Acids, Bases and Salts: Class 7 Science Chapter 5 Notes

Key Terms and Definitions

Acid

A chemical substance that is sour in taste. Examples: Curd (lactic acid), lemon juice (citric acid), vinegar (acetic acid).

Base

A chemical substance that is bitter in taste and feels soapy to touch. Examples: Baking soda (sodium bicarbonate), soap, lime water (calcium hydroxide).

Indicator

A special substance used to test whether a substance is acidic or basic. It changes its colour when added to a solution containing an acidic or a basic substance.

Neutral Substance

A substance that is neither acidic nor basic. Example: Distilled water, sugar solution, salt solution.

Neutralisation

The reaction between an acid and a base. This reaction produces salt and water, and heat is evolved.

Salt

A substance formed by the neutralisation reaction between an acid and a base. Salts can be acidic, basic, or neutral in nature.

Litmus

A natural dye extracted from lichens, used as an indicator. It turns red in acidic solutions and blue in basic solutions.

Acids vs. Bases: Key Differences

Understanding Indicators

How do we know if a substance is an acid or a base without tasting it? Tasting unknown chemicals is extremely dangerous! This is where indicators come in. Indicators are special substances that change their color when they come into contact with an acid or a base. This color change acts as a signal, telling us the chemical nature of the substance.

There are two main types of indicators: natural and synthetic. In this chapter, we focus on natural indicators.

1. Litmus: This is the most common indicator, extracted from lichens. It is available as a solution or as strips of paper (litmus paper), which come in two colors: red and blue. A simple rule to remember is BRA (Blue litmus turns Red in Acid). Consequently, bases turn red litmus blue.
2. Turmeric: The yellow spice in your kitchen is also a brilliant indicator! A turmeric stain on a white cloth turns reddish-brown when soap (which is basic) is applied to it. In acidic solutions, turmeric remains yellow.
3. China Rose Petals (Gudhal): An indicator can be prepared by soaking China rose petals in warm water. This light pink solution turns dark pink (magenta) in an acidic solution and green in a basic solution.

By observing these color changes, we can safely classify substances as acidic, basic, or neutral (which show no color change with any indicator).

Must-Remember Facts

• {"point":"Acids are sour. Bases are bitter and soapy."}
• {"point":"Acids turn blue litmus paper to red. Memory tip: Acid turns litmus Red (A comes before B, R comes before U in 'blue')."}
• {"point":"Bases turn red litmus paper to blue. Memory tip: Base turns litmus Blue."}
• {"point":"The reaction between an acid and a base is called a neutralisation reaction."}
• {"point":"Products of a neutralisation reaction are always Salt and Water. Heat is also evolved."}
• {"point":"General equation for neutralisation: Acid + Base → Salt + Water + Heat."}
• {"point":"Turmeric turns reddish-brown in basic solutions."}
• {"point":"China rose indicator turns magenta in acids and green in bases."}
• {"point":"Antacids (like milk of magnesia) are bases used to neutralize excess acid in the stomach."}
• {"point":"An ant's sting injects formic acid, which can be neutralized by applying a mild base like baking soda."}

The Neutralisation Reaction Explained

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Neutralisation in Everyday Life

• {"example":"Indigestion","explanation":"Problem: Our stomach produces hydrochloric acid to digest food. Too much acid causes indigestion (acidity). \nSolution: We take an antacid like Milk of Magnesia, which contains a base (Magnesium Hydroxide). This base neutralizes the excess acid, providing relief."}
• {"example":"Ant Bite","explanation":"Problem: When an ant bites, it injects acidic liquid (Formic Acid) into the skin, causing pain and swelling. \nSolution: Rubbing a mild base like moist baking soda (Sodium Hydrogen carbonate) or calamine solution (containing Zinc Carbonate) on the affected area neutralizes the acid."}
• {"example":"Soil Treatment","explanation":"Problem: Excessive use of chemical fertilizers can make the soil too acidic, which is bad for plant growth. \nSolution: Farmers treat acidic soil with bases like quicklime (calcium oxide) or slaked lime (calcium hydroxide) to neutralize the excess acid and improve soil fertility."}

Exam Traps and Scoring Tips

A very common area for mistakes is memorizing the color changes for different indicators. Create a simple table and review it often. Examiners love to ask questions like, 'What color will China rose indicator be in a soap solution?'. You must remember that soap is a base, and China rose turns green in a base. Another key point is the neutralisation reaction. Always remember the two main products: salt and water. Simply writing 'salt' might not get you full marks if the question asks for all products. Mentioning that heat is evolved is a bonus point!

Quick Revision Check

• Q: Name one acid and one base commonly found in a kitchen. A: Acid: Vinegar (acetic acid) or Lemon Juice (citric acid). Base: Baking soda (sodium bicarbonate).
• Q: What happens when you add a drop of soap solution to turmeric paste? Why? A: The turmeric paste will turn reddish-brown. This is because soap solution is basic, and turmeric acts as an indicator that changes to reddish-brown in the presence of a base.
• Q: You are given three unlabelled test tubes containing distilled water, an acidic solution, and a basic solution. How will you identify them using only red litmus paper? A: Dip the red litmus paper in all three. The solution that turns the red litmus paper blue is the base. Now, use this blue litmus paper to test the other two. The one that turns it back to red is the acid. The solution that causes no change in either red or blue litmus paper is distilled water.
• Q: What are the products of a reaction between Hydrochloric Acid (HCl) and Sodium Hydroxide (NaOH)? A: The products are Sodium Chloride (NaCl, a salt), Water (H₂O), and heat is evolved.

Frequently Asked Questions

Frequently Asked Questions