Acids, Bases and Salts
When you bite into a lemon, your mouth puckers up. When you touch soap, your fingers feel slippery. When you put baking soda on an ant bite, the stinging eases. These are all examples of acids and bases at work — right in your kitchen and your body!
Let’s understand what acids and bases are, how we identify them, and what happens when they meet each other.
What Are Acids?
Acids are substances that taste sour and have certain chemical properties.
You’ve already tasted acids without knowing it:
- Lemon juice contains citric acid
- Vinegar contains acetic acid
- Curd (yoghurt) contains lactic acid
- Tomatoes contain tartaric and citric acid
- Tamarind contains tartaric acid
In science, we never taste or touch unknown substances to identify them. We use safe tests called indicators. The “taste sour” rule is just to help you remember from daily life.
Properties of Acids
- They taste sour.
- They turn blue litmus paper red.
- They do NOT change red litmus paper.
- They have a pH less than 7.
- They react with metals to produce hydrogen gas.
- They are corrosive — strong acids can burn skin. Handle with care!
What Are Bases?
Bases are substances that taste bitter, feel slippery, and neutralise acids.
Common bases you’ll find at home:
- Baking soda (sodium bicarbonate)
- Washing soda (sodium carbonate)
- Lime water (calcium hydroxide)
- Soap (contains sodium or potassium hydroxide)
- Milk of magnesia (used as antacid)
- Toothpaste (mildly basic)
Properties of Bases
- They taste bitter.
- They feel slippery or soapy to touch.
- They turn red litmus paper blue.
- They do NOT change blue litmus paper.
- They have a pH greater than 7.
- Strong bases are also corrosive.
Memory trick: Acids = sour, change Blue litmus to Red (remember: “ABR” — Acid, Blue, Red). Bases = bitter, change Red litmus to Blue (remember: “BRB” — Base, Red, Blue).
What Is a Neutral Substance?
A substance that is neither acidic nor basic is called neutral.
Examples: Pure water, common salt (sodium chloride) solution, sugar solution.
Neutral substances:
- Have a pH of exactly 7.
- Do NOT change the colour of either red or blue litmus paper.
What Are Indicators?
An indicator is a substance that changes its colour in the presence of an acid or a base. Indicators help us identify whether a substance is acidic, basic, or neutral.
Litmus
Litmus is the most commonly used indicator. It is extracted from lichens (a type of plant-like organism).
Litmus comes in two forms:
- Blue litmus paper — turns red in acids, stays blue in bases
- Red litmus paper — stays red in acids, turns blue in bases
| Substance | Blue litmus | Red litmus |
|---|---|---|
| Acid | Turns red | No change |
| Base | No change | Turns blue |
| Neutral | No change | No change |
Litmus is also available as a solution:
- Litmus solution is purple.
- In acid: turns red.
- In base: turns blue.
Turmeric (Haldi)
Turmeric is a natural indicator that you have in your kitchen!
- In neutral or acidic substances: remains yellow.
- In bases: turns red-brown.
You may have seen yellow turmeric stains on clothes turn red-brown when they come in contact with soap (which is basic). That’s the indicator reaction happening!
We can make turmeric indicator paper by dipping filter paper strips in turmeric solution and letting them dry.
China Rose (Gudhal)
China rose petals contain a natural dye that acts as an indicator.
- In acids: turns dark pink (magenta).
- In bases: turns green.
We make china rose indicator by soaking the petals in warm water.
Litmus vs Natural Indicators
| Indicator | Made from | Acid colour | Base colour |
|---|---|---|---|
| Litmus | Lichen | Red | Blue |
| Turmeric | Turmeric root | Yellow (no change) | Red-brown |
| China rose | Flower petals | Dark pink/magenta | Green |
Testing Household Substances
We can test common substances from home to classify them.
| Substance | Nature |
|---|---|
| Lemon juice | Acid |
| Vinegar | Acid |
| Curd | Acid |
| Tomato juice | Acid |
| Baking soda solution | Base |
| Washing soda solution | Base |
| Soap solution | Base |
| Milk | Slightly acidic |
| Tap water | Nearly neutral |
| Sugar solution | Neutral |
| Salt solution | Neutral |
In exams, you may be asked: “Which indicator will you use to test if a substance is acidic or basic?” The safest answer is litmus. Turmeric is also acceptable — but remember turmeric can’t distinguish between neutral and acidic (both remain yellow).
