CBSE Class 7 Science Advanced Chemistry Acids, Bases and Salts Short Answer Questions Set 02

Short Answer Questions

Question 1. Two friends Neela and Leela went to a shop, brought mango and lemon pickle of 1 kg each, respectively. Neela stored the pickle in a tin-coated iron vessel, whereas Leela stored her pickle in a glass vessel. After a few days, Neela found that her pickle got spoiled, whereas Leela’s pickle did not. Explain the reason behind it.
Answer:
Pickles contain organic acids.
These acids react with tin and iron and form harmful salts; they also corrode the vessel.
Glass is inert towards organic acids, so the pickle in the glass vessel was not spoiled.
In simple words: Pickle acids attack metals but not glass.

Teacher's Note:
1. Never store acidic food in metal containers.
2. Plastic is also safe.

Question 2. Hydrochloric acid is used to clean the metal surface before the metal surface is subjected to galvanization. Explain.
Answer:
The surface of iron has impurities such as oxides and carbonates.
These impurities are basic in nature, so hydrochloric acid reacts with them and removes them.
A clean surface lets the coating (zinc) stick evenly and firmly.
In simple words: The acid removes the basic coating so the metal coat sticks well.

Teacher's Note:
1. Galvanization means coating iron with zinc.
2. The same reaction is used before tinning.

Question 3. Why is nitric acid used in the purification of gold?
Answer:
Nitric acid is a good oxidising agent.
It oxidises and dissolves the impurities, while gold does not dissolve in it, so pure gold is left behind.
In simple words: Nitric acid eats away the impurities but leaves gold untouched.

Teacher's Note:
1. Gold and platinum are insoluble in nitric acid.
2. This property is called the purification of noble metals.

Question 4. Hydrated sodium sulphate and hydrated copper sulphate are taken in two air-tight containers X and Y, respectively. When blue-coloured \(\ce{CoCl2}\) is introduced into the two containers, there is colour change in one container. Identify the container and give reasons in support of your answer.
Answer:
Hydrated sodium sulphate is \(\ce{Na2SO4.10H2O}\) and hydrated copper sulphate is \(\ce{CuSO4.5H2O}\).
Sodium sulphate decahydrate is efflorescent; it loses water to the air in container X.
Blue \(\ce{CoCl2}\) absorbs this moisture and turns pink: \(\ce{CoCl2 -> CoCl2.2H2O}\) (on absorbing moisture).
Blue vitriol does not lose its water in the air-tight container Y, so there is no change there.
The colour change takes place in container X.
In simple words: Only the efflorescent salt gives water vapour for cobalt chloride to absorb.

Teacher's Note:
1. Cobalt chloride is a moisture detector.
2. Blue means dry; pink means it has taken up water.

Question 5. A teacher demonstrated action of acids on metals in a laboratory. After the activity was over, one naughty student took an aluminium rod. He dipped it in a bottle containing concentrated nitric acid. Later, he dipped it in a beaker containing dil \(\ce{H2SO4}\). But he did not find any bubbles coming out. He was surprised because he was able to see bubbles coming out when a metal rod is dipped in sulphuric acid when the teacher conducted the experiment. He then approached the teacher and asked what is wrong with his activity. Predict the answer given by the teacher.
Answer:
Concentrated nitric acid is highly oxidising. It forms a thin oxide layer on the aluminium rod.
This layer is a passive layer. It stops further reaction between aluminium and dilute sulphuric acid, so no hydrogen gas bubbles are released.
If the rod had been dipped directly in sulphuric acid, bubbles of hydrogen would have been seen.
In simple words: Nitric acid coated the aluminium with an oxide film that blocked the reaction.

Teacher's Note:
1. This is called passivation.
2. Hydrogen bubbles show metal-acid reaction.

Question 6. In a chemistry practical examination, three friends DEAN, BEAN and SEAN were given test tubes A, B, and C, respectively filled with solutions. DEAN added blue litmus, BEAN added methyl orange and SEAN added the litmus solution to the given test tube, respectively. DEAN found the blue litmus turning to red, BEAN observed methyl orange turning to yellow but SEAN found no action in the solution provided to him in the test tube. Comment on the nature of the three solutions given.
Answer:
Test tube A turned blue litmus red, so solution A is acidic.
Test tube B turned methyl orange yellow, so solution B is basic.
Test tube C showed no change with litmus, so solution C is neutral (for example a normal salt solution).
In simple words: Red litmus means acid, yellow methyl orange means base, and no change means neutral.

Teacher's Note:
1. Remember the colours of each indicator.
2. A normal salt solution is neutral.

