What happens when acids and bases mix?

What’s happening?

pH is a measure of how acidic or basic a solution is. As shown in the image above, different amounts of acids and bases in solutions will produce a different color when exposed to an indicator. Red cabbage juice is an excellent, easy-to-come-by indicator solution that shows a variety of colours across a wide range of pH. The indicator will turn reddish-pink in the presence of an acid, greenish-yellow in the presence of a base and blue in the presence of a neutral solution.

When acids and bases are combined in the right amounts, they will react together to form a new substance that has a neutral pH. Adding Milk of Magnesia to vinegar should show a colour change from pink, through blue and then to green as the base becomes in excess (i.e. when there is more base present then acid). Similarly, adding vinegar to Milk of Magnesia should show a colour change from green, through blue and then to pink as the acid is added in excess.

What you think is happening when the acid and base are mixed together?

Why does it matter?

Acid-base reactions are very common chemical reactions used every day to perform a variety of functions. Indicators are solutions of pigments that react with solutions of acids and/or bases and change colour. Indicators are very useful to measure the pH (the measure of acidity) caused by the presence of certain chemicals in a solution. For example, the pH of a swimming pool is tested using indicators. Water that is too acidic or basic can hurt our eyes, so pool operators must ensure that chlorinated pool water is safe for our eyes. These pH indicators may be liquids that are added to a water sample or they may be embedded in strips of paper that are dipped into the water. The resulting colour that develops on the test strip or in the water sample is then compared to a standard colour chart that tells you what pH a colour represents.

Many common household substances are acids or bases, as are many of the foods we eat. In foods, the amount of acid/base affects the taste of the food, including which foods taste good together. Foods that taste sour contain acids; those that taste bitter often contain bases. Many household cleaners, soaps, shampoo, laundry cleaners, bleach, etc., are bases. Many people think acids are more dangerous than bases, while in fact both are equally as hazardous when in concentrated form. Many household cleaners can cause burns to skin and damage clothing and should be handled with care. Normally the strongest acid found in households is vinegar. While it is not dangerous per se, it will sting if it gets in cuts or scrapes or in your eyes. However, prolonged exposure to even a weak acid like vinegar can damage your skin and lead to blisters and lesions.

Investigate further

• Repeat the experiment using other liquids like apple juice, orange juice, other fruit juices, soft drinks, or club soda.
• What happens when you add the red cabbage indicator to these liquids? What does it tell you about the contents of fruit juices and non-diet soft drinks? Why is it important to brush your teeth regularly if you drink these liquids?
• Check other common household liquids and foods to determine if they are acids or bases. CAUTION: household cleaners are dangerous and should only be tested by an adult.

For more information on this topic check out these Let's Talk Science resources:

• How can I write secret messages? (Hands-on Activities) - Learn how you can write and reveal secret messages in this hands-on chemistry activity.
• What is Acid rain? (STEM in Context) - Acid rain is any precipitation that has an unusually low pH. It can be rain, snow, fog, etc. But what is a low pH and why is this a problem?