Class 10 Science
Chapter 2 Acids, Bases and Salts
Activities Solutions
Activity 2.1
Procedure: Activity 2.1 asks us to test various acids and bases with various reagents one by one and see the result.
Explanation: Various reagents mentioned here are PH indicators. We find its use them in chemistry practicals.
Blue Litmus Paper: It is a reagent to test for basic PH. In acidic PH it turns red.
Red litmus paper: It turns blue in basic PH.
Phenolphthalein: (Pronunciation: L Silent) It is a colourless reagent. It turns pink in basic PH.
Methyl Orange: It is a narrow range PH indicator. At PH below 3.1 it is red and PH above 4.4 it is Yellow.
Observation: Stronger acids like Hydrochloric acid, Sulphuric acid, Nitric acid have very acidic PH. So, it turns blue litmus into red and gives a red solution with methyl orange.
Similarly, Stronger bases like sodium hydroxide, calcium hydroxide, potassium hydroxide, magnesium hydroxide, and ammonium hydroxide turn the red litmus into the blue. With phenolphthalein, they give a pink solution and with methyl orange they give a yellow solution.
For weak acid and bases:
The PH of weak acids and bases depends on their concentration.
For example
Acetic acid: The 1M solution has a PH of 2.88. So It turns Blue litmus Red. Phenolphthalein indicates only basic PH so there is no change in the colour with phenolphthalein. It turns methyl orange into the red.
Inference/conclusion: Different PH indicators give different colour with the acids and bases. An acid turns the blue litmus paper red while a base reveres this. Bases give red colour with phenolphthalein while with methyl orange it gives a yellow solution. Similarly, acids give red colour with methyl orange.
Application: We use indicators in an acid-base reaction to finding the endpoint. After completion of a reaction, any addition of a reactant changes the PH of the solution. If a suitable PH indicator is present in the solution, it changes the colour which marks the endpoint.
Some extra point: Salt of a strong acid and strong bases like sodium chloride have PH near 7.
Salt of Strong acid and a weak base, e.g. copper Sulphate (CuSO4) have acidic PH.
Similarly Salt of a Strong base and weak acid, e.g. Calcium carbonate (CaC03) have basic PH.
Activity 2.2
Brief Procedure: Activity 2.2 asks us to check the change in odour and colour of cloth soaked with onion and vanilla with acids and bases.
Observation: Bases deodorise onion and vanilla but do not change the colour.
Explanation:
The odour and colour of Onion:
Onion contains sulphurous allium. It gives the peculiar onion odour. It is acidic and reacts with bases like sodium hydroxide to give the odourless compound. So when we add bases to onion soaked clot, its smell disappears.
Allium does not react with acids, so when we add acids to onion soaked cloth smell remains in the cloth.
Colour of onion: There is no relation to the reaction and colour. So the colour remains the same.
The odour and colour of Vanilla:
Vanilla contains an aldehyde which gives it a pleasant odour. The aldehyde is also weak acids. They react with bases, and the smell disappears.
Similar to onion vanilla colour also has no relation with bases and acids. As a result, we do not see any changes in the colour of vanilla with acids or bases.
Application: You must have seen onion dipped in a red colour solution in home or restaurant. This red colour solution is potassium permanganate. It is a slightly basic salt. It deodorises the onion smell and also helps in preserving onion for a long duration.
Activity 2.3
Brief procedure: Activity 2.3 asks us to react zinc granules with various acids and observe the flammability of gas formed.
Observation: We see bubbles coming out vigorously with strong acids. This gas burn when we bring the candle to it.
Zinc also react with weak acids like acetic acid, but here gas formation is slow.
Explanation: Zinc reacts with acids and forms its salt and hydrogen gas. Hydrogen gas liberates as the bubble. When we bring the flame to this gas, it burns as hydrogen is highly flammable.
For example
Zn + H2SO4 ———> ZnSO4 + H2
Weak acid does not dissociate quickly in water. So its reaction with zinc is slower as compared to strong acids like Hydrochloric acid and sulphuric acid.
Activity 2.4
Procedure: Activity 2.4 asks us to treat zinc granules with bases like sodium hydroxide.
Observation: Bubbles come out from zinc granules. These bubbles catch fire when the flame is brought towards the test tube.
Explanation: Just like a metal reacts with acid (see Activity 2.3), metal also react to bases and produce hydrogen gas.
Here, zinc reacts with sodium hydroxide and forms the oxide of zinc with the evolution of hydrogen. The reaction is vigorous. So we use dilute bases to moderate the reaction.
Zn(s) + 2NaOH(aq) ———-> Na2ZnO2(s) + H2(g)
Inference: Metals react with bases and form hydrogen gas bubbles.
Activity 2.5
Procedure: Activity 2.5 asks us to react metal carbonates and metal hydrogen carbonate with acids and see if any gas evolves; check also if gas gives a precipitate with quick lime.
Observation:
When we add acid to carbonates and hydrogen carbonates of metal, bubbles start appearing from the carbonates. When we pass this air to a quick lime solution, it turns the lime water milky.
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