apple juice
  • soy sauce
PART TWO OF THE LIST - A little more dangerous so take care (better for tweenies and teenagers) but you can also use things like:
  • Hydrochloric Acid
  • Alcohol
  • Isopropyl
  • Surgical spirit/rubbing alcohol 
  • Bleach - hydrogen peroxide is a good bleach to use
  • Cleaning agents that are in a liquid form
These are all just examples there are many more.

How to set up your experiment...

Remember if you are doing this with little ones, anything under 6 I would recommend that you do as much setting up of the experiment as possible before involving them and leave out the second part of the list of substances to keep the experimenting as safe as possible.


First prepare your substances for testing!

Fill all your first test tube with 20ml of your first substance - we used salt solution in our first one.
Now fill each test tube with a different substance from the above list.  Prepare at least 6 substances for testing.
TIP: label as you go so you know what you have in each test tube.
Make a simple table like this...


Before Testing:
Hypothesis – Do you think it will be an acid or an alkali
After testing:
Was it an Acid or an Alkali?









Write down your predictions: Do you think it will be an acid, an alkali or neutral.

Get your pH indicator papers and test each. 

Did you guess right?

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