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Karnataka State Syllabus Class 7 Science Chapter 5 Acids, Bases and Salts
Class 7 Science Acids, Bases and Salts NCERT Textbook Questions and Answers
State differences between acids and bases.
|i) These are sour to taste.
|i) These are bitter to taste.
|ii) Acids turn blue litmus red.
|ii) Bases do, not change the colour of blue litmus.
|iii) Acids do not change the colour of red litmus.
|iii) Bases turn red litmus blue.
|iv) with china rose indicator, these give dark pink colour.
|iv) Bases give green colour with china rose indicator.
|v) Acids do not change the colour of turmeric indicator.
|v) Bases turn the colour of turmeric indicator to red.
|vi) Examples: curd, lemon juice and vinegar.
|vi) Examples: Baking soda and soap.
Ammonia is found in many household products, such as window cleaners. It turns red litmus blue. What is its nature?
Bases turn red litmus blue, so nature of ammonia is basic.
Name the source from which litmus solution is obtained. What is the use of this solution?
Litmus is a natural indicator. It is extracted from lichens. It has a purple colour in distilled water. It is available in the form of a solution or in the form of strips of paper. It is available as red and blue litmus paper. It is used to test the nature of a substance. Acid turns blue litmus solution red and base turns red litmus solution blue.
Is the distilled water acidic/basic/ neutral? How would you verify it?
Distilled water is neutral in nature. The same can be verified by using red and blue litmus papers.
Neither will show a colour change with distilled water. This proves that distilled water is neutral.
Describe the process of neutralization with the help of an example.
The reaction between an acid and a base is known as neutrilization reaction. In this reaction, both acid and base cancel each other’s effect.
The Neuralization reaction results in the formation of salt and water. During this reaction, energy in the form of heat is evolved.
Acid + Base ➝ Salt + water + Heat
For example, when sodium hydroxide (NaOH) is added, to hydrochlaric acid (HCl), sodium chloride. (Nacl) and water (H2O) are obtained.
NaOH + HCL ➝ Nacl + H2O + Water.
Mark ‘T’ if the statement is true and ‘F’ if it is false:
(i) Nitric acid turn red litmus blue. (T/F)
(ii) Sodium hydroxide turns blue litmus red. (T/F)
(iii) Sodium hydroxide and hydrochloric acid neutralise each other and form salt and water. (T/F)
(iv) Indicator is a substance which shows different colours in acidic and basic solutions. (T/F)
(v) Tooth decay is caused by the presence of a base. (T/F)
Dorji has a few bottles of soft drink in his restaurant. But, unfortunately, these are not labelled. He has to serve the
drinks on the demand of customers. One customer wants acidic drink, another wants basic and third one wants neutral drink. How will Dorji decide which drink is to be served to whom?
Since the drikts are edible, Dorji can take the decision by tasting the drinks. Acidic drinks will be sour to taste where as basic drinks will be bitter to taste and neutral drinks will have no taste. He can also use litmus paper to identify the acid, base and neutral drink, by just dropping few drops of drink on litmus paper and can take the decision according to the following :
- If it turns blur, drink is basic.
- If it turns red, drink is acidic.
- If it turns any other colour, drink is neutral.
(a) An antacid tablet is taken when you suffer from acidity.
Due to release of excess of hydrochloric acid, we suffer from acidity. An antacid tablet consists of a base like milk of Magnesia (magnesium hydroxide). It neutralizes the effect of excessive acid and brings relief.
(b) Calamine solution is applied on the skin when an ant bites.
The sting of an ant contains formix acid which caues irrition on the skin. Calamine solution contains, zinc carbonation which is a base. Calamine solution neutralizes the acid effect of the ant bite when applied on the skin.
(c) Factory waste is neutralised before disposing it into the water bodies.
The wastes of many factories contain acids. If they are allowed to flow into the water bodies, the acids will kill aquatic organisms, The factory wastes are, therefore, neutralized by adding basic substances.
Three liquids are given to you. Once is hydrochloric acid, another is sodium hydroxide and third is a sugar solution. How will you identify them? You have only turmeric indicator
Turmeric is yellow in colour. When a base is added to it, the solution turns into red colour. However turmeric remains yellow when an acid or neutral solution is added to it. We shall perform the following steps to identify neutral.
- Take few drops from each solution and test it with turmeric solution. If the solution turn into red colour that solution is lease i.e., Sodium Hydroxide.
- Now, we will put a drop of sodium hydroxide on a drop of each of the other two liquids seperately, After that we will put the drops of these mixtures on turmeric indicator. The drop which will change the colour of the turmeric indicator to red contains sugar solution. This is because the mixture of basic and neutral solutions is basic in nature.On the other hand, the drop which will not change the colour of turmeric indicator contains hydrochloric acid. This is because hydrochloric acid reacts with sodium hydroxide to from a neutral solution.
Blue litmus paper is dipped in a solution. It remains blue. What is the nature of the solution? Explain.
A solution which does not change the colour of blue litmus paper can be either basic or neutral in nature. The nature of the solution can be obtained by adding a few drops of it on a red litmus paper. If the colour of the red litmus changes to blue, then the solution is basic in nature. If no colour change is observed, then the solution is neutral in nature.
Consider the following statements:
a. Both acids and bases change colour of all indicators.
b. If an indicator gives a colour change with an acid, it does not give a change with a base.
c. If an indicator changes colour with a base, it does not change colour with an acid.
d. Change of colour in an acid and a base depends on the type of the indicator.
Which of these statements are correct?
i. All four
ii. a and d
iii. b, c, and d
iv. only d
iv. only d
Class 7 Science Acids, Bases and Salts Additional Important Questions and Answers
What are the indicators ?
Some special Substances which are used to test whether a substance is acidic or basic very easily are called indicators.
Name some natural indicators.
Turmeric, litmus, china rose petals etc., are some of the natural indicators.
Define neutral Solutions.
The solutions which do not change the colour of either red or blue litmus are known as
Neutral Solutions: These solutions are neither acidic nor basic in nature.
What is phenolphthalein ?
Phenolphthalein is one of the chemical indicators. If the solution is acidic the phenolphthalein is colourless, when the solution is basic it gives pink color.
Where do we see neutrolisation in everyday life?
We can see neutralisation in everyday life are
- Ant bite
- Soil treatment
- Factory waste.
Define Neutralisation. Give example.
The reaction between an acid and a base is known as neutralisation.
Hydrochloric acid + Sodium Hydroxide } ➝ Sodium chloride + water.
HCl + NaOH ➝ NaCl + H2O.
Write a table of acids and bases in which it obtains.
II. Fill in the blanks :
- When an ant bites it injects formic acid into the skin.
- Our stomach contains hydrochloric.
- When soil is acidic, it is treated with quick lime (calcium oxide.
- If the soil is basic organic matter compost is added to it.
- China rose indicator turns acidic solutions to dark pink (majenta) and basic solutions to green.
- Litmus is extracted from lichens.
- The solutions which do not change the colour of either red or blue litmus are known as neutral solutions.
- The word acid comes from the Latin word acere.
- The chemical name of milk of magnesia is magnesium hydroxide.
- Sodium hydroxide or Potassium hydroxide is found in soap.