Exploring Substances: Acidic, Basic, and Neutral – Study module with Revision Notes

1. What Are Acids, Bases and Neutral Substances?

All substances around us have certain chemical properties. Some taste sour, some feel soapy, and some are neither sour nor soapy. One of the most important ways of grouping substances in chemistry is into acids, bases and neutral substances.

1.1 Acids – Sour-Tasting Substances

Substances which taste sour are generally acids. The sour taste comes from special particles called hydrogen ions (H⁺), but at Class 7 level we simply remember that acids are sour and react in a particular way with indicators.

Strong mineral acids like hydrochloric acid (HCl), sulphuric acid (H₂SO₄) etc. are used in laboratories and industries. These are corrosive and must be handled with great care.

1.2 Bases – Substances that Feel Soapy

Substances that taste bitter and feel soapy or slippery when touched are generally bases. They show characteristic changes with indicators and can also be corrosive.

Strong bases like sodium hydroxide (NaOH) and potassium hydroxide (KOH) are widely used in factories to make soap, detergents and other products. They can also burn the skin.

1.3 Neutral Substances

Some substances are neither acidic nor basic. They do not change the colour of most indicators and are called neutral substances.

Neutral substances are very important in nature. For example, our body tries to keep the blood almost neutral so that all organs can work properly.

2. Indicators – How Do We Identify Acids and Bases?

We cannot always taste or touch a substance to know whether it is acidic or basic because that may be unsafe. Scientists use special substances called indicators which change colour in acids and bases.

2.1 Natural Indicators – Litmus, Turmeric, China Rose

Many plants contain coloured chemicals that act as natural indicators. Some commonly used ones in Class 7 are:

• Litmus – obtained from lichens (a type of plant-like organism).
• Turmeric – a common kitchen spice.
• China rose (hibiscus) petals – gives a coloured extract.

Turmeric paste or solution remains yellow in acids and neutral solutions. It turns reddish-brown when a base is added. Many “soap vs. no soap” tests are done using turmeric.

3. Neutralisation – When Acids and Bases React

When an acid and a base are mixed in the right amounts, they react with each other and form a salt and water. The effect of both acid and base is cancelled. This process is called neutralisation.

During neutralisation, the resulting solution may become neutral or slightly acidic/basic depending on the amounts used. Neutralisation has many uses in our everyday life.

3.1 Neutralisation in Daily Life

• Indigestion: Our stomach produces hydrochloric acid for digestion. Sometimes, excess acid causes a burning sensation or pain (acidity). Doctors prescribe antacids like milk of magnesia which are basic and neutralise the extra acid.
• Soil treatment: Some soils become too acidic due to acid rain or excessive use of fertilisers. Farmers spread quicklime (calcium oxide) or slaked lime (calcium hydroxide) to neutralise the acidity so plants can grow well.
• Bee sting: Some insect stings inject acidic substances into our skin, causing pain and swelling. Applying a mild base like baking soda solution helps neutralise the acid.
• Wasp sting: Wasp stings are often basic. A mild acid like vinegar (acetic acid) is sometimes used to reduce the effect.

4. Simple Idea of pH (Conceptual)

In higher classes, you will learn in detail about a scale called the pH scale. It is used to measure how acidic or basic a solution is. For Class 7, we just need a simple idea:

• Solutions with pH less than 7 are acidic.
• Solutions with pH equal to 7 are neutral.
• Solutions with pH more than 7 are basic.

Strong acids have very low pH, and strong bases have very high pH. Our body fluids work properly only in a narrow pH range, so any big change can be harmful.

5. Acids, Bases and the Environment

The chapter also helps us understand how acidic and basic substances affect our environment. One important example is acid rain.

5.1 Acid Rain

When factories and vehicles release gases like sulphur dioxide (SO₂) and nitrogen oxides (NOₓ) into the air, these gases dissolve in rainwater and form acids. This makes the rainwater more acidic and is called acid rain.

To reduce acid rain, we must control air pollution by using cleaner fuels, following emission norms and planting more trees.

6. Uses of Acids and Bases in Daily Life and Industry

Acids and bases are not only present in food and cleaning materials; they are also used in many industries. Knowing about them helps us understand how science supports our daily life.

6.1 Uses of Acids

• Hydrochloric acid is present in our stomach to help digest food.
• Sulphuric acid is used in car batteries and in manufacturing fertilisers.
• Acetic acid (in vinegar) is used as a food preservative and in cooking.
• Citrus acids in fruits provide sour taste and are rich in vitamin C.

6.2 Uses of Bases

• Sodium hydroxide is used in making soap and detergents.
• Calcium hydroxide (lime water) is used in whitewashing walls and in treating acidic soil.
• Magnesium hydroxide is used in antacid medicines to neutralise excess stomach acid.
• Ammonium hydroxide is used in cleaning solutions and in fertilisers.

7. Safety While Handling Acids and Bases

Because strong acids and bases are corrosive, some basic safety rules must always be followed in the laboratory and at home.

• Never taste or smell any chemical directly.
• Always wear safety goggles and, if needed, gloves in the lab.
• Handle acid and base bottles with both hands and keep them closed after use.
• If acid is to be diluted, always add acid slowly to water, never the other way round (to avoid sudden heating and splashing).
• If any chemical spills on the skin, wash immediately with plenty of water and inform the teacher.

8. Quick Revision Summary for Exams

• Substances are grouped into acids, bases and neutral substances based on their properties.
• Acids are usually sour and turn blue litmus red.
• Bases are bitter, soapy to touch and turn red litmus blue.
• Neutral substances are neither acidic nor basic and do not change the colour of litmus.
• Indicators (litmus, turmeric, china rose) help us identify acids and bases safely.
• Neutralisation is the reaction between an acid and a base to form salt and water.
• Neutralisation is used in indigestion, soil treatment, insect stings and many other cases.
• Acid rain is caused by pollutants and harms plants, animals and buildings.
• Acids and bases have many uses in industry, medicine, food and cleaning.
• Always follow lab safety rules while handling corrosive substances.

If you understand these key ideas with examples, you will be able to answer short answer, long answer, MCQ and application-based questions from this chapter confidently in your CBSE Class 7 exams.