Exploring Substances: Acidic, Basic, and NeutralClass 7 Science Notes

Exploring Substances: Acidic, Basic, and Neutral

Nature - Our Science Laboratory

Litmus as an indicator

Litmus is a natural substance obtained from lichens. It is used as an acid-base indicator, meaning it shows different colors in acidic and basic solutions. Litmus is available as a solution and in the form of paper strips called litmus paper. Litmus paper comes in two colors blue and red.

To test if a substance is acidic or basic using litmus paper:

1. Collect samples of the substances you want to test (e.g., lemon juice, soap solution, vinegar).
2. Take a strip of blue litmus paper and a strip of red litmus paper.
3. Place a drop of the sample on each strip of litmus paper.
4. Observe any color changes.

• Substances that turn blue litmus paper to red are acidic in nature.
• Substances that turn red litmus paper to blue are basic in nature.
• Substances that do not change the color of either litmus paper are neutral.

1. Collect samples of lemon juice, soap solution, amla juice, tamarind water, vinegar, baking soda solution, lime water, tap water, washing powder solution, sugar solution, and salt solution.
2. Take a strip of blue litmus paper and cut it into small pieces. Spread these pieces on a clean and dry white tile.
3. Using a dropper, put one drop of each of the samples, one-by-one, on these litmus paper pieces.
4. Observe any change in the colour of the blue litmus pieces.
5. Repeat the same activity with pieces of red litmus paper.

How to prepare lime water?

Lime water (solution of calcium hydroxide in water) can be easily prepared by mixing lime (chuna, i.e. calcium oxide) in water and leaving it undisturbed for some time, say an hour. Filter the liquid into another container and use it as lime water. Do not confuse lime water with the word lime, which is a fruit similar to lemon.

Grouping Samples

After testing with litmus paper, samples can be sorted into three groups:

• Group A Samples that turn blue litmus paper to red (acidic).
• Group B Samples that turn red litmus paper to blue (basic).
• Group C Samples that do not affect either litmus paper (neutral).

What are acids?

Substances that taste sour tend to contain acids and are acidic in nature.

• Lemon contains Citric acid.
• Orange contains Ascorbic acid (Vitamin C) and Citric acid.
• Grapes contain Tartaric acid.
• Tomatoes contain Citric acid and Oxalic acid.

What are bases?

Bases generally taste bitter and feel soapy or slippery to touch. However, not everything that tastes bitter is a base. For example, bitter gourd (karela) possesses a bitter taste but is not basic in nature.

Red rose as an indicator

Red rose extract can also be used as an acid-base indicator.

To prepare red rose extract:

1. Collect fallen petals of red roses.
2. Wash the petals with water.
3. Crush the petals using a mortar and pestle.
4. Place the crushed petals in a glass tumbler.
5. Pour some hot water into the glass tumbler to ensure that the crushed flower petals are completely immersed.
6. Cover the glass tumbler with a lid. Wait for 5-10 minutes until the water becomes colored, and filter it. The filtrate is the red rose extract.

To test substances with red rose extract:

1. Place 10-20 drops of the prepared red rose extract in each of two small transparent bottles or test tubes. Mark them $A$ and $B$.
2. Add 20-30 drops of lemon juice in test tube A and 20-30 drops of soap solution in test tube B with the help of droppers.
3. Observe and record any color changes.
4. Repeat the same with the other samples used in Activity 2.1 and record your observations.

• Red rose extract gives a red color in acidic solutions.
• Red rose extract gives a green color in basic solutions.

Turmeric as an indicator

Turmeric can also be used as a natural indicator.

To prepare turmeric paper:

1. Take a spoonful of turmeric (haldi) in a petri dish or container and add a little water to make a paste. You may also grind a piece of fresh turmeric.
2. Carefully dip a piece of filter paper in the turmeric paste until it gets a yellow color.
3. Take it out and allow it to dry.
4. Cut this yellow paper into thin strips, which are used as 'turmeric paper'.

To test substances with turmeric paper:

1. Using a dropper, put a drop of each of the samples used in Activity 2.1, one by one, on separate pieces of turmeric paper.
2. Record your observations.

• Turmeric paper turns red in basic solutions.
• Turmeric paper does not change color in acidic and neutral solutions.

Turmeric paper can be used to test basic substances but cannot differentiate between acidic and neutral substances.

Olfactory indicators

Olfactory indicators are substances whose odors change in acidic or basic mediums.

1. Take some finely chopped onions in a container, along with some strips of clean cotton cloth or filter paper.
2. Tightly close the container and leave it overnight.
3. Take two of the cotton cloth or filter paper strips from the container and check their odor.
4. Keep them on a clean surface and put a few drops of tamarind water on one strip and a few drops of baking soda solution on the other. Allow the drops to spread on the strips.
5. Check the odor again.
6. Observe any change in the odor of the onion strips before and after putting tamarind water and baking soda solution on them.
7. Similarly, test the change in the odor with other acidic and basic substances and record your observations.

What Happens When Acidic Substances Mix with Basic Substances?

When the solution of an acid is mixed with the solution of a base in sufficient quantity, the resulting solution is neither acidic nor basic. Such reactions are called neutralisation reactions. In a neutralisation reaction, salt and water are formed with the evolution (i.e., release) of heat.

The general equation for a neutralization reaction is:

Acid + Base $\rightarrow$ Salt + Water + Heat

1. Take one drop of lemon juice in a test tube and add around twenty drops of water to it. Observe the colour.
2. Add a drop of blue litmus solution to it.
3. Observe any colour change.
4. Slowly add drops of lime water to this test tube with the help of a dropper and swirl it well.
5. Observe any change in the colour of the solution.
6. A stage comes when the colour of the solution changes from red to blue.
7. Again, add one drop of lemon juice to the above solution.

Neutralisation in Daily Life

Neutralisation processes are used in many everyday situations.

Situation 1: Ant Bites

When an ant bites, it injects an acidic liquid (formic acid) into the skin. The effect of the acid can be neutralised by rubbing moist baking soda, which is a base, on the affected area.

Situation 2: Soil Treatment

Excessive use of chemical fertilizers can make the soil acidic. To treat acidic soil, lime (a base) can be added. If the soil is basic, organic matter like manure and composted leaves are added to it. Organic matter releases acids that neutralise the basic nature of the soil.

Situation 3: Factory Waste

Factory waste can be acidic. To save aquatic life, factory waste can be neutralised by adding basic substances before releasing it into lakes and rivers.