Question 7. One molecule of \(\ce{H3PO4}\) is treated with one molecule of NaOH, two molecules of NaOH and three molecules of NaOH separately. Comment on the nature of products formed and write their names.
Answer:
\(\ce{H3PO4 + NaOH -> NaH2PO4 + H2O}\): sodium dihydrogen phosphate, an acidic salt.
\(\ce{H3PO4 + 2NaOH -> Na2HPO4 + 2H2O}\): disodium hydrogen phosphate, an acidic salt.
\(\ce{H3PO4 + 3NaOH -> Na3PO4 + 3H2O}\): sodium phosphate, a normal (neutral) salt.
In simple words: More \(\ce{NaOH}\) replaces more hydrogens until the normal salt forms.

Teacher's Note:
1. Count the H left in the formula.
2. Only the full replacement gives the normal salt.

Question 8. An element X on treatment with oxygen gives Y which on hydrolysis produces Z. Z on treatment with higher oxide of carbon “A” gives a milky white precipitate “B”. Identify X, Y, Z, A and B?
Answer:
The milky white precipitate is calcium carbonate, so B is \(\ce{CaCO3}\), made from lime water Z and carbon dioxide A.
\(\ce{2Ca + O2 -> 2CaO}\)  (X = Ca, Y = CaO)
\(\ce{CaO + H2O -> Ca(OH)2}\)  (Z = lime water)
\(\ce{Ca(OH)2 + CO2 -> CaCO3 + H2O}\)  (A = \(\ce{CO2}\), B = \(\ce{CaCO3}\))
So X = calcium, Y = calcium oxide, Z = calcium hydroxide, A = carbon dioxide, B = calcium carbonate.
In simple words: Lime water turns milky with carbon dioxide because insoluble calcium carbonate forms.

Teacher's Note:
1. Work backwards from the milky precipitate.
2. \(\ce{CO2}\) is the higher oxide of carbon.

Question 9. X is an atom of an element in which fourth shell is the valence shell and the number of electrons are in 1 : 1 ratio in the first, fourth and second, third shells respectively. Its corresponding oxide Y on hydrolysis gives Z. Explain the changes that take place when phenolphthalein and methyl orange are added to Z separately.
Answer:
The first and fourth shells have equal numbers of electrons (2), and the second and third shells have equal numbers (8). So the configuration is 2, 8, 8, 2 and X is calcium.
\(\ce{2Ca + O2 -> 2CaO}\) (Y = CaO), and \(\ce{CaO + H2O -> Ca(OH)2}\) (Z = calcium hydroxide).
Z is a base. Phenolphthalein turns pink and methyl orange turns yellow in it.
In simple words: Calcium gives a base, and a base turns phenolphthalein pink and methyl orange yellow.

Teacher's Note:
1. Fill the shells to find the element first.
2. Calcium burns with a brick-red flame.

Question 10. Calcium hydroxide on heating gives a solid product “X” and on treatment with hydrochloric acid gives another solid product Y. What do you observe when X and Y are exposed to atmosphere? Explain.
Answer:
\(\ce{Ca(OH)2 -> CaO + H2O}\) (on heating) (X = CaO)
\(\ce{Ca(OH)2 + 2HCl -> CaCl2 + 2H2O}\) (Y = \(\ce{CaCl2}\))
CaO is hygroscopic: it absorbs moisture and stays solid.
\(\ce{CaCl2}\) is deliquescent: it absorbs moisture and changes into a solution.
In simple words: Quicklime absorbs water but stays solid, while calcium chloride becomes wet and turns liquid.

Teacher's Note:
1. Compare the final physical state of each.
2. Hygroscopic and deliquescent are different.

Question 11. A nonmetal “X” with electronic configuration 2, 8, 6 forms corresponding -ous and -ic acids. Identify both the acids and explain the method of dilution of the corresponding -ic acid.
Answer:
Configuration 2, 8, 6 has 16 electrons, so X is sulphur.
The '-ous' acid is sulphurous acid, \(\ce{H2SO3}\), and the '-ic' acid is sulphuric acid, \(\ce{H2SO4}\).
Dilution of sulphuric acid: add the acid slowly to water with constant stirring. Adding water to the acid releases a large amount of heat at once, which can cause a violent, explosive splash.
In simple words: Always pour acid into water, never water into acid.

Teacher's Note:
1. Remember: 'Add acid to water'.
2. Stir slowly and keep the container cool.

