Kerala Syllabus Class 8 Basic Science: Chapter 13 Acids, Bases, Salts - Questions and Answers
Std 8: Physics: Chapter 13: Acids, Bases, Salts: Questions and Answers
♦ Acid in the school laboratory.
Hydrochloric acid (HCl), sulphuric acid (H₂SO₄) and nitric acid (HNO₃) are common acids used in school laboratories.
♦ Reaction of acids with metals.
♦ Experiment
Take a small piece of zinc in a test tube. Add a little dilute hydrochloric acid to it. Hold a burning matchstick at the mouth of the test tube.
Observation: A gas evolves from the test tube. The gas catches fire with a 'pop' sound when a burning matchstick is shown.
Inference: Hydrogen gas is produced.
♦ Repeat the above experiment by adding sulphuric acid instead of hydrochloric acid. Record the observation.
Repeat the experiment using magnesium and hydrochloric acid, and also with magnesium and sulfuric acid. Record the observation.
| Experiments - Materials | Observation & Inference |
|---|---|
| • Magnesium and hydrochloric acid • Zinc and sulfuric acid • Magnesium and sulfuric acid | Gas is liberated from the test tube. This gas catches fire with a 'pop' sound when a burning matchstick is shown. The liberated gas is Hydrogen |
♦ Complete the table given below.
♦ When acids react with highly reactive metals, --------- gas is produced.
Hydrogen
♦ Let's do an experiment to understand how acids react with carbonates.
♦ Experiment
Take some calcium carbonate (marble pieces) in a boiling tube. Add dilute
hydrochloric acid to it through a tistle funnel. Pass the gas that comes out through clear lime water in a test tube.
Inference: Carbon dioxide (CO₂) gas is liberated. This CO₂ gas turns lime water milky.
♦ Which gas is released through the delivery tube?
Carbon dioxide (CO₂)
♦ Repeat the experiment using sodium carbonate and dilute sulphuric acid. Record your observations.
If the same experiment is repeated with sodium carbonate (washing soda) or any other carbonates, and dilute sulphuric acid, the same observation will be made.
♦ When acids react with carbonates, ............... gas is liberated.
Carbon dioxide (CO₂)
♦ What are the properties of acids
• Have a sour taste
• Turns blue litmus red.
• Carbon dioxide gas is formed by reacting with carbonates.
• Hydrogen gas is liberated by reacting with highly reactive metals such as zinc and magnesium.
♦ Hydrogen is a common component in acids
• What is the symbol for hydrogen?
H
• Hydrogen ion (H⁺) is the ion formed from hydrogen.
• What is the chemical formula of hydrochloric acid?
HCl
• When hydrochloric acid dissolves in water, hydrogen and chloride ions with opposite charges are formed.
HCl → H⁺ + Cl⁻
• Which are the ions formed when nitric acid (HNO₃) dissolves in water?
HNO₃ → H⁺ + NO₃⁻
♦ Complete the table given below.
| Name of acid | Chemical formula | Ions liberated when dissolved in water |
|---|---|---|
| Hydrochloric acid | HCl | H⁺, Cl⁻ |
| Sulfuric acid | H₂SO₄ | 2H⁺, SO₄²⁻ |
| Carbonic acid | H₂CO₃ | 2H⁺, CO₃²⁻ |
| Phosphoric acid | H₃PO₄ | 3H⁺, PO₄³⁻ |
| Hydrogen ions are the basis of the general properties of acids. Acids are substances that can increase the concentration of hydrogen ions (H⁺) in aqueous solution. |
|---|
Acids
♦ What is the basis of the general properties of acids?
The presence of hydrogen ions (H⁺) in aqueous solution.
♦ How do acids affect the concentration of hydrogen ions in water?
Acids increase the concentration of hydrogen ions (H⁺) in aqueous solution.
♦ Why are hydrogen ions important in determining the strength of an acid?
The strength of an acid depends on how easily it releases hydrogen ions in solution.
♦ What happens when hydrogen ions (H⁺) are present in water?
H⁺ ions combine with water molecules (H₂O) to form hydronium ions (H₃O⁺).
♦ Write the chemical equation showing the formation of hydronium ions.
H⁺ + H₂O → H₃O⁺
♦ Substances with sour taste that we use in everyday life, such as lemon juice, yoghurt, and vinegar contain organic acids. Note their list in the table.
| Substances | Acid responsible for the sour taste |
|---|---|
| • Lemon juice | Citric acid |
| • Yoghurt/ Buttermilk | Lactic acid |
| • Tamarind | Tartaric acid |
| • Venegar | Acetic acid |
♦ Which is the gas liberated when a soda bottle is opened?
Carbon dioxide (CO₂)
♦ Find out the nature of soda water using litmus paper.
