The Class 9 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 9 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 9 Science Book syllabus.
1. Identify the odd one out and justify.
(a) Chloride, nitrate, hydride, ammonium
(b) Calcium oxide, magnesium oxide, zinc oxide, sodium oxide
(c)Crystalline blue vitriol, crystalline common salt, crystalline ferrous sulphate, crystalline sodium carbonate.
Answer: (a) Ammonium is the odd one out as it is cation and the rest are anions.
Ammonium = NH4+
Nitrate = NO3–
Hydride = H–
Chloride = Cl–
(b) ZnO is amphoteric in nature, while other ions are basic ions.
Calcium oxide = CaO
Magnesium oxide = MgO
Zinc oxide = ZnO
sodium oxide = Na2 O
(c) Common salt does not change color on heating, while in the rest of the compounds, there is change in colour.
2. If 50ml water is added to a 50ml solution of copper sulphate, what are the changes seen and explain the reason behind it.
Answer: Upon adding 500ml water to 50ml copper sulphate solution, reversible reaction takes place and the colour changes from pale blue to white and on adding water once more it will change back to blue again. Anhydrous Copper sulphate changes to hydrated copper sulphate.
CuSO4 + H2 0 → CuSO4 . 5H2 O(BLUE VITRIOL)
3. A litmus paper was dropped into 2ml dilute HCl. Then 2ml concentrated NaOH was added to it and stirred. Write the changes seen in the instance and explain the reason.
Answer: The litmus paper dropped into HCl turns red, while with NaOH, the litmus paper turns blue. HCl is an acid with pH less than 7. Hence, its colour changes to red. NaOH, in the meantime is a base with pH greater than 7. So, its colour changes to blue.
4. Dilute H2SO4 was taken in an electrolytic cell and electric current was passed through it. What are the changes seen in it and give reason?
Answer: On electrolysing dilute solutions of gases are produced at both the anode and the cathode electrode. The gas developed at the cathode is hydrogen as it burns with a pop sound when a sample is lit with a lighted splint. Meanwhile, the gas at the anode is Oxygen and relights a glowing splint dipped into a sample of gas. When the ions move towards the electrode , the gases are produced.
At the Cathode,
2H+ + 2e– → H2
At the Anode,
40H– –4e– → 2H2 O+O2 .
5. Classify the following oxides into three types and name the types.
(CaO, MgO, CO2 , SO3 , Na2 O, ZnO, Al2O3 , Fe2O3 )
Answer: Oxides are of three types. Find the types with the examples:
1) Acidic Oxide- Carbon Dioxide (CO2 ) and Sulphur trioxide (SO3 ).
2) Basic Oxides- Calcium Oxide (CaO), Magnesium Oxide (MgO) and Sodium Oxide (Na2 O)
3) Amphoteric Oxides- Zinc Oxide (ZnO), Aluminum Oxide (Al2O3 ) and Ferric Oxide (Fe2O3 ).
6. Show the dissociation of the following compounds on dissolving in water, with the help of chemical equation and write whether the proportion of dissociation is small or large. (Sodium chloride)
Answer: NaCl (s)+ H2 0(aq)→ Na+ (aqueous) + Cl– (aqueous) + H2 O(l)
When NaCl reacts with the water the Na+ part of the solution is attracted to the oxygen side of the water molecules and the Cl– part moves towards the hydrogen side of the water molecules.
7. Write down the concentration of the solution in g/L and mol/L.
7.3g HCl in 100ml solution
Answer: Concentration of HCL in g L-1 = mass of solute in grams / volume of HCl in litre = (7.3 × 1000) / 100 = 73 g L-1
Thus, 7.3g HCl in 100ml solution7.3g HCl in 100ml solution = 73 g L-1
Molecular mass of HCl = 1 + 35.5= 36.5
Meanwhile. Molarity = mass of the solute in moles / volume of HCl in L
= (7.3 × 1000) / (36.5 × 100) = 2 mol L-1
Therefore, in moles per litre 7.3g HCl in 100ml solution7.3g HCl in 100ml solution = 2 mol L-1 .
8. Classify the acids according to their basicity and give one example
of each type.
Answer: Basicity is the number of H+ ions obtainable by the dissociation of one molecule of an acid. On the basis of basicity, acids are divided into monobasic, dibasic and tribasic acids.
Mono-basic Acids –Acids that on ionisation produce one hydronium ion, which further produces one hydrogen ion when it reacts with water. Examples are HCl and HNO3 .
