Acids, Bases and Salts – Class 7 Lecture

These flashcards review definitions, indicators, properties, examples, reactions and everyday applications of acids, bases, salts and neutralisation as covered in the lecture.

What is an acid?

A substance that reacts with a base to form a salt and water; acids taste sour and turn blue litmus red.

What is a base?

A substance that neutralises an acid to form a salt and water; bases taste bitter, feel soapy, and turn red litmus blue.

What is a salt?

The product formed when an acid reacts with a base during neutralisation.

What is an acid-base indicator?

A dye that changes colour in acidic and basic solutions, helping identify their nature.

Name three natural indicators.

Litmus, China rose (Hibiscus) extract, Turmeric.

Give one synthetic indicator used in the lab.

Phenolphthalein.

From which organism is litmus obtained and what is its natural colour?

Litmus is extracted from lichens and is naturally purple in neutral solution.

What colour does blue litmus turn in an acid?

Red.

What colour does red litmus turn in a base?

Blue.

How can you identify an unknown solution with both blue and red litmus papers?

If it turns blue litmus red, it is acidic; if it turns red litmus blue, it is basic; no change indicates neutrality.

China rose indicator colour in acidic solution?

Magenta (deep pink).

China rose indicator colour in basic solution?

Green.

Turmeric paper colour in acid?

Remains yellow.

Turmeric paper colour in base?

Turns red.

Phenolphthalein colour in acid?

Colourless.

Phenolphthalein colour in base?

Pink.

What are neutral substances?

Materials whose aqueous solutions do not change the colour of any indicator; they are neither acidic nor basic.

Give three examples of neutral substances.

Distilled water, common salt solution, glucose solution.

Define organic (natural) acids.

Acids occurring naturally in plants or animals, usually weak (e.g., citric acid in lemon).

Define mineral acids.

Man-made acids prepared from earth’s minerals, generally strong, e.g., hydrochloric acid.

List four organic acids with their sources.

Citric acid – citrus fruits; Lactic acid – curd; Tartaric acid – tamarind; Formic acid – ant sting.

Name the three common laboratory (mineral) acids.

Hydrochloric acid, sulphuric acid, nitric acid.

What is a strong acid? Give two examples.

An acid that ionises completely in water; examples: HCl and H2SO4.

What is a weak acid? Give two examples.

An acid that ionises partially in water; examples: acetic acid and citric acid.

Name two strong bases.

Sodium hydroxide (NaOH) and potassium hydroxide (KOH).

Name two weak bases safe enough for medicinal use.

Magnesium hydroxide (in milk of magnesia) and sodium hydrogencarbonate (baking soda).

What is an alkali?

A base that is soluble in water.

Give three household examples of alkalis.

Sodium hydroxide in soap, calcium hydroxide in lime water, ammonium hydroxide in window cleaners.

Describe the taste and feel of bases.

They taste bitter and feel slippery or soapy to touch.

State the litmus rule for acids and bases.

Acids turn blue litmus red; bases turn red litmus blue.

Define neutralisation.

The reaction in which an acid reacts with a base to produce salt and water, releasing heat.

What are the products of a neutralisation reaction?

Salt, water, and heat.

Write the word equation for the reaction of HCl with NaOH.

Sodium hydroxide + Hydrochloric acid → Sodium chloride + Water.

Why does a test-tube feel warm after a neutralisation reaction?

Because the reaction releases heat (it is exothermic).

What causes indigestion and how is it treated?

Excess hydrochloric acid in the stomach; treated with basic medicines called antacids.

Name a common antacid and its active base.

Milk of magnesia containing magnesium hydroxide.

Which acid is injected during an ant sting and how can it be neutralised?

Formic acid; neutralise with a mild base such as baking soda paste or calamine (zinc carbonate) lotion.

How is too acidic soil treated in agriculture?

By adding bases like quicklime (calcium oxide) or slaked lime (calcium hydroxide) to neutralise excess acid.

How is too basic soil corrected?

By adding decaying organic matter (manure/compost) which produces acids to neutralise excess base.

Why must acidic factory effluents be treated before discharge?

To neutralise acids with bases, preventing harm to aquatic life and the environment.

Define acid rain.

Rainwater with higher-than-normal acidity due to dissolved sulphuric, nitric, and carbonic acids formed from pollutant gases.

Name three gases responsible for acid rain.

Sulphur dioxide, nitrogen dioxide, carbon dioxide.

List two damages caused by acid rain.

Kills aquatic animals and corrodes buildings/monuments.

What are neutral salts? Give two examples.

Salts whose aqueous solutions are neutral, e.g., sodium chloride (NaCl) and sodium sulphate (Na2SO4).

What are acidic salts? Give two examples.

Salts whose solutions are acidic, formed from strong acids + weak bases, e.g., ammonium chloride (NH4Cl) and ammonium sulphate [(NH4)2SO4].

What are basic salts? Give two examples.

Salts whose solutions are basic, formed from strong bases + weak acids, e.g., sodium carbonate (Na2CO3) and sodium hydrogencarbonate (NaHCO3).

How is the name of a salt related to its parent acid and base?

First part from the base (metal or ammonium), second part from the acid (e.g., chloride from HCl).

Salts of hydrochloric acid are called ____.

Chlorides.

Salts of sulphuric acid are called ____.

Sulphates.

Why is vinegar used as a food preservative?

It contains acetic acid, a weak organic acid that inhibits microbial growth.

Which weak acid gives fizz and taste to soft drinks?

Carbonic acid (H2CO3).

Why is soap solution basic?

It contains sodium or potassium salts of fatty acids that behave as bases; turmeric turns red in its presence.

How does toothpaste prevent tooth decay?

Being basic, it neutralises acids produced by mouth bacteria, protecting enamel.

Define ‘strong’ vs ‘weak’ in the context of acids and bases.

Strong acids/bases ionise completely in water; weak ones ionise only partially.

What is the difference between a base and an alkali?

All alkalis are bases that dissolve in water; not all bases are soluble, so not all are alkalis.

Give two household products containing bases.

Baking soda (sodium hydrogencarbonate) and milk of magnesia (magnesium hydroxide).

What colour does phenolphthalein turn when a neutralised solution becomes slightly basic?

Light pink, signalling completion of neutralisation.

Why are concentrated mineral acids dangerous?

They are highly corrosive and can burn skin and clothes, requiring careful handling.

What does the yellow-to-red change of turmeric stain by soap indicate?

Soap is basic, as bases turn turmeric red.