Acid, Base and Salt: Understanding Our Everyday Chemistry (Class 7 Science)

What are Acids? - The Sour Truth!

Imagine biting into a fresh lemon or tasting a juicy raw mango. That tangy, sour taste you experience is characteristic of acids. In chemistry, acids are substances that typically taste sour, though you should never taste chemicals in a lab! Many fruits like oranges, grapes, and tamarind contain natural acids. Acids can also be corrosive, meaning they can eat away at certain materials, which is why we handle them with care. A key property of acids is that they turn blue litmus paper red. Litmus paper is a special paper called an indicator that helps us tell if a substance is an acid or a base. Common acids you might know include citric acid (in citrus fruits), acetic acid (in vinegar), and lactic acid (in curd). Understanding acids helps us explain why some foods are sour and why certain cleaners work the way they do. For example, stomach acid (hydrochloric acid) helps digest our food!

What are Bases? - The Bitter, Soapy Feel!

Have you ever accidentally gotten soap in your mouth? It tastes bitter, right? And when you wash your hands, soap feels slippery or soapy. These are classic characteristics of bases. Bases are chemical substances that typically have a bitter taste and a soapy or slippery feel to the touch. Just like with acids, you should never taste or touch unknown chemicals. Instead of blue litmus, bases have the opposite effect: they turn red litmus paper blue. This is a simple and safe way to identify them. Common examples of bases include baking soda (sodium bicarbonate), washing soda (sodium carbonate), and household cleaning agents like ammonia solution and soap. Many antacids, which we take for stomach acidity, are also bases because they help neutralise the excess acid in our stomach. Knowing about bases helps us understand why certain cleaning products are effective and how remedies for indigestion work.

Indicators: Your Chemical Detectives!

1. Understanding Indicators — Indicators are special substances used to test whether a solution is acidic or basic. They change colour when added to an acidic or basic substance. This colour change acts as a signal, helping us identify the nature of the substance without tasting or touching it.
2. Litmus Paper Test — Litmus paper is the most common indicator. It comes in two colours: blue and red. 1. For Acids: Dip a strip of blue litmus paper into the solution. If it turns red, the solution is acidic. 2. For Bases: Dip a strip of red litmus paper into the solution. If it turns blue, the solution is basic. 3. For Neutral: If neither blue nor red litmus paper changes colour, the solution is neutral (neither acidic nor basic).
3. Turmeric Test (Natural Indicator) — Turmeric is a natural indicator! Its yellow colour changes in bases. 1. Prepare Turmeric Paper: Make a paste of turmeric powder with a little water and apply it on a blotting paper or filter paper. Let it dry. 2. Test with Base: If you put a drop of soap solution (a base) on the turmeric paper, it will turn reddish-brown. 3. Test with Acid/Neutral: It will remain yellow in acidic or neutral solutions.
4. China Rose Petal Solution (Natural Indicator) — China rose (Hibiscus) flowers can also make an indicator! 1. Prepare Solution: Collect some China rose petals and place them in a beaker. Add some warm water and keep it for some time till the water becomes coloured. 2. Test with Acid: Add a few drops of this solution to an acidic solution. It turns the acidic solution to dark pink (magenta). 3. Test with Base: Add a few drops of this solution to a basic solution. It turns the basic solution to green.

Neutralisation Reaction: The Balancing Act!

