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Class 7_ Acid base and salt Solved Questions
Q. why are the sting of an ant is very painful?
Answer: Ant sting contains formic acid. When an ant bites, it injects this formic acid into the skin. This causes pain.
Q. Why does wasp sting should be treated with vinegar and ant sting with should be treated with baking soda?
Answer: wasp stings are alkaline, and can therefore be neutralised with vinegar(acetic acid). Ant sting contains an acid , and can therefore be neutralised with base like baking soda.
Q. why does an antacid tablet is taken when you suffer from acidity?
Ans: An antacid tablet consists of a base like Milk of Magnesia (magnesium hydroxide). It neutralises the effect of excessive hydrochloric acid released during indigestion.
Q. Why does factory waste treated before disposing into the water bodies?
Ans: The wastes of many factories contain acids. If they are allowed to flow into the water bodies, the acids will kill fish and other organisms. The factory wastes are, therefore, neutralised by adding basic substances.
Q. Why is calamine solution on the skin when an ant bites?
Ans: Ant sting contains formic acid. When an ant bites, it injects this formic acid into the skin. This causes pain. Therefore to give relief, it can be neutralised by rubbing the ant bite with moist baking soda, which contains sodium hydrogen carbonate (NaHCO3), or with a solution of calamine, which contains zinc carbonate (ZnCO3).
Q. Some acid indigestion tablet contains calcium carbonate. How do they help to make indigestion go away? why does their effect often wear off after a while?
Answer: During indigestion there is a secretion of HCl in stomach. The antacid like Mg(OH)2 or NaHCO3contain base that neutralize acid and give relief. Since more acid are released in stomach and require more base so effect of indigestion tablet often wears off after a while
Q. Ammonia is found in many household products, such as window cleaners. It turns red litmus blue. What is its nature?
Answer: Since it turns red litmus blue, Ammonia is basic in nature.
Q. Name the source from which litmus solution is obtained. What is the use of this solution?
Answer: Litmus is extracted from plant lichens. It has a mauve (purple) colour when dissolved in distilled water.
Q. Define acid and base?
Ans: According to Arrhenius any compounds that contain hydrogen and can dissolve in water to release hydrogen ions into solution is called acid. For example, hydrochloric acid (HCl) dissolves in water as follows:
HCl +H2O→ H+ (aq) + Cl-(aq)
According to Arrhenius any compounds that dissolve in water to release hydroxide ions (OH-) into solution is called base. For example, sodium hydroxide (NaOH) is a base dissolves in water as follows:
NaOH + H2O→ Na+(aq) + OH-(aq)
According to The Brønsted-Lowry definition acid is any substance that can donate a hydrogen ion . (Under the Brønsted definition, acids are often referred to as proton donors because an H+ ion, hydrogen minus its electron, is simply a proton).
According to The Brønsted-Lowry definition bases is any substance that can accept a hydrogen ion. NaOH and KOH, are bases because they can accept an H+ from an acid to form water.
However, the Brønsted-Lowry definition also explains why substances that do not contain OH- can act like bases. Baking soda (NaHCO3), for example, acts like a base by accepting a hydrogen ion from an acid as illustrated below:
HCl + NaHCO3 → H2CO3 + NaCl
In this example, the carbonic acid formed (H2CO3) undergoes rapid decomposition to water and gaseous carbon dioxide, and so the solution bubbles as CO2 gas is released.
Q. If bottle full of concentrated sulphuric acid is left open in the air by accident, the acid starts flowing out of the bottle of its own.
Answer : Concentrated sulphuric acid is highly hygroscopic. It absorbs moisture from air and gets diluted. Since the volume increases, the acid starts flowing out of bottle.
Q. Explain why?
(a) Common salt becomes sticky during the rainy season. (b) Blue vitriol changes to white upon heating.
Answer : (a) common salt contains impurity of magnesium chloride which is of deliquescent nature. When exposed to the atmosphere, it becomes moist. Therefore it becomes sticky during rainy season.
(b) Blue vitriol (CuSO4.5H2O) upon heating changes to anhydrous copper sulphate (CuSO4) which is white in colour
Q. Why acetic acid is called a weak acid though there are 4 'H' atoms in the molecule?
Answer: Acetic acid is called a monobasic acid because only one of the four 'H' atoms of the acid is released as H+ ion in solution.
Q. What happens when a base is put on turmeric paste? What will happen when acid is added to the same?
Answer. When base is added to turmeric paste, then its colour changes to red. If we put an acid on the same then changes to yellow again.
Q. How should tooth paste help in preventing tooth decay?
Ans: When bacteria digest food in our mouths, they release weak acids, especially when we eat sugars. Our teeth have a hard covering called enamel that is weakly basic. When bacteria make our mouth acidic, the acid reacts with our enamel, removing hydroxyl ions, causing tooth decay.
The active ingredient in toothpaste is a fluoride salt. When toothpaste dissolves in our mouths, negatively charged fluoride ions are released. The fluoride ions are similar to other hydroxyl ions in that they are negatively charged and can take the place of hydroxyl ions that have reacted with acid. Also, fluoride ions don't react with acid as easily as the hydroxyl ions, making enamel more resistant to decay.
Q. why should acid not stored in metal container?
Ans: Acids cannot be stored in metal containers as they will react with the metal, forming metal salt and liberating Hydrogen gas.
Q. Which shampoo will be better for hair- One with high pH or one with low pH?
Human hair has a pH balance of between 4.5 and 5.5 ie. One with low pH. This natural hair acidity prevents fungi and bacteria in the hair
Q. Why should you not try to wash off turmeric stain on your cloth with soap?
Ans: Turmeric is a kind of indicator which turns red when comes in contact with basic solution and as we know soap solution is basic therefore cloth get red patches.
Q. How soap is prepared?
Ans: soap is prepared by boiling oil or animal fat with caustic soda (Sodium hydroxide)
Q. Name a base that is used to remove ink spot from cloth?
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