Di-Basic Acids- They produce two hydronium ions on ionisation and react with water to produce two hydrogen ions. Examples: H2 SO4 , H2 CO3 .
Tri-basic Acids- produces three hydronium ions on ionisation, which further produces three hydrogen ions when it reacts with water. H3 PO4 , H3 PO3 are examples.
9. What is meant by neutralization? Give two examples from everyday life of the neutralization reaction.
Answer: A reaction in which an acid and a base react to produce salt and water is known as neutralization. This involves the combination of H+ ions and OH– ions.
Neutralization of a strong base and strong acid has pH = 7.
Neutralization of a strong acid and a weak base has pH <7.
Neutralization of a strong base and a weak acid has pH >7.
HX + BOH → BX + H2 O
Acid + Base → Salt + Water
When a solution is neutralized salts are formed from equal weights of base and acid.
Examples of application of Neutralization in daily life.
10. Explain what is meant by electrolysis of water. Write the electrode reactions and explain them.
Answer: When electric current is passed through water, it decomposes into oxygen and oxygen gases. This process is called Electrolysis of water. The equation to represent electrolysis of water is H2 0(l).
In pure water, reduction occurs at the negatively charged cathode with (e– ) electrodes from the cathode being transferred to hydrogen cations to form hydrogen gas.
Cathode (Reduction Occurred ) 2H+ + 2e– → H2 .
Meanwhile, an oxidation occurs at the positively charged anode, which generates oxygen by giving electrons to the anode.
Oxidation at Anode 2H2 0→ 02 + 4H+ + 4e–
These reactions can also be balanced with base as given below: These half reactions have to be balanced with either an acid or base.
Cathode Reaction 2H2 0(l)l + 2e– → H2 (g) + 2OH– (aq)
Anode oxidation 40H– (aq)→ 02 (g) + 2H2 0(aq) + 4e–
Combining either of these half reaction pairs results in the same decomposition of water into hydrogen and oxygen. Hence, the overall reaction would be
2H2 0(l)→ 2H2 (g) + 02 (g)
Here, the number of hydrogen is twice that of the oxygen produced. So, the hydrogen gas produced will also have twice the volume of oxygen gas produced. Also the number of electrons passed through the water is twice that of the hydrogen molecules generated and four times that of the generated oxygen molecules.
11. Hydronium ions are always in the form H3 O+ . . Give reasons.
Answer: Hydronium ions are unstable and are normally bound to a neutral molecule, like the solvent. Hence, in water soluble, hydronium is bound to water molecules, thus forming the hydronium ions.
H+ + H2 O → H3 O+
12. Why do Buttermilk spoil if kept in a copper or brass container?
Answer: Buttermilk if kept in a brass or copper vessel spoils easily, as it contains lactic acid, which when it reacts with the copper or brass container produces toxic salts. These toxic salts spoil the buttermilk, thus causing food poisoning and making it harmful for human consumption.
13. NaOH solution was added to HCl solution. Write the chemical equation for this activity.
Answer: When Sodium Hydroxide (NaOH) is added to Hydrogen Chloride (HCl), a neutralisation reaction takes place, thus producing the salt Sodium Chloride (NaCl) and some water (H2 0). This reaction is because when acid and base react together they nullify each other. Thus, the pH of the solution will be 7, neutral, which is neither acidic nor basic.
NaOH(aq) + HCl(aq)→ NaCl(aq)+ H2 0(l)
14. Carbon dioxide gas was passed through the KOH solution. Write the chemical reaction for this.
Answer: The chemical reaction that occurs when Carbon dioxide (CO2) gas is passed through Potassium Hydroxide (KOH) solution is given below: To balance the equation, 2 is added before KOH.
2KOH + CO2 → K2 O3 + H2 O
Here, the base Potassium Hydroxide absorbs carbon dioxide the gas to generate potassium carbonate and water. This is just another example of acidic oxide like CO2 reacting with a strong alkali KOH to form salt and water.
15. Dilute nitric acid was added to calcium oxide. Write the equation.
Answer: Calcium oxide (CaO), when it reacts with nitric acid (HNO3 ) produces aqueous calcium nitrate [Ca(NO3 )2 ] and water (H2 0).
CaO (s) + 2HNO3 (aq) → Ca(NO3 )2 (aq) + H2 O(l).
