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State Board · Class 8 · General Science

Lesson 12: Introduction to Acid and Base

5 concepts · 40 practice questions with answers · Free to read

This lesson is also in: मराठी

Acids and their properties

From daily life

A squeeze of lemon makes a stain on a copper vessel shine, and a drop of curd turns milk sour. A sour taste is the sign of a family of substances called acids.

Simple definition

An acid (आम्ल) is a substance that gives hydrogen ions (H+) in water. It tastes sour, is corrosive and turns blue litmus red.

Lemonsour tasteTamarindsour tasteVinegarsour tasteAmlasour taste

In the figure : In the figure lemon, tamarind, vinegar and amla taste sour because each contains an acid.

Rule 1: Natural and mineral acids — Acids in food are natural (organic) acids and are weak. Strong acids like sulphuric acid (H2SO4), hydrochloric acid (HCl) and nitric acid (HNO3) are mineral acids; concentrated solutions burn the skin and their vapours are harmful.

  • Easy : Vinegar contains a weak natural acid.
  • Medium : HCl is a strong mineral acid.
  • Exam-type : Never taste or touch a laboratory acid. Concentrated acids burn the skin.
HClin waterH⁺Cl⁻An acid gives H⁺ ions in water

In the figure : In the figure in water an acid such as HCl breaks up and gives H+ ions: HCl → H+ + Cl−.

Rule 2: Ions of an acid — An acid gives H+ ions in solution. Properties: sour taste; reacts with metals to give hydrogen gas; reacts with carbonates to give CO2; turns blue litmus red.

  • Easy : HCl → H+ + Cl−.
  • Medium : HNO3 → H+ + NO3−: it also gives H+.
  • Exam-type : Every acid gives H+ in water; that is the main constituent of an acid.
Acid into waterslowlyWater into acidquickly

In the figure : In the figure a strong acid is diluted by adding the acid slowly into water. Adding water into concentrated sulphuric acid gives a lot of heat and may explode.

Rule 3: Diluting an acid — Always add the acid slowly into water, never water into concentrated sulphuric acid. Dilute acids are less harmful.

  • Easy : Rule: acid into water.
  • Medium : Water into concentrated H2SO4 gives enormous heat.
  • Exam-type : Adults do this in a lab with care; students should not try it alone.
H₂Metalgives H₂CO₂Carbonategives CO₂Blue litmusturns red

In the figure : In the figure an acid reacts with a metal to give hydrogen gas, with a carbonate to give carbon dioxide, and turns blue litmus red.

Rule 4: Reactions of acids — Acid + metal → salt + hydrogen. Acid + carbonate → salt + water + CO2.

  • Easy : Zn + 2HCl → ZnCl2 + H2.
  • Medium : CaCO3 + 2HCl → CaCl2 + H2O + CO2.
  • Exam-type : Fizzing when an acid meets chalk (a carbonate) is the CO2 escaping.
Remember

Acid: sour, H+, blue litmus → red. Dilute by adding acid to water.

Common mistake

Adding water to concentrated acid. Always acid into water.

Shortcut

Memory clue: "Add Acid to Aqua" (A to A).

Spot the difference

PointDetail
TasteSour
IonH+
With metalHydrogen gas
With carbonateCO2
Blue litmusTurns red

Practice questions

Easy
  1. 1What is an acid?
  2. 2Name three mineral acids.
  3. 3Why do lemon and tamarind taste sour?
Medium
  1. 4What colour does blue litmus turn in an acid?
  2. 5Write the equation for HCl giving ions in water.
  3. 6What gas is released when an acid reacts with a metal?
Challenging
  1. 7How do you dilute concentrated sulphuric acid safely?
  2. 8A friend pours water into concentrated sulphuric acid. Explain what can happen and correct him.
Show answers (try them yourself first)
  1. 1A substance that gives H+ ions in water.
  2. 2Sulphuric, hydrochloric and nitric acid.
  3. 3They contain acids.
  4. 4Red
  5. 5HCl → H+ + Cl−
  6. 6Hydrogen
  7. 7Add the acid slowly into water.
  8. 8Enormous heat is released and it may explode; always add the acid slowly to water.

Bases and their properties

From daily life

Soap feels slippery, and baking soda tastes bitter. These are the opposite of sour. A wash with soap can remove a lemon stain's sourness.

Simple definition

A base (आम्लारी) is a substance that gives hydroxide ions (OH−) in water. It is bitter, slippery and turns red litmus blue.

