Acids, Bases and Salts — CBSE Class 7 Science NCERT

Have you ever wondered why curd or lemon juice tastes sour, while soap feels slippery and tastes bitter? Or why a pinch of baking soda bubbles up instantly when mixed with vinegar? The answers lie in the fascinating world of acids, bases, and salts. In this chapter, we will dive deep into these three distinct families of chemical substances. You will master how to identify them safely without tasting them, using clever natural materials called indicators like turmeric, litmus, and China rose. We will also explore neutralization reactions—what happens when acids and bases battle it out—and how this simple reaction affects our daily lives, from treating indigestion to curing ant bites. With YoLearn AI's interactive guidance, you will easily learn these fundamental chemical reactions and score beautifully in your school exams. Let's make chemistry fun and highly intuitive together!

What are Acids and Bases?

The word acid comes from the Latin word acere, which means sour. Acids are chemical substances that taste sour and turn blue litmus paper red. Common natural acids include citric acid (found in lemons and oranges), lactic acid (found in curd), and acetic acid (found in vinegar). On the other hand, bases are substances that taste bitter and feel soapy or slippery when rubbed between your fingers. Soap, baking soda, and lime water (calcium hydroxide) are classic examples of bases. Since we cannot taste or touch every substance to find out if it is acidic or basic (as some can be highly corrosive and dangerous!), we use special substances called indicators to test them. Indicators change their color when added to a solution containing an acidic or a basic substance.

How Natural Indicators Work (Step-by-Step)

  1. Litmus Paper Test — Litmus is a natural dye extracted from lichens. To test a solution, dip a strip of blue litmus paper into it. If it turns red, the solution is acidic. If you dip red litmus paper into a basic solution, it turns blue. Neutral solutions do not change the color of either litmus paper.
  2. Turmeric Indicator Test — Turmeric is a bright yellow natural indicator. Mix turmeric powder with water to make a paste, then deposit it on paper to create test strips. When you add a basic solution (like soap water) to turmeric, it turns red. It remains yellow in acidic or neutral solutions.
  3. China Rose (Gudhal) Test — Collect some China rose petals and place them in warm water until the water becomes colored. This light pink water acts as an indicator. When added to an acidic solution, it turns dark pink (magenta). When added to a basic solution, it turns green.

Neutralization in Everyday Life

  • Indigestion Treatment: Our stomach contains hydrochloric acid (HCl) to help digest food. But too much acid causes painful indigestion. To neutralize this excess acid, we take mild bases called antacids, such as Milk of Magnesia (which contains magnesium hydroxide).
  • Ant Bite Relief: When an ant bites, it injects acidic formic acid into our skin, causing irritation. This can be neutralized by rubbing a moist basic substance, such as baking soda (sodium hydrogen carbonate) or calamine solution (which contains zinc carbonate), on the skin.
  • Soil Treatment: Plants do not grow well if the soil is too acidic or too basic. Acidic soil is treated with quicklime (calcium oxide) or slaked lime (calcium hydroxide), which are bases. Basic soil is treated with organic matter, which releases acids to neutralize it.
  • Factory Waste Neutralization: Factory waste often contains harmful acids. If discharged directly into water bodies, it can kill fish. Therefore, factories treat and neutralize these wastes with basic substances before disposing of them.

Smart Tips for Exam Success

  1. Don't Mix Up Colors: Students often get confused between litmus paper changes. Remember the acronym BAR: Base turns litmus Blue, Acid turns litmus Red.
  2. Neutralization Equation: In exams, always write down the full equation: Acid + Base -> Salt + Water + Heat. This shows the evaluator that you understand both the product and the energy change.
  3. Is Salt always Neutral?: A common mistake is thinking all salts are neutral. Salts can actually be acidic, basic, or neutral depending on the strength of the reacting acid and base!

Practice Questions with Solutions

  • Q: State the differences between acids and bases.
    A: Step 1: Compare their tastes. Acids taste sour, while bases taste bitter.
    Step 2: Compare their textures. Acids do not have a specific texture, but bases feel soapy or slippery when rubbed between fingers.
    Step 3: Compare their reactions with litmus. Acids turn blue litmus red, whereas bases turn red litmus blue.
    Final answer: Acids taste sour and turn blue litmus red. Bases taste bitter, feel slippery, and turn red litmus blue.

  • Q: A blue litmus paper is dipped in a solution and it remains blue. What is the nature of the solution? Explain.
    A: Step 1: Analyze how acids react. An acidic solution would turn blue litmus paper red.
    Step 2: Since the paper remained blue, the solution is not acidic.
    Step 3: Analyze the behavior of bases and neutral substances. Basic solutions turn red litmus blue but do not change blue litmus. Neutral solutions do not change the color of either litmus paper.
    Final answer: The solution is either basic or neutral.

  • Q: Why is an antacid tablet taken when you suffer from acidity?
    A: Step 1: Identify the chemical cause of indigestion. The stomach secretes excess hydrochloric acid, leading to a burning sensation.
    Step 2: Identify the type of compound in an antacid. Antacids contain mild bases like magnesium hydroxide (Milk of Magnesia).
    Step 3: Explain the reaction. The base reacts with the excess acid, neutralising its effect and producing harmless salt and water.
    Final answer: Antacids contain mild bases that neutralize excess hydrochloric acid in the stomach to relieve acidity.

  • Q: Three liquids are given to you: hydrochloric acid, sodium hydroxide, and a sugar solution. How will you identify them using only turmeric indicator?
    A: Step 1: Test all three liquids with the turmeric indicator. Turmeric paper turns red only in a basic solution. The liquid that turns turmeric red is sodium hydroxide (base).
    Step 2: Take a small amount of the identified sodium hydroxide in two separate test tubes and mix a few drops of the remaining two liquids into them respectively to allow neutralization.
    Step 3: Test both mixtures with turmeric paper. The mixture containing the acid (hydrochloric acid) will be neutralized and will not turn turmeric red. The sugar solution mixture will remain basic and turn turmeric red.
    Final answer: Sodium hydroxide turns turmeric red; the acid is identified because mixing it with sodium hydroxide neutralizes the basic effect, keeping turmeric yellow.