pH Scale

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Chemists use the pH scale to determine whether a substance is acidic or basic in nature. This is very useful as it can give us an idea of how certain chemicals react with each other as well as knowing how careful we have to be when using them.

The pH scale ranges from 1-14, and we can categorise different substances based on where on this scale they fall. We can determine the acidity/basicity of different substances in a variety of ways:

  • Universal indicator paper: This is a piece of paper which changes colour when a substance is added to it (either as a solution or as a vapour) depending on whether the substance is acidic or basic.
  • Universal indicator solution: This is a solution that changes colour when added to an acid or base
  • pH meter: This is a very sensitive machine that uses a probe to detect the concentration of H+ ions in the solution. The more H+ ions there are, the more acidic the substance will be.

We can determine whether something is an acid or a base by finding its pH:

pH 1-6 : ACIDS  

These include things like vinegar, also known as acetic acid, citric acid (found in lemons and oranges) as well as much stronger acids such as stomach acid (HCl) and battery acid (H2SO4). When using universal indicator, acids appear as yellow (roughly pH 5) to red (pH 1).

close up photo of sliced lemons
Image of lemons, which contain citric acid
close up photo of batteries
Image of batteries, which contain sulphuric acid








clean clear cold drink
Image of water being poured into a glass. Water is pH 7, which is neutral

This is the term used for substances that exist at an equilibrium between acid and base. The best example of this is water, but other examples, such as blood (pH 7.4) can be found. When using universal indicator, neutral compounds appear green.

pH 8-14: BASIC 

Also known as alkalis, these substances include household bleach, hand soap and toothpaste. When using universal indicator, bases appear as blue (roughly pH 8) to dark purple (pH 14). 

person washing his hand
Image of a person washing their hands with soap, which is alkaline
clean mouth teeth dentist
Image of toothpaste being added to a toothbrush.

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