The Class 8 is an important year in a student’s life and Maharashtra State Board Science is one of the subjects that require dedication, hard work, and practice. It’s a subject where you can score well if you are well-versed with the concepts, remember the important formulas and solving methods, and have done an ample amount of practice. Worry not! Home Revise is here to make your Class 8 journey even easier. It’s essential for students to have the right study material and notes to prepare for their board examinations, and through Home Revise, you can cover all the fundamental topics in the subject and the complete Maharashtra State Board Class 8 Science Book syllabus.
Q1. Identify the following solutions, whether they are acid or base.
| Solution | Change in Indicator | Acid / Base | ||
| Litmus | Phenolphthalein | Methyl orange | ||
| 1 | No change | |||
| 2 | Orange colour turns red | |||
| 3 | Red litmus turns blue | |||
Answer:
| Solution | Change in Indicator | Acid / Base | ||
| Litmus | Phenolphthalein | Methyl orange | ||
| 1 | No change | Acid | ||
| 2 | Orange colour turns red | Acid | ||
| 3 | Red litmus turns blue | Base | ||
Q2. Write chemical names from given formulae.
H2 SO4
Ca(OH)2
HCl
NaOH
KOH
NH4 OH
Answer: H2 SO4 –Sulphuric Acid
Ca(OH)2 –Calcium hydroxide/Lime water
HCl –Hydrochloric Acid
NaOH –Sodium Hydroxide
KOH –Potassium Hydroxide
NH4 OH –Ammonium hydroxide
Q3. Sulphuric acid has the highest importance in the chemical Industry. Why?
Answer: In the chemical industry sulfuric acid is one of the important compounds. This chemical is used in making hundreds of compounds by the chemical industry. By far the largest amount of sulfuric acid is used to make phosphoric acid, used in turn to make phosphate fertilizer, calcium dihydrogen phosphate and aluminum phosphate. lt is also used to make ammonium sulfate, which is particularly important fertilizer in sulfur deficient. Due to the following reasons sulphuric acid is highly used in chemical industry.
Q4. Give answers.
a. Which acid is used for getting chloride salt?
b. By squeezing lemon on a piece of rock the gas liberated turned lime water milky. Which compound is present in the rock?
c. The label on the bottle of chemical is spoiled. How will you find whether the chemical is acidic or not?
Answer a: Hydrochloric acid is used to get chloride salt. It is a strong acid as it is completely dissociated in water. Therefore, it is used to prepare salts containing the Cl– anion called chlorides.
Answer b: When lemon is squeezed onto the rock, a gas is liberated. This gas turned the lime water milky. This confirms that Carbon dioxide is present in the rock. This is because insoluble calcium carbonate that is CaCO3 is formed.
Answer c: We can find the nature of that chemical by using Acid-Base indicators like litmus, Phenolphthalein, China rose extract, etc. If the stored chemical is acid then the blue litmus will turn into red and if the chemical was base in nature then the red litmus turn into blue color.
Q5. Answer the following questions.
a. Explain the difference between acid and base.
b. Why does the indicator not get affected by salt?
c. Which substances are produced by neutralization process?
d. Which are the industrial uses of acids?
Answer a: The difference between acid and base are:
| Basis | Acid | Base |
| Definition | An acid is any chemical compound once dissolved in water produces a solution with hydrogen ion activity more than purified water | A base is an aqueous substance that could absorb hydrogen ions. |
| Strength | Relies on the concentration of the hydronium ions | Relies on the concentration of the hydroxide ions |
| Examples | Acetic acid CH3COOH and sulphuric acid | Sodium Hydroxide (NaOH) and Ammonia |
| Characteristics (Physical) | Would depend upon the temperature, acids would look solid, liquid or in the form of gas. It would also have a sour taste. | Bases would feel slippery and solid in nature except ammonia which is gaseous. It would have a bitter taste. |
| Disassociation | Acids would release hydrogen ions (h+) when mixed with water | Bases would release hydroxide ions(OH-) when mixed with water |
| Test with Litmus | Would turn litmus paper red | Would turn litmus paper blue |
Answer b: The substances, which change their colours in presence of acid or base are called ‘Indicators’. From the reaction of acids and bases salt is produced and due to this reason it is considered as neutral. For this reason, Indicators are not affected by salt as salt is the product of acid and base which is neutral.
Answer c: We have seen that acid contains (H+) hydrogen ions and base contains(OH-) hydroxide ions. Salt and water are formed by combination of acid and base.
This chemical reaction is called neutralisation.
Answer d: The industrial uses of acids:
Q6. Select proper word given in brackets and fill in the blanks.
a. Main constituent of acid is ………..
b. Main constituent of base is …….
c. Tartaric acid is a …………. acid.
Answer a: Main constituent of an acid is H+ ions
Answer b: Main constituent of base is OH- ions
Answer c: Tartaric acid is a white, crystalline organic acid.
