NCERT Solutions for Class 10 Science Chapter 2- Acids, Bases And Salts
PAGE NO – 18
Q1. You are given three test tubes. The three test tubes contain distilled water, an acidic solution and the basic solution, respectively. There is only red litmus paper available in order to identify what is there in each test tube. How will you find out what is in each of the test tubes?
Answer-
We can identify the solutions using red litmus paper by observing the colour changes.
· We dip red litmus paper into all three test tubes one by one.
· The solution which turns red litmus paper to blue is the basic solution.
· We take that blue litmus paper and cut it into two pieces.
· We then test the other two solutions using the blue litmus paper.
· The one that turns blue litmus to red is the acidic solution.
· The test tube that does not change the colour of either red or blue litmus paper contains distilled water.
PAGE NO- 22
Q1. Why should curd and sour substances not be kept in brass and copper vessels?
Answer:
We should not keep curd or sour substances in brass or copper vessels because they contain acids. These acids react with the metal and form harmful substances. This can make the food toxic and unsafe to eat, which can affect our health.
Q2. Which gas is usually liberated when an acid reacts with a metal? Illustrate with an example. How will you test for the presence of this gas?
Answer:
When an acid reacts with a metal, hydrogen gas is usually released.
Metal + Acid → Salt + Hydrogen gas
For example:
Zinc + Hydrochloric acid → Zinc chloride + Hydrogen gas
(Zn + 2HCl → ZnCl₂ + H₂↑)
To test the gas, we can bring a burning matchstick near it. If the gas is hydrogen, it will burn with a 'pop' sound. This sound confirms the presence of hydrogen gas.
Q3. Metal compound A reacts with dilute hydrochloric acid to produce effervescence. The gas evolved extinguishes a burning candle. Write a balanced chemical equation for the reaction if one of the compounds formed is calcium chloride.
Answer:
The gas that puts out a burning candle is carbon dioxide, so the metal compound A must be calcium carbonate.
When calcium carbonate reacts with dilute hydrochloric acid, it gives calcium chloride, carbon dioxide, and water.
The balanced chemical equation is:
CaCO₃ (s) + 2HCl (aq) → CaCl₂ (aq) + CO₂ (g) + H₂O (l)
PAGE NO- 25
Q. Why do HCl, HNO₃, etc., show acidic characters in aqueous solutions while solutions of compounds like alcohol and glucose do not show an acidic character?
Answer:
Acids like HCl and HNO₃ show acidic character because they release H⁺ ions when mixed with water. And acids are substances which upon dissociating with water gives H⁺ ions in the aqueous solution. These H⁺(aq) ions are what make the solution acidic.
Compounds like glucose and alcohol also contain hydrogen atoms, but they do not release H⁺ ions in water. The hydrogen present in these compounds is tightly bonded and does not come out as ions. That’s why they don’t show acidic properties, even though they have hydrogen in their structure.
Q2. Why does an aqueous solution of acid conduct electricity?
Answer:
An acid conducts electricity in water because it forms ions. When we dissolve an acid in water, it releases H⁺ ions along with other charged particles. These ions help carry electric current, which is why an acid solution can conduct electricity.
Q3. Why does dry HCl gas not change the colour of the dry litmus paper?
Answer:
Dry HCl gas does not change the colour of dry litmus paper because there is no water present. Without water, HCl does not release H⁺ ions, which are needed to show acidic behaviour. Since no ions are formed, it cannot affect the litmus paper, and the colour stays the same.
Q4. While diluting an acid, why is it recommended that the acid should be added to water and not water to the acid?
Answer:
When we dilute an acid, we should always add acid to water slowly, not water to acid. This is because adding water to a concentrated acid produces huge amount of heat very quickly. This sudden heat can cause the acid to splash out and may lead to burns or injuries.By adding acid to water slowly, the heat gets spread out and it becomes much safer to handle.
Q5. How is the concentration of hydronium ions (H₃O⁺) affected when a solution of an acid is diluted?
Answer:
When we add water to an acid (i.e., dilute it), the total number of hydronium ions stays the same, but the amount of water increases. This means the hydronium ions get more spread out in the larger volume of solution.
As a result, the concentration of hydronium ions (H₃O⁺) decreases when an acid is diluted.
Q6. How is the concentration of hydroxide ions (OH⁻) affected when excess base is dissolved in a solution of sodium hydroxide?
Answer:
When we add more base to a sodium hydroxide solution, the number of OH⁻ (hydroxide) ions in the solution increases at first.
But after a certain point, the solution becomes saturated, which means it can’t dissolve any more base. After that, the concentration of OH⁻ ions stays the same, even if we keep adding more base.
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