NaOHin waterNa⁺OH⁻A base gives OH⁻ ions in water

In the figure : In the figure in water a base such as NaOH breaks up and gives OH− ions: NaOH → Na+ + OH−.

Rule 1: Properties of bases — Bases like lime water, baking soda (NaHCO3), caustic soda (NaOH) and soap are bitter and slippery. Their properties are opposite to acids. Distilled water is tasteless: neither acidic nor basic. NaOH and KOH are strong and caustic; concentrated solutions burn the skin by decomposing proteins. Metal oxides are generally basic.

  • Easy : Soap is a base.
  • Medium : Distilled water is neutral.
  • Exam-type : Never touch caustic soda; it burns by decomposing the proteins of the skin.
NaOHsoapKOHshampooCa(OH)₂whitewashMg(OH)₂antacidNH₄OHfertilizers

In the figure : In the figure bases have many uses: NaOH and KOH in soap and shampoo, Ca(OH)2 in white washing, Mg(OH)2 in antacids and NH4OH in fertilizers.

Rule 2: Uses of bases — NaOH (caustic soda): washing soap. KOH (potash): soap and shampoo. Ca(OH)2 (lime water): lime or white washing. Mg(OH)2 (milk of magnesia): antacids. NH4OH: fertilizers.

  • Easy : Milk of magnesia is used in antacids.
  • Medium : Lime water is used for white washing walls.
  • Exam-type : Match: soap: NaOH or KOH; antacid: Mg(OH)2; fertilizer: NH4OH.
Remember

Base: bitter, slippery, gives OH−, red litmus → blue.

Common mistake

Thinking every slippery thing is safe because soap is used daily. Strong bases like NaOH burn.

Shortcut

Memory clue: "Base = Bitter, Brings OH".

Spot the difference

PointAcidBase
TasteSourBitter
TouchNot slipperySlippery
IonH+OH−
Red litmusNo changeTurns blue

Practice questions

Easy
  1. 1What is a base?
  2. 2Name two bases used at home.
  3. 3Write the equation for NaOH giving ions in water.
Medium
  1. 4What does a base do to red litmus?
  2. 5Which base is used in antacids?
  3. 6Which base is used for white washing?
Challenging
  1. 7Is distilled water acidic, basic or neutral? Why?
  2. 8Why are NaOH and KOH called caustic?
Show answers (try them yourself first)
  1. 1A substance that gives OH− ions in water.
  2. 2Baking soda and soap.
  3. 3NaOH → Na+ + OH−
  4. 4Turns it blue.
  5. 5Mg(OH)2 (milk of magnesia)
  6. 6Ca(OH)2 (lime water)
  7. 7Neutral; it is tasteless and neither acidic nor basic.
  8. 8Their concentrated solutions burn the skin by decomposing proteins.

Indicators

From daily life

It is not safe to taste or touch every liquid in the lab to find if it is an acid or a base. So scientists use "colour detectives" which change colour and tell the answer.

Simple definition

An indicator (दर्शक) is a substance that changes colour in the presence of an acid or a base.

In acidIn alkaliLitmusredblueMethyl orangepinkyellowPhenolphthaleincolourlesspinkMethyl redredyellow

In the figure : In the figure indicators change colour with acids and alkalis: litmus is red in acid and blue in alkali, and phenolphthalein is colourless in acid and pink in alkali.

Rule 1: Laboratory indicators — Litmus (from lichen; red or blue paper): blue litmus turns red in acid, red litmus turns blue in alkali. Methyl orange: orange; pink in acid, yellow in alkali. Phenolphthalein: colourless; colourless in acid, pink in alkali. Methyl red: red; red in acid, yellow in alkali. Universal indicator is a liquid that changes colour with acid and base.

  • Easy : Blue litmus in acid: red.
  • Medium : Phenolphthalein in a base: pink.
  • Exam-type : A solution turns methyl orange yellow and phenolphthalein pink: it is a base.
Turmericred with soapSoda + juicefizzingRed cabbagerose, hibiscus

In the figure : In the figure turmeric turns red with soap; baking soda with an acidic juice fizzes; red cabbage, rose and hibiscus also work as natural indicators.

Rule 2: Natural indicators — Turmeric (a yellow stain turns red with soap), red cabbage, radish, tomato, hibiscus and rose can be used as indicators. Baking soda added to acidic fruit juice gives bubbles (effervescence).