Neutralisation — When Acid Meets Base
What happens when we mix an acid with a base?
They react with each other! The acid and base cancel each other out, and we get a salt and water.
Acid + Base → Salt + Water
This reaction is called neutralisation because the acidic and basic properties are neutralised (cancelled out).
The solution becomes neutral (or close to neutral) after neutralisation.
Example:
Hydrochloric acid + Sodium hydroxide → Sodium chloride + Water
(Common salt is produced — the same salt we eat!)
How Does It Feel?
Neutralisation produces heat. If you mix a strong acid with a strong base, the mixture becomes warm. This heat production is called the heat of neutralisation.
Everyday Examples of Neutralisation
1. Antacids for Stomach Upset
Your stomach contains hydrochloric acid (HCl) to digest food. When it produces too much acid, you feel acidity or heartburn.
The solution? Take an antacid like milk of magnesia or ENO. Antacids are basic substances. They neutralise the extra acid in your stomach, giving relief.
Extra stomach acid + Antacid (base) → Salt + Water
2. Ant Bite Treatment
An ant injects formic acid when it bites you — that’s why it stings.
To relieve the sting, apply baking soda (which is basic). The baking soda neutralises the formic acid.
Formic acid (from ant) + Baking soda (base) → Salt + Water
3. Bee Sting vs Wasp Sting
Bee stings are acidic (formic acid) — treat with a base like baking soda.
Wasp stings are basic (alkaline) — treat with a mild acid like vinegar.
4. Treating Acidic Soil
Sometimes soil becomes too acidic due to rainfall or fertilisers, and crops don’t grow well.
Farmers add lime (calcium hydroxide) — a base — to the soil. This neutralises the excess acid and makes the soil suitable for farming.
5. Factory Waste Treatment
Factories that produce acidic waste treat it with lime or other basic substances before releasing it into water bodies. This prevents pollution and harm to aquatic life.
6. Tooth Decay
Bacteria in our mouth act on leftover food particles and produce acids. These acids slowly dissolve the enamel of our teeth, causing tooth decay.
Toothpaste is basic in nature. When we brush, the toothpaste neutralises the acids produced by bacteria, protecting our teeth.
What Is a Salt?
When an acid reacts with a base, the product (apart from water) is called a salt.
Common salt (NaCl = Sodium Chloride) is formed when hydrochloric acid reacts with sodium hydroxide.
Different acid-base pairs produce different salts. Salts are neither acidic nor basic — most are neutral, though some salts can be slightly acidic or basic.
5 Common Mistakes to Avoid
Mistake 1: Saying turmeric turns blue in bases
Turmeric turns RED-BROWN (or reddish) in bases — not blue. Blue is what happens to litmus in acids. Don’t mix them up!
Mistake 2: Confusing what litmus does in acids vs bases
A classic exam trap. Remember: Blue litmus turns Red in Acids. Red litmus turns Blue in Bases. The change always goes “towards the other colour.” An easy way: Acids and Blue don’t get along — blue turns red. Bases and Red don’t get along — red turns blue.
Mistake 3: Forgetting that neutral substances don’t change litmus
Many students say neutral substances turn litmus green or yellow. Wrong! Neutral substances cause NO change in litmus paper (neither blue nor red litmus changes colour in a neutral solution).
Mistake 4: Writing “acid + base = acid” or “acid + base = base”
Neutralisation produces SALT + WATER, not more acid or more base. The point of neutralisation is that both are cancelled out.
Mistake 5: Treating bee and ant stings the same way
Ant bite = acid → treat with base (baking soda). Bee sting is also acidic (formic acid) → treat with base. Wasp sting = basic → treat with acid (vinegar).
Don’t say “all insect stings are treated with vinegar” — that’s wrong for ant/bee stings.
Practice Questions
Question 1: What happens when blue litmus paper is dipped in lemon juice?
Lemon juice is acidic (contains citric acid). Blue litmus paper turns red in an acid.
