How to Determine the Acid or the Base Nature of an Unknown Solid

You know this is an acid -- but an unknown substance can be trickier.

If you have a sample of an unknown solid substance and you want to find out whether it's an acid or a base, the simplest way is to dissolve it in water. An acid in solution decreases pH, while a base in solution elevates pH. When you dip a strip of pH paper in the water, it will change color to reflect the pH of the solution. By matching the new color of the paper with the color key in the pH kit, you can determine whether the substance is alkaline or acidic.

Things You'll Need

• Gloves
• Scoop
• Beaker
• pH paper kit

Instructions

1. Put on the gloves and use the scoop to add some of the solid to a little water in a beaker. Stir or swirl gently until it dissolves. Do not handle the substance with your bare hands, especially if you don't know what it is.

2. Dip the end of a strip of pH paper in the water. Watch to see whether it changes color.

3. Compare the color of the pH paper with the color key in the pH paper kit. If the solution has become acidic, your unknown substance is an acid. If it has become alkaline, your unknown substance is a base.

Tips & Warnings

• If the substance is edible, you can often tell whether it's an acid or base judging by the taste. Acids tend to be sour and tart like lemon juice, while bases tend to be bitter.

• Never taste an unknown substance to figure out what it is. Many chemicals in a lab are toxic and could poison you if you try to taste-test them.

References

• University of Illinois at Urbana-Champaign: Virtual Chemistry Lab for Acid-Base Titration