• What is Neutralisation? Neutralisation is a special type of chemical reaction where an acid reacts with a base to form salt and water. It's like a balancing act where the acidic nature and the basic nature cancel each other out. This reaction is often accompanied by the release of heat, making the reaction mixture warm. Acid + Base → Salt + Water ( + Heat is usually produced)
• Example 1: Hydrochloric Acid and Sodium Hydroxide Let's consider a common example. If you mix hydrochloric acid (a strong acid) with sodium hydroxide (a strong base), they neutralise each other. Reactants: Hydrochloric Acid (HCl) + Sodium Hydroxide (NaOH) Products: Sodium Chloride (NaCl) + Water (H₂O) * Equation (simplified for Class 7): Acid + Base → Salt + Water Here, sodium chloride is common table salt!
• Example 2: Stomach Antacids When we have indigestion, our stomach produces too much hydrochloric acid, causing pain. We take antacids (like milk of magnesia), which contain bases such as magnesium hydroxide. The base in the antacid reacts with the excess acid in the stomach, neutralising it and relieving the pain. * Process: Excess Stomach Acid + Antacid (Base) → Neutralised Stomach + Water + Salt (formed from the reaction) This is a perfect example of neutralisation in our daily lives!

Acids, Bases, and Salts in Daily Life

• Indigestion Relief: Antacids (bases) are used to neutralise excess stomach acid.
• Ant Bite Treatment: Ant bites inject formic acid. Calamine solution (a base) or moist baking soda (a base) is applied to neutralise the acid and reduce irritation.
• Soil Treatment: Farmers use quicklime (calcium oxide, a base) or slaked lime (calcium hydroxide, a base) to treat overly acidic soil, making it suitable for crops. If the soil is too basic, organic matter (which releases acids) is added.
• Factory Waste: Waste from factories often contains acids. These are neutralised with basic substances before being discharged into water bodies to prevent harm to aquatic life.
• Food Preservation: Salts are widely used as preservatives, for example, in pickles and cured meats, to inhibit microbial growth.

Practice Questions with Solutions

• Q: What is an indicator? Give two examples of natural indicators and how they react with acids and bases. A: Step 1: Define indicator. An indicator is a substance that shows different colours in acidic and basic solutions. Step 2: Provide examples and reactions. Two natural indicators are: 1. Litmus: Turns red in acid and blue in base. 2. Turmeric: Remains yellow in acid, but turns reddish-brown in base. 3. China Rose solution: Turns dark pink (magenta) in acid and green in base. Final answer: An indicator helps us identify acids and bases through a colour change. Litmus paper turns red in acid and blue in base. Turmeric stays yellow in acid but turns reddish-brown in a basic solution.
• Q: Describe the neutralisation reaction with an example from daily life. A: Step 1: Define neutralisation. Neutralisation is a reaction between an acid and a base, which produces salt and water. The acidic and basic properties cancel each other out. Step 2: Give a daily life example. When we have indigestion, our stomach produces too much hydrochloric acid. To relieve this, we take an antacid like milk of magnesia (which contains a base, magnesium hydroxide). The base reacts with the excess stomach acid, neutralising it and relieving the discomfort. Final answer: Neutralisation is the reaction where an acid and a base combine to form salt and water, cancelling out their properties. A common example is using antacids (bases) to neutralise excess stomach acid.
• Q: You are given three unknown liquids: A, B, and C. Liquid A turns blue litmus red. Liquid B turns red litmus blue. Liquid C has no effect on either red or blue litmus. Identify the nature of each liquid. A: Step 1: Analyze Liquid A. Liquid A turns blue litmus red. This is a characteristic property of acids. Step 2: Analyze Liquid B. Liquid B turns red litmus blue. This is a characteristic property of bases. Step 3: Analyze Liquid C. Liquid C has no effect on either litmus paper. This indicates it is a neutral substance. Final answer: Liquid A is acidic. Liquid B is basic. Liquid C is neutral.
• Q: Why is it advised not to eat very sour or very bitter substances to identify them? A: Step 1: Recall properties. Sour taste is characteristic of acids, and bitter taste is characteristic of bases. Step 2: Explain the danger. While these are properties, many acids and bases, especially in concentrated forms or unknown substances, can be corrosive or harmful if ingested or touched. Tasting unknown chemicals can be very dangerous and cause severe burns or poisoning. Final answer: It is advised not to taste sour or bitter substances to identify them because many acids and bases can be highly corrosive or toxic. Tasting unknown chemicals is dangerous and can cause severe harm.

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