On testing with litmus paper, soda water turns blue litmus red.
♦ Is soda water acidic or basic?
Soda water is acidic.
(This is because dissolved carbon dioxide (CO₂) reacts with water to form carbonic acid (H₂CO₃), which releases hydrogen ions (H⁺).)
♦ Which is the acid in soda water?
Carbonic acid (H₂CO₃)
♦ Write the chemical formula of the acid in soda water.
H₂O + CO₂ → H₂CO₃
(Carbon dioxide (CO₂), a nonmetal oxide, reacts with water to form carbonic acid (H₂CO₃).)
♦ Complete the chemical equation for the reaction of sulphur dioxide (SO₂) gas in water to form acid.
H₂O + SO₂ → H₂SO₃ (Sulphurous acid)
♦ Some nonmetals and their important oxides are given in the table below.
| Element | Main oxide of element |
|---|---|
| • Carbon | Carbon dioxide (CO₂) |
| • Nitrogen | Nitrogen dioxide (NO₂) |
| • Phosphorus | Phosphorus pentoxide (P₂O₅) |
| • Sulphur | Sulphur trioxide (SO₃) |
♦ Write the chemical formula and chemical name of the acids formed when the oxides given in the table react with water.
The substances formed when nonmetal oxides react with water generally show acidic properties.
♦ What are the general properties of bases?
• Turns red litmus blue.
• Have a bitter taste
• Solutions of bases are slippery in nature
♦ Basic nature of metal oxides
♦ Experiment
Burn a well-scrubbed and cleaned magnesium ribbon. A white powder of magnesium oxide is formed. Collect it and dissolve in water. When tested with litmus, it turns red litmus paper blue.
The chemical equation of this reaction - MgO + H₂O → Mg (OH)₂ (Magnesium hydroxide)
• Magnesium hydroxide, formed by dissolving Magnesium oxide in water, is basic in nature.
♦ Now let's do another activity.
Add a little bit of quicklime (calcium oxide) to the water in a beaker and stir. Take a little clear solution from the beaker into a test tube and add a drop of red litmus solution to it. What do you observe? What does this litmus test shows about the nature of this substance?
When red litmus is added to the clear solution, it turns blue, showing that the solution is basic. Quicklime reacts with water to form calcium hydroxide (Ca(OH)₂), a base. In general, metal oxides dissolve in water to produce bases.
♦ What is the substance formed when calcium oxide reacts with water? Complete the chemical equation of the reaction.
Quicklime (calcium oxide) is the oxide of calcium metal. Quicklime (CaO) react with water to produce slaked lime, which is the hydroxide of calcium (Calcium hydroxide - Ca(OH)₂). This is also a base.
CaO + H₂O → Ca(OH)₂)
♦ Are CaO and MgO metal oxides or nonmetal oxides?
CaO and MgO are metal oxides that produce bases when they react with water.
♦ Observe the chemical equation of the reaction that occurs when sodium hydroxide dissolves in water.
NaOH → Na⁺ + OH⁻ (hydroxide ion)
Complete the ionisation equation that occurs when calcium hydroxide dissolves in water.
Ca(OH)₂ → Ca²⁺ + 2OH⁻
♦ What is the common ion liberated when alkalies dissolve in water?
When alkalies (like NaOH, KOH, Ca(OH)₂) dissolve in water, the common ion liberated is the hydroxide ion (OH⁻).
♦ What are alkalies?
Alkalies are substances that can increase the concentration of hydroxide ions in aqueous solution.
♦ Give two examples of alkalies.
Sodium hydroxide (NaOH) and potassium hydroxide (KOH).
♦ Some alkalies
| Common name | Chemical name | Chemical formula |
|---|---|---|
| Caustic soda | Sodium hydroxide | NaOH |
| Milk of lime | Calcium hydroxide | Ca(OH)₂ |
| Caustic potash | Potassium hydroxide | KOH |
♦ Neutralisation reaction
The chemical reaction in which an acid and a base react and lose their properties to form water and a salt is known as neutralisation reaction.
♦ What happens when dilute hydrochloric acid and dilute sodium hydroxide solution are mixed?
Experiment
Take 50 mL of dilute hydrochloric acid in a burette. Take 20 mL of dilute sodium hydroxide solution in a conical flask using a pipette. Add one or two drops of phenolphthalin into it. The solution became pink coloured. This shows the alkaline nature of NaOH.
Slowly add dilute HCl to the conical flask. Keep stirring the solution in the conical flask. The pink colour appears to fade when it fades too much. Add HCl drop by drop and stir. When the colour disappears completely on adding one drop of HCl, stop adding acid. That means the basic nature of the solution is completely changed. At this stage, the solution is said to be neutralised. This is called neutralisation.
