Solution: We can identify the content in each of the test tubes using red litmus paper. This can be done by noticing the color change of the red litmus paper.
* If the red litmus paper changes to blue color the solution is a basic solution.
* If the red litmus paper experience no change in acidic solution.
* If the red litmus paper changes to purple color the solution is distilled water.

Solution: Curd and sour food substances contain acids; these acidic substances combine with metal. This reaction turns food to poison which damage people’s health.

Solution: When an acid reacts with any metal, salt and hydrogen gas are formed.
Metal + Acid → Salt + Hydrogen gas

Solution: As metal compound released is Calcium Chloride the gas evolved here is CO₂. Hence metal A should be Calcium Carbonate. Hence the reaction between Calcium Carbonate and HCl is.
CaCO₃ (s) + 2HCl (Aq) → CaCl₂( Aq) + CO₂ (g) + H₂O (l)

Acids are sour in taste, turn blue litmus red, and dissolve in water to release H+ ions.
Example: Sulphuric acid (H2SO4), Acetic Acid (CH3COOH), Nitric Acid (HNO3), etc.

Acids have a sour taste.
Turns blue litmus red.
Acid solution conducts electricity.
Release H+ ions in aqueous solution.

Natural Acids
Mineral Acids

Indicators are substances which indicate the acidic or basic nature of the solution by the color change.

Solution. : Bases are bitter in taste, have soapy touch, turn red litmus blue and give hydroxide ions (OH–) in aqueous.
Examples: Sodium hydroxide (caustic soda) – NaOH
Calcium hydroxide – Ca(OH)2
Potassium hydroxide (caustic potash) – (KOH)

Have a bitter taste.
Soapy to touch.
Turns red litmus blue.
Conducts electricity in solution.
Release OH– ions in Aqueous Solution

pH of a given solution is the negative logarithm to the base 10 of the hydrogen ion concentration, [H+] expressed in g ions/lit or moles/lit. Thus
pH=- log[H+]

Being alkaline in nature, cold milk is used to neutralise the acidity produced by HCl present in the gastric juice in the stomach.
Astronauts in space ships use this reaction to neutralise the dangerous levels of CO₂.
Farmers add slaked lime (calcium hydroxide) to reduce acidity of soil.
Sting of ants and bees contains formic acid. This can be neutralised by rubbing soap, which contains free sodium hydroxide.
Persons suffering from acidity are given antacid tablets, containing magnesium hydroxide which neutralises excess HCl produced, in stomach. Alternately, they are advised to sip cold milk, which neutralises HCl.

Let us take the example of potassium sulphate, which is a salt of strong acid [sulphuric acid] and strong base [potassium hydroxide solution].
From the above equation, it is clear that water is always feebly ionised and hence solution of potassium sulphate is neutral in nature.