Acids, bases and metalsPrint
When an acid is dissolved in water we get an acidic solution, and alkalis make alkaline solutions. If a solution is neither acidic nor alkaline we call it neutral. Pure water is neutral, and so is paraffin.
Indicators are substances that change colour when they are added to acidic or alkaline solutions. You can prepare homemade indicators from red cabbage or beetroot juice - these will help you see if a solution is acidic or alkaline.
Litmus and universal indicator are two indicators that are commonly used in the laboratory.
Litmus indicator solution turns red in acidic solutions and blue in alkaline solutions - and it turns purple in neutral solutions.
Litmus paper is usually more reliable, and comes as red litmus paper and blue litmus paper. The table shows the colour changes it can make.
|Red Litmus||Blue Litmus|
|Acidic solution||Stays red||Turns red|
|Neutral solution||Stays red||Stays blue|
|Alkaline solution||Turns blue||Stays blue|
Notice how we say 'stays red'. This is better than saying 'nothing' or 'stayed the same', because it tells us the colour we actually see.
Universal indicator is a mixture of several different indicators. Unlike litmus, universal indicator can show us exactly how strongly acidic or alkaline a solution is. This is measured using the pH scale. The pH scale runs from pH 0 to pH 14.
Universal indicator has many different colour changes, from red for strong acids to dark purple for strong bases. In the middle, neutral pH 7 is indicated by green.
These are the important points about the pH scale:
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