Exploring Substances : Acidic, Basic and Neutral - Class 7 Science Chapter 2 (Short Notes)

• Acids → Substances that taste sour, feel drying and are corrosive to touch. 
•  Bases → Substances that taste bitter and feel soapy/slippery to touch. 
•  Neutrals → Substances that are neither sour nor bitter and feel normal to touch.

Indicators and their types

Indicators are substances that show different results (color or smell) in acidic and basic solutions.

• On the basis of source, indicators can be natural or synthetic.
• On the basis of action, indicators can be visual or olfactory.

Types of Indicators based on Source

1. Natural Indicators 🌿: Indicators obtained from natural sources like plants, vegetables or fruits are called Natural Indicators.
Examples: Litmus, Turmeric, Red rose extract, onion, beetroot, red cabbage

2. Synthetic Indicators 🧪: Indicators made in laboratories by chemical processes are called Synthetic Indicators.
Examples: Phenolphthalein, Methyl Orange, Universal indicator

Types of Indicators Based on Action

3. Visual Indicators 👀: Indicators that show a colour change in the presence of acids or bases are called Visual or colour Indicators.
Examples: Litmus, Turmeric, Phenolphthalein, Methyl orange. A table given below contains many visual indicators along with the colours they show in acidic and basic medium.

S. No.	Indicator	In Acid	In Base
1.	Blue litmus	Turns red	No change
2.	Red litmus	No change	Turns blue
3.	Phenolphthalein	Remains Colourless	Pink
4.	Methyl orange	Red	Yellow
5.	Universal indicator	Yellow/Orange/Red	Blue/Green/Violet
6.	Turmeric	Remains yellow	Turns red-brown
7.	Red rose extract	Red	Green
8.	Hibiscus extract	Red	Green
9.	Beetroot extract	Pink/Red	Yellowish
10.	Purple cabbage extract	Reddish	Green/Yellow

4. Olfactory Indicators 👃: Indicators that show a change in smell/odour in the presence of acids or bases are called Olfactory Indicators. Examples: Onion → Smell disappears in base, remains in acid Vanilla essence → Smell present in acid, disappears in base Clove oil → Smell present in acid, disappears in base

Steps to Make Red Rose Indicator 🌹

The steps needed for making Red-rose indicator are mentioned below: 

1. Collect petals – Take a few fresh red rose petals. 
2. Crush petals – Put them in a mortar and crush them well. 
3. Add solvent – Add a small amount of water or ethanol to extract the colour. 
4. Filter – Filter the mixture to remove solid parts and collect the coloured liquid. 
5. Store – Keep the extract in a clean glass bottle; this is the red rose indicator. 
6. Test – 
•  Add to an acid → solution turns red. 
•  Add to a base → solution turns green.

Why are rose petals crushed while making indicator?

There are colourful pigments inside the flower petals which are responsible for the colour change when the indicator react with acid or base. Crushing the petals helps to extract those pigments inside the solvent. If flower petals are not crushed, the pigments will not react with acid or base properly.

Invisible Ink Experiment 🧪

1. Prepare the Ink: Use baking soda solution (basic) as ink. 
2. Write the Message: Dip a cotton swab, toothpick, or paintbrush into the solution and write a message on a clean, white sheet of paper. The message will appear wet at first but disappear once dry. 
3. Prepare the Developer: Choose phenolphthalein as the indicator that will react with your invisible ink and show pink color. 
4. Reveal the Message: Lightly spray or brush the indicator over the entire paper. The areas where the invisible ink was will become pink, revealing your secret message.

Common Acids (in substances) 🌡️

• Hydrochloric acid (HCl) → in gastric juice (stomach) 
• Acetic acid (CH₃COOH) → in vinegar 
• Citric acid → in lemon, orange, amla 
• Tannic acid → Tea, Coffee 
• Lactic acid → in curd, milk 
• Tartaric acid → in tamarind, grapes 
• Oxalic acid (C₂H₂O₄) → in spinach, tomato 
• Carbonic acid (H₂CO₃) → in cold drinks

Common Bases (in substances) 🧂

• Sodium hydroxide or Caustic soda (NaOH) → in soap, cleaning agents 
• Calcium hydroxide or lime water Ca(OH)₂ → in whitewash 
• Magnesium hydroxide Mg(OH)₂ → in milk of magnesia (used for antacid) 
• Ammonium hydroxide NH₄OH → in window cleaners 
• Baking soda (sodium bicarbonate) NaHCO₃ → in cooking, fire extinguishers 
• Potassium hydroxide (KOH) → in soft soaps, alkaline batteries

Acid-Base Neutralisation Reaction

Reaction in which acid and base are mix together to form a neutral substance alongside water is called as Neutralization reaction.
Acid + Base → Neutral Substance + water 

• HCl + NaOH → NaCl + H₂O 
• H₂SO₄ + Ca(OH)₂ → CaSO₄ + 2 H₂O

Examples of Neutralisation in Daily Life

1. Ant bite → Ant injects formic acid which causes swelling and irritation in skin. Applying baking soda (base) gives relief by neutralizing the effect of acid. 
2. Soil treatment → Due to repetitive farming, soil loos nutrients quality and become either too acidic or too basic. It can be treated using neutralization reaction. 
1. Acidic soil → Add lime (base) or basic fertilizers. 
2. Basic soil → Add organic matter (releases acids) or acidic fertilizers. 
3. Treatment of factory waste: Factory waste causes problems to aquatic life when disposed into the ocean. The effect of Acidic waste is neutralised by adding bases before release. 
4. To treat acidity in stomach: When extra hydrochloric acid is released in our stomach, we feel acidity. Drinking Eno (contains bases such as baking soda and milk of magnesia) neutralizes the effect of acid.

• Acids: These are the substances which release H+ ions in water. 
• Bases: These are the substances which release OH– ions in water. 
• pH (Power of Hydrogen ion) : It is a number from 0 to 14 which depicts how strong an acid or base is. 
• Universal Indicator: It is a mixture of many indicators of different pH range. It shows different colour at different pH levels.