Acids & Alkalis

In our activity kids were shown a plate full of different food items. This plate mainly had items with different types of acids in them such as tamarind with tartaric acid, banana with malic acid etc., .

After which we had a  discussion around why some items taste sour, salty,  bland etc. Also, in baking, how cake dough rises to become soft and fluffy, the chemical reactions that take place due to baking soda ( sodium bi carbonate) and buttermilk in the recipe. Kids were able to guess acids are sour and baking soda , baking powder are bases. Then we talked about what is pH (power of hydrogen) scale and what it means in chemistry.

What is a pH scale ? 

Scientists use the pH scale to measure how acidic or basic a liquid is. pH scale has a range from 0 to 14. From 0 to 7 are acids with 0 being the strongest acid. From 7 to 14 are bases with 14 being the strongest base.

Since we can not be tasting every item to find out if it is a acid or base, the easiest way to find in chemistry lab is by using a neutral item called 'indicators'.

 What are 'indicators'?

Indicators are substances that change colour when they are added to acidic or alkaline solutions. In chemistry labs litmus paper, methyl orange and phenolphthalein are commonly used. 

Here in this activity we have used Purple cabbage as an 'acid-base indicator' .

Purple (sometimes called red ) cabbage contains an anthocyanin based dye that can be used as an indicator.

 Indicators such as purple cabbage show reactions when it mixes with acid or base. It turns red, pink for acids ( depending on the acidity level) to blue, green brown (depending on the alkaline level) for bases.

Materials needed

Purple cabbage - 5-6 leaves
Water - 4 cups
Lemon juice - 2  tsp
Vinegar - 3 tsp
Baking soda - 2 tsp
Washing powder - 2 tsp
Bleach - 2-3 tsp

Procedure

Add 4 cups of water with 5-6 leaves of purple cabbage.
Using a blender, blend it together for few minutes.

Strain the mix using a strainer to get the purple cabbage juice.

Purple cabbage juice ready!

 Take a quarter cup of this purple  cabbage juice and mix it up with a teaspoon full of lemon juice and you will notice the change in color from purple to red.

Note the color change from purple to red..

To test the effect of alkali (base) on this indicator we added  1 teaspoon of baking soda (sodium bicarbonate) to the purple cabbage juice. You can see the color change from purple to blue as we were mixing itself.

Due to alkali mixed in the juice, it changed from purple to blue

Lemon juice  (acid) when mixed with cabbage juice turns red, whereas sodium bi carbonate (baking soda) when mixed with cabbage juice turns blue. Kids will be thrilled to see the color change spontaneously.

To see the effects of other acids such as vinegar and bases such as washing powder and bleach we added in separate cups.
You can see the color change from red, pink,purple,blue,green brown as the pH level increases from 0 to 14.

pH levels from 0 - 14

 After seeing the effects of acids and alkali with purple cabbage juice kids were curious to know  if in nature we would be able to see the acids and alkali effects.

To show them that nature has it too, we went on a garden trip. There we  found these  'Hydrangeas' blooming with blue flowers where the soil is acidic, and has pink flowers where the soil content is more of  alkaline.

Blue 'Hydrangeas' when the soil is acidic

Pink hydrangeas when soil is alkaline

 When the soil has both alkaline and acid in equal portion it has purple flowers. Kids were able to relate to it as they had just then completed the experiment. 

Mix of blue and pink to make purple flowers

Isn't it amazing to note that nature is also busy in chemistry experiments :-).