16. State the difference between acids and bases.
Answer: A chemical compound that when dissolved in water can form a solution with a hydrogen ion activity greater than in pure water is known as an acid. Products with pH from 0 upto 7 are known as acidic. Meanwhile, an aqueous substance that can accept hydrogen ions is called base and it has a pH value between 7 to 14. Acids are proton donors and can accept electron pairs while bases are electron donors and can accept protons. Acids are corrosive in nature and sour in taste. Bases, on the other hand, are soapy in nature and bitter in taste. Acids can turn a blue litmus paper red and base can turn a red litmus blue. Also, acids will react with metals while bases cannot.
17. What is the difference between cations and anions?
Answer: An atom or a molecule that is negatively charged is an anion and has more number of electrons than protons. At the same time, the cation is an atom or a molecule that is positively charged and has more number of protons than electrons. Cations are metals and anions are non-metals.
18. What are the differences between Negative electrode and positive electrode ?
Answer: Negative electrode is also known as cathode and is connected to the negative terminal of the battery. Similarly, the positive electrode is known as the anode and is connected to the positive terminal of the battery. Also, during the electrolysis procedure, the metals are deposited at the cathode, while gases are liberated at the anode. Oxidation process takes place at the negative electrode and reduction occurs at the positive electrode.
19. What is an ionic bond?
Answer: Molecule of an ionic compound has two constituents, that is the cation and anions. Ionic bond is known as the force of attraction found between these ions. The force of attraction between one negative ion (cation) and one positive ion (anion) makes one ionic bond.
20. What indicates the stable state in an electronic configuration?
Answer: An electronic configuration with complete octet indicates a stable state.
21. Who put forth the theory of Acids and Bases in the year 1887?
Answer: Arrhenius, a Swedish Scientist put forth the theory of Acids and Bases in the year 1887.
22. What are strong acids? Give examples.
Answer: Strong acids dissociate completely when dissolved in water and the resulting aqueous solution consists of chiefly H+ ions and the concerned acidic radical. Examples are HCl, H2 SO4 , HBr.
23. What are weak bases? Give examples.
Answer: A weak base does not dissociate completely if mixed with water and the resulting aqueous solution contains a small portion of the OH- ions along with the concerned basic radical as well as a large proportion of the undissociated molecules of the base. Example NH4 0H.
24. What are alkali?
Answer: Bases that are highly soluble in water are known as Alkali. Examples include NaOH, KOH, NH3 and so on.
25. What does the taste of the foodstuff depend on? Give an example.
Answer: The taste of foodstuff depends upon the nature of the taste-giving ingredient as well as its proportion in the foodstuff. To confirm this, see the example given: Cut a lemon into two equal parts and take the juice of each part into two separate beakers. In one beaker, pour 10ml of drinking water and 20ml in the second one. Now, stir the solutions in both the beakers and then taste them. Both the solutions taste sour, because of the solute, lemon juice, in them. However, the quantity of lemon juice differs in each beaker. Even if both the beakers carry the same amount of solute, the quantity of the solvent differs as does the taste. The solution in the beaker with more amount of the solvent tastes more bitter.
26. What is molarity?
Answer: There are several units used to express the concentration of a solution. One is the mass of solute in grams dissolved in one litre of the solution. (grams per litre, g/L). Second one is the number of moles of the solute dissolved in one litre of the solution also called the molarity (M) of the solution. The molarity of a solute is indicated by writing its molecular formula inside a square bracket. Take the example of [NaCl] = 1 indicating that the molarity of this solution of common salt is 1M (1 Molar).
27. What is pH scale?
Answer: A convenient new scale to express H+ ion concentration was introduced by Soresen and found to be useful in chemical and biochemical processes. This new scale is called the pH scale, where pH indicates the power of hydrogen and extends from 0 to 14. This pH is midpoint of the scale. pH 7 indicates a neutral solution, while pH of an acidic solution is less than 7 and that of a basic solution is greater than 7.
28. How are basic and acidic salts formed?
Answer: The neutralization reaction between a strong acid and a weak base results in an acidic salt, while basic salt is produced by a neutralization reaction between a weak acid and a strong base. The pH of the aqueous solution of an acidic salt is less than 7, while that of a basic salt is greater than 7.
29. What are electrolytes?
Answer: A liquid or a solution should have a large number of dissociated ions if electrolysis is to take place. Electrolytes are these substances that undergo dissociation to such an extent in the liquid state or a solution. Electrolytes are also good conductors of electricity in their liquid or solution state.
30. How can pH of a solution be measured?
Answer: It is possible to determine the pH of a solution with the help of a universal indicator solution or the pH paper manufactured from it. Nevertheless, the most accurate method to measure the pH of a solution is to use an electrical instrument known as the pH meter. In this method, pH is measured by dipping electrodes into the solution.