Q7. Match the pairs.
| Group A | Group B |
| Tamarind | Acetic acid |
| Curd | Citric acid |
| Lemon | Tartaric acid |
| Vinegar | Lactic acid |
Answer:
| Group A | Group B |
| Tamarind | Tartaric acid |
| Curd | Lactic acid |
| Lemon | Citric acid |
| Vinegar | Acetic acid |
Q8. State true or false.
a. Oxides of metals are acidic in nature.
b. Table salt is acidic.
c. Metal corrodes due to salts.
d. Salts are neutral.
Answer a: True
Answer b: False
Answer c: False
Answer d: True
Q9. Classify following substances into acidic, basic and neutral group –
HCl, NaCl, MgO, KCl, CaO, H2 SO4 , HNO3 , H2 O, Na2 CO3
Answer: Acidic group –HCl, KCl and NaCl
Basic group –H2 SO4 , HNO3 , Na2 CO3̣
Neutral group –MgO, CaO, H2 O
Q10. Define acid.
Answer: Acid is such a substance, which gives H+ ions in solution state. e.g. HCl dissociates in water solution. Examples of some acids : Hydrochloric acid (HCl), Nitric acid (HNO3), Sulphuric acid (H2SO4 ), Carbonic acid (H2 CO3 ) (in cold aerated drinks), ascorbic acid, citric acid in lemon and other fruits, acetic acid in vinegar, etc.
Q11. Define natural acids or organic acids.
Answer: Acids present in food are called natural acids or organic acids. These acids being weak in nature are called weak acids.
Q12. Define mineral acids.
Answer: Some acids are strong in nature. They are caustic/inflammatory, e.g. sulphuric acid
(H2SO4 ), hydrochloric acid (HCl) and nitric acid (HNO3 ). These acids are also called ‘mineral acids’. The skin gets burnt when their concentrated solution falls on skin, similarly their vapours if inhaled can be harmful to health. Strong concentrated acids are converted into their dilute acids by slowly adding them into water. Such dilute acids are less harmful than their corresponding strong acids.
Q13. What are Indicators?
Answer: The substances, which are neither acidic nor basic and are chemically inert in nature. Touching or tasting of acid or base is very harmful and hence typical indicators are used to recognise them. The substances which change their colours in presence of acid or base are called ‘Indicators’.
Q14. Write the properties of acid?
Answer: The properties of acid are as follows:
Q15. Discuss the uses of acids?
Answer: The uses of acids are as follows:
Q16. What is pH scale?
Answer: In order to find the numeric value of the level of acidity or basicity of a substance, the pH scale (wherein pH stands for ‘potential of hydrogen’) can be used. The pH scale is the most common and trusted way to measure how acidic or basic a substance is. A pH scale measure can vary from 0 to 14, where 0 is the most acidic and 14 is the most basic a substance can be.
Q17. Define litmus paper.
Answer: Litmus paper is filter paper, which has been treated with a natural water-soluble dye obtained from lichens. The resulting piece of paper, called “litmus paper ”, can be used as a pH indicator. Blue litmus paper turns red under acidic conditions (pH below 4.5) while red litmus paper turns blue under alkaline conditions (pH above 8.3).
Q18. Define base with examples.
Answer: Base is a substance whose water solution gives hydroxide (OH-) ion e.g.
Q19. Write the uses of bases?
Answer: The uses of bases are:
Q20. What is alkali and discuss its properties?
Answer: Alkali is a base that dissolves in water. Not all bases are alkali but all alkali is base. It is a basic salt alkali earth metal or alkali metal. On adding alkali to acid the pH of the mixture increases. On adding acid to alkali the pH decreases due to the removal of alkali. Neutralization is a reaction in which removal of alkalinity or acidity occurs.
Properties of alkalies:
Q21. What is neutralization?
Answer: Acid contains (H+) hydrogen ions and base contains(OH-) hydroxide
ions. Salt and water are formed by a combination of acid and base.
This chemical reaction is called neutralization.
Q22. Difference between alkali and base.
Answer: The difference between alkali and base are:
| Difference between Alkali and Base | |
| Base | Alkali |
| Bases do not dissolve in water | Bases that dissolve in water are alkali |
| All bases are not alkali | All alkali are bases |
| It neutralizes acids | It releases OH- ions, accepts a proton |
| Example: Zinc hydroxide, copper oxide | Example: potassium hydroxide, sodium hydroxide |
Q23. Why are bases called alkali?
Answer: In contact with water, alkali metals form alkali (base) and form metal oxides as water reacts. Alkali metal oxides are also bases and when dissolved in water, they form an alkaline solution. During reaction with water, Group 2 metals also form bases and their oxides are basic and found in the earth’s crust.
Q24. Write the uses of Sodium hydroxide?
Answer: The uses of sodium hydroxide are:
Q25. Write the uses of Calcium hydroxide?
Answer: Calcium hydroxide is a white powder, which does not have any characteristic odour. It is used in industrial settings such as the treatment of waste, the manufacturing of paper, building and processing of food. There are also several medical and dental applications of this compound. For instance, fillings used in root canal treatments often contain calcium hydroxide.