Question 12. Between \(\ce{H2SO4}\) and \(\ce{H3PO4}\), which solution possesses greater number of \(\ce{H+}\) ions in the given volume assuming that the number of molecules of both acids is equal? Give reasons.
Answer:
\(\ce{H2SO4}\) has more \(\ce{H+}\) ions.
\(\ce{H2SO4}\) is a strong acid and ionizes completely: \(\ce{H2SO4 -> 2H+ + SO4^2-}\).
\(\ce{H3PO4}\) is a weak acid and ionizes only partially, so it gives fewer \(\ce{H+}\) ions than the 3 per molecule that its formula suggests.
In simple words: A strong acid ionizes fully, so it gives more H+ than a weak acid even if it has fewer hydrogens.

Teacher's Note:
1. Strength matters more than the count of H atoms.
2. Compare the extent of ionization.

Question 13. Sunny and Bunny are neighbours staying in the same apartment. In summer vacation, one day they wanted to prepare some snacks for eating. During that process, the food material fell on the floor and the beautiful marble flooring became dirty. Sunny was afraid that his mother would scold him and in a hurry, brought some acid to clean the floor. But Bunny stopped him and said that it will spoil the flooring permanently. Why did Bunny say so? Give reason.
Answer:
Marble is mainly calcium carbonate, \(\ce{CaCO3}\), which is basic in nature.
It reacts with acids and gives a salt, water and carbon dioxide: \(\ce{CaCO3 + 2HCl -> CaCl2 + H2O + CO2}\).
So the acid would eat into the floor and spoil its polish permanently.
In simple words: Acid reacts with the carbonate in marble and damages it.

Teacher's Note:
1. Never use acidic cleaners on marble.
2. Carbonates fizz with acids.

Question 14. Tartaric acid is one of the components in baking powder. What is its role?
Answer:
Baking powder contains baking soda. Baking soda is alkaline in water and gives a bitter taste and yellow colour to bread and cakes.
Tartaric acid neutralizes this alkali, removing the bitter taste and the yellow colour. It also helps release the carbon dioxide that makes the batter rise.
In simple words: Tartaric acid cancels the bitterness of baking soda.

Teacher's Note:
1. Acid + base = neutralization.
2. The same acid is found in grapes and tamarind.

Question 15. Calcium hydroxide is added to soils before applying fertilizers. Justify the purpose.
Answer:
Soils are generally slightly acidic.
Calcium hydroxide is a base, so it neutralizes the acidity of the soil and makes it suitable for the crops and fertilizers.
Calcium hydroxide is prepared cheaply from naturally occurring limestone.
In simple words: Slaked lime reduces the acidity of soil.

Teacher's Note:
1. Neutralization is the idea.
2. Do not add too much lime.

Question 16. What is the role of calcium hydroxide used in making mortar and white washing?
Answer:
Mortar is a mixture of calcium hydroxide, sand and water. In air, calcium hydroxide reacts with carbon dioxide and forms hard calcium carbonate, which strengthens the construction: \(\ce{Ca(OH)2 + CO2 -> CaCO3 + H2O}\).
White washing: the same reaction forms white calcium carbonate, which gives the wall a white coat.
In simple words: Slaked lime turns into hard, white calcium carbonate on the wall.

Teacher's Note:
1. Both uses depend on reaction with \(\ce{CO2}\).
2. Write the equation once.

Question 17. Why is Chile salt petre used in the manufacture of gunpowder?
Answer:
Gunpowder is a mixture of nitrate, carbon and sulphur.
On heating, the nitrate breaks down and releases oxygen. This oxygen supports the rapid burning of carbon and sulphur.
A large amount of heat and gases is produced suddenly in a small space, and the pressure causes an explosion.
In simple words: The nitrate supplies the oxygen that makes the powder burn explosively.

Teacher's Note:
1. Nitre and Chile saltpetre are both nitrates.
2. They are oxidisers.

Question 18. Write formulae and names of the various salts formed when magnesium hydroxide is treated with sulphurous acid.
Answer:
\(\ce{Mg(OH)2}\) is diacidic and \(\ce{H2SO3}\) is dibasic, so more than one salt forms.
\(\ce{Mg(OH)2 + H2SO3 -> Mg(OH)HSO3 + H2O}\): magnesium hydroxy bisulphite (basic and acidic salt)
\(\ce{Mg(OH)HSO3 + H2SO3 -> Mg(HSO3)2 + H2O}\): magnesium bisulphite (acidic salt)
\(\ce{Mg(HSO3)2 + Mg(OH)2 -> 2MgSO3 + 2H2O}\): magnesium sulphite (normal salt)
In simple words: Step-by-step neutralization gives hydroxy, hydrogen and finally normal salts.

Teacher's Note:
1. Replace the OH and the H one at a time.
2. The normal salt is \(\ce{MgSO3}\).