  • Easy : A turmeric stain turns red when washed with soap.
  • Medium : A pinch of baking soda in lemon juice fizzes, so lemon juice is acidic.
  • Exam-type : If a hibiscus extract turns pink in a clear liquid, the liquid is likely acidic.
Live figure

Indicators

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Open in the app
Remember

Acid: blue litmus red, methyl orange pink. Base: red litmus blue, phenolphthalein pink.

Common mistake

Mixing up phenolphthalein: it is colourless in acid, pink in base.

Shortcut

Memory clue: "Blue Becomes Red in acid" (B-B-R); "Phenol is Pink in base" (P-P).

Spot the difference

IndicatorIn acidIn base
LitmusBlue to redRed to blue
Methyl orangePinkYellow
PhenolphthaleinColourlessPink
TurmericYellowRed

Practice questions

Easy
  1. 1What is an indicator?
  2. 2From what is litmus obtained?
  3. 3What colour does red litmus turn in a base?
Medium
  1. 4What colour is phenolphthalein in an acid?
  2. 5Name two natural indicators.
  3. 6What happens when baking soda is added to lemon juice?
Challenging
  1. 7A solution turns methyl orange yellow and phenolphthalein pink. Is it an acid or a base?
  2. 8Why are indicators used instead of tasting or touching a liquid?
Show answers (try them yourself first)
  1. 1A substance that changes colour with an acid or a base.
  2. 2From lichen
  3. 3Blue
  4. 4Colourless
  5. 5Turmeric and red cabbage.
  6. 6It fizzes, because the juice is acidic.
  7. 7A base.
  8. 8Tasting or touching acids and bases is harmful.

Natural acids and uses of acids

From daily life

A glass of orange juice, a bowl of curd and a fizzy drink all hold acids that we eat or drink daily. Our own body has acids too: the juice in the stomach and even the DNA in every cell.

Simple definition

Natural acids (नैसर्गिक आम्ले) are acids found in foods and living things.

Vinegaracetic acidOrangeascorbic acidTamarindtartaric acidTomatooxalic acidCurdlactic acidLemoncitric acid

In the figure : In the figure natural acids in foods are: acetic in vinegar, ascorbic in orange, tartaric in tamarind, oxalic in tomato, lactic in curd and citric in lemon.

Rule 1: Natural acids — Vinegar: acetic acid. Orange: ascorbic acid. Tamarind: tartaric acid. Tomato: oxalic acid. Curd: lactic acid. Lemon: citric acid. Aerated drinks: carbonic acid.

  • Easy : Curd has lactic acid.
  • Medium : Lemon: citric acid; orange: ascorbic acid (vitamin C).
  • Exam-type : A fizzy drink contains carbonic acid, so it has bubbles of CO2.
Fertilizersmanufacture+-Batterydilute H₂SO₄Waterdilute HClWhite paperfrom pulpMedicinesand dyesPaintsand colours

In the figure : In the figure acids are used to make fertilizers, medicines, dyes, paints and white paper. Dilute H2SO4 is used in batteries and dilute HCl to sterilize water.

Rule 2: Uses of acids — Acids are used in chemical fertilizers, explosives, oil purification, medicines, dyes and paints. HCl makes chloride salts and (dilute) sterilizes water. Dilute H2SO4 is used in batteries. Acids are used for making white paper from wood pulp. DNA, an acid in our body, decides heredity; proteins are made of amino acids; fat is formed by fatty acids.

  • Easy : Batteries use dilute sulphuric acid.
  • Medium : DNA is an acid that decides heredity.
  • Exam-type : Proteins are made of amino acids, fats of fatty acids.
Remember

Natural acids: acetic (vinegar), citric (lemon), lactic (curd), tartaric (tamarind), oxalic (tomato), ascorbic (orange).

Common mistake

Thinking all acids are dangerous. Natural acids in food are weak and safe.

Shortcut

Memory clue: "V-O-T-T-C-C": Vinegar–acetic, Orange–ascorbic, Tamarind–tartaric, Tomato–oxalic, Curd–lactic, Citrus/lemon–citric.