Answer: The blue litmus paper will turn red.
Question 2: Name two natural indicators other than litmus.
- Turmeric (haldi) — turns red-brown in bases, stays yellow in acids
- China rose (gudhal/hibiscus) — turns magenta/dark pink in acids, turns green in bases
Question 3: Classify the following as acid, base, or neutral: (a) Curd (b) Baking soda (c) Pure water (d) Vinegar
(a) Curd → Acid (contains lactic acid, tastes sour) (b) Baking soda → Base (sodium bicarbonate, used in antacids) (c) Pure water → Neutral (pH = 7) (d) Vinegar → Acid (contains acetic acid, tastes sour)
Question 4: What happens when turmeric paper is dipped in soap solution? Why?
Soap solution is basic in nature. Turmeric indicator turns red-brown in the presence of a base.
So the turmeric paper will change from yellow to red-brown when dipped in soap solution.
Question 5: How does an antacid provide relief from acidity?
When the stomach produces excess hydrochloric acid, it causes a burning sensation (acidity).
Antacids (like milk of magnesia, baking soda) are basic substances. When we take an antacid, it neutralises the excess acid in the stomach:
Acid (stomach) + Base (antacid) → Salt + Water
This brings the pH back to normal and provides relief from the burning sensation.
Question 6: A factory releases acidic waste water. How can this be treated before releasing into a river?
We can add a base (like lime — calcium hydroxide) to the acidic waste water.
The base neutralises the acid: Acid (waste water) + Lime (base) → Salt + Water
This reduces the acidity of the water to safe levels before it is released into the river, preventing harm to aquatic plants and animals.
Question 7: Why does the farmer add lime to soil that is too acidic?
Lime (calcium hydroxide) is a base.
When soil is too acidic, crops don’t grow well because most crops need a slightly acidic to neutral soil.
Adding lime neutralises the excess acid in the soil (acid + lime → salt + water), bringing the soil pH to a range suitable for farming. This improves crop growth.
Question 8: Explain why brushing teeth helps prevent tooth decay.
Bacteria in the mouth break down food particles (especially sugar) and produce acids. These acids attack tooth enamel, slowly dissolving it — this is tooth decay.
Toothpaste is basic. When we brush, the toothpaste neutralises the acids produced by bacteria: Acid (from bacteria) + Toothpaste (base) → Salt + Water
This neutralisation protects the enamel and prevents tooth decay. That’s why dentists recommend brushing twice a day — especially after meals.
Frequently Asked Questions
Q1: Is milk acidic or basic?
Milk is slightly acidic — its pH is around 6.5-6.7. It turns blue litmus very slightly pink. However, for most Class 7 purposes, we can say milk is nearly neutral. If milk turns sour (curd), it becomes more acidic.
Q2: Can water be acidic?
Pure water is neutral (pH 7). But rainwater is slightly acidic (pH ~5.6) because it dissolves carbon dioxide from the air to form carbonic acid. Acid rain has a pH below 5.6 — it’s harmful to plants and buildings.
Q3: What is the difference between an acid and a strong acid?
Acids vary in strength. Lemon juice and vinegar are weak acids — they are safe to consume in small amounts. Hydrochloric acid (HCl) and sulphuric acid (H₂SO₄) are strong acids — they are highly corrosive and dangerous. In Class 7, we mainly deal with weak, everyday acids.
Q4: What is pH?
pH is a scale from 0 to 14 that measures how acidic or basic a solution is.
- pH < 7 = Acidic (lower = more acidic)
- pH = 7 = Neutral
- pH > 7 = Basic (higher = more basic)
You’ll study pH in more detail in Class 10.
Q5: Are all bases soluble in water?
No. Bases that dissolve in water are called alkalis. Sodium hydroxide and potassium hydroxide are alkalis. Copper hydroxide is a base but is insoluble in water. So all alkalis are bases, but not all bases are alkalis.
Q6: Why does our skin feel slippery when we touch soap?
Soap is basic. Bases react with the oils and fats on our skin surface. This reaction gives a slippery, smooth feeling. That slippery sensation is actually a clue that the substance is basic!