Spot the difference

FoodAcid
VinegarAcetic
LemonCitric
CurdLactic
TamarindTartaric
TomatoOxalic

Practice questions

Easy
  1. 1Which acid is present in vinegar?
  2. 2Which acid is present in curd?
  3. 3Which acid is present in aerated drinks?
Medium
  1. 4Name two uses of acids in industry.
  2. 5Which acid is used in batteries?
  3. 6Which acid in our body decides heredity?
Challenging
  1. 7Match: orange, tamarind, tomato.
  2. 8Why is a fizzy drink fizzy, and which acid does it contain?
Show answers (try them yourself first)
  1. 1Acetic acid
  2. 2Lactic acid
  3. 3Carbonic acid
  4. 4Making fertilizers and medicines (or dyes, paints).
  5. 5Dilute sulphuric acid
  6. 6DNA
  7. 7Ascorbic, tartaric, oxalic acid.
  8. 8It contains carbonic acid and dissolved CO2 bubbles out.

Neutralization

From daily life

After a heavy, spicy meal you may feel a burning in the chest. A spoon of milk of magnesia or a tablet of antacid calms it. A base is cancelling the extra acid in the stomach.

Simple definition

Neutralization (उदासीनीकरण) is the reaction of an acid with a base to give salt and water.

HClacid+NaOHbaseneutralizationNaClsalt+H₂OwaterH⁺ + OH⁻ → H₂O

In the figure : In the figure acid + base → salt + water: the H+ of the acid and the OH− of the base combine to form water, which is neutralization.

Rule 1: The reaction — Acid contains H+ and base contains OH−. Their combination gives salt and water: Acid + Base → Salt + Water, e.g. HCl + NaOH → NaCl + H2O.

  • Easy : HCl + NaOH → NaCl + H2O.
  • Medium : H2SO4 + 2NaOH → Na2SO4 + 2H2O (balanced: 2 Na, 6 O, 4 H on each side).
  • Exam-type : Mg(OH)2 + 2HCl → MgCl2 + 2H2O is what an antacid does to stomach acid.
Excess acidindigestionAntacidMg(OH)₂Acidneutralised

In the figure : In the figure an antacid containing milk of magnesia neutralises the excess HCl of the stomach and relieves indigestion.

Rule 2: In the body — HCl in the stomach helps digestion, but excess acid causes indigestion. Antacids containing milk of magnesia neutralise it.

  • Easy : Antacid + excess acid → salt + water.
  • Medium : Stomach acid is useful in the right amount, but harmful in excess.
  • Exam-type : Frequent heartburn should be shown to a doctor, not treated only with antacids.
Fertilizersacidic soilLimeneutralisesSoilneutral

In the figure : In the figure excess chemical fertilizers make the soil acidic, and mixing limestone or lime water in the soil neutralises it.

Rule 3: In the soil — Excess use of chemical fertilizers makes soil acidic. Limestone or lime water is mixed in the soil to neutralise it.

  • Easy : Acidic soil is treated with lime.
  • Medium : A farmer adds limestone because it is a base that cancels the soil acid.
  • Exam-type : Overusing fertilizer for years can acidify soil, so the farmer must correct it.
Remember

Acid + base → salt + water. Used in antacids and soil treatment.

Common mistake

Writing the products as only salt. Water is also formed.

Shortcut

Memory clue: "A + B = S + W" (Acid + Base = Salt + Water).

Spot the difference

WhereProblemNeutralised by
StomachExcess acidAntacid (milk of magnesia)
SoilAcidic soilLimestone or lime water

Practice questions

Easy
  1. 1What is neutralization?
  2. 2Write the word equation of neutralization.
  3. 3Write the equation for HCl and NaOH.
Medium
  1. 4Why do we take an antacid for indigestion?
  2. 5What happens to soil if too much chemical fertilizer is used?
  3. 6How is acidic soil neutralised?
Challenging
  1. 7Check that H2SO4 + 2NaOH → Na2SO4 + 2H2O is balanced.
  2. 8A farmer's field has become acidic after years of heavy fertilizer use. Explain what he should do and why it works.
Show answers (try them yourself first)
  1. 1The reaction of an acid with a base to form salt and water.
  2. 2Acid + Base → Salt + Water
  3. 3HCl + NaOH → NaCl + H2O
  4. 4It neutralises the excess HCl in the stomach.
  5. 5It becomes acidic.
  6. 6By mixing limestone or lime water.
  7. 7Left: 2 Na, 6 O, 4 H, 1 S. Right: 2 Na, 6 O, 4 H, 1 S. Balanced.
  8. 8Mix limestone or lime water; it is a base and neutralises the acid in the soil.

Quick revision

  • Acids: sour, give H⁺, turn blue litmus red. Bases: bitter, slippery, give OH⁻, turn red litmus blue.
  • Acid + base → salt + water (neutralization).
  • Never add water to concentrated sulphuric acid; add the acid slowly to water.