Analysis of the experiment
• Colour of NaOH solution when phenolphthalein is added: Pink (This shows the basic nature of NaOH).
• Change in colour when HCl is added: Pink colour faded. (This means the basic nature of solution is decreasing).
• Presence of NaOH in the solution when the colour has completely disappeared: Absence of NaOH
• When more NaOH is added to the colourless solution: Pink colour appears again
• If more HCl is added at this stage: Pink colour disappears.
Neutralisation: Acid + Alkali → Salt + Water
♦ Which was the colour of NaOH solution when phenolphthalein was added?
Pink
♦ What property of NaOH solution does this indicate?
It indicates the basic (alkaline) nature of sodium hydroxide solution. Phenolphthalein turns pink in a basic medium.
♦ What can be understood from the fact that the colour of the NaOH solution decreases as HCl is added?
The addition of hydrochloric acid gradually neutralises the base. As the acid reacts with NaOH, the solution becomes less alkaline, so the pink colour fades.
♦ Will NaOH remain in the conical flask when the colour completely disappears?
No. When the pink colour disappears, it means the NaOH has been completely neutralised by HCl, so no NaOH remains in the solution.
♦ Add a little NaOH to the solution that has completely changed colour. What do you see? What is the reason for the observation?
• Observation: Pink colour reappears.
• Reason: The solution becomes basic again, and phenolphthalein shows pink in a basic medium.
♦ Add dilute HCl drop by drop to it again and mix. What is your observation?
• Observation: Pink colour fades and disappears.
• Reason: HCl neutralises NaOH; the solution becomes neutral, so phenolphthalein turns colourless.
♦ What is the role of hydrochloric acid in our stomach?
Hydrochloric acid helps in the digestion of food by breaking it down and activating digestive enzymes.
♦ What problems can occur if the acid content in the stomach is too high?
Excess acid can cause heartburn and indigestion.
♦ What are antacids?
Antacids are medicines used to reduce acidity in the stomach.
♦ Name a commonly used antacid.
Magnesium hydroxide, also known as milk of magnesia, is a commonly used antacid.
♦ How do antacids work?
Antacids neutralise excess stomach acid, giving relief from acidity and indigestion.
♦ pH scale
The pH value determines the degree of acidic or basic nature of soil and other substances.
♦ Who developed the pH scale?
The pH scale was developed by the Danish scientist Sorensen.
♦ On what basis was the pH scale developed?
The pH scale is based on the concentration of H⁺ ions in a solution.
♦ What does a low pH value indicate?
A low pH value indicates that the substance is acidic.
♦ What does a high pH value indicate?
A high pH value indicates that the substance is basic (alkaline).
♦ What is the pH value of a neutral solution?
The pH value of a neutral solution is 7.
♦ What characteristics do solutions with a pH value greater than 7 show?
Solutions with a pH greater than 7 are basic (alkaline) in nature.
♦ What characteristics do solutions with a pH value less than 7 show?
Solutions with a pH less than 7 are acidic.
♦ Find the pH values of the following substances using pH paper and complete the table.
| Name of substance | pH value | Acidic/Basic/Neutral |
|---|---|---|
| Lemon juice | 2-3 | Acidic |
| Dilute Hydrochloric acid | 3.5 | Acidic |
| Water | 7 | Neutral |
| Common salt solution | 7 | Neutral |
| Sodium hydroxide solution | 13-14 | alkaline |
| Soap solution | 9-10 | alkaline |
♦ As the pH value increases, does it become more acidic or more basic?
As the pH value increases, the solution becomes more basic (alkaline).
♦ Why is the pH of soil important for agriculture?
The pH of the soil is one of the factors that affects the proper growth of agricultural crops. Acidic soil is suitable for some crops while basic soil is suitable for others.
♦ What is added to reduce the acidity of acidic soil?
Slaked lime (Ca(OH)₂) is added to reduce soil acidity.
♦ What substances are added to reduce the basic nature of soil?
Acidic substances such as ammonium sulphate and aluminium sulphate are added to reduce soil alkalinity.
♦ What are the products formed when sodium hydroxide and dilute hydrochloric acid react?
Sodium chloride and water.
NaOH + HCl → NaCl + H₂O
♦ Here, what is the product formed when the common component of the acid and the common component of the alkali combine?
Water (H₂O)
♦ What is the product formed when the positive ion in sodium hydroxide and the negative ion in hydrochloric acid combine?
Sodium chloride (NaCl).
♦ Experiment
Take 5 mL of sodium chloride solution in a test tube. Add 2 mL of silver nitrate solution to it.
Observation: A white precipitate is formed. The white precipitate is the result of a combination of Cl⁻ ions from NaCl and Ag⁺ ion from AgNO₃, to form AgCl (Silver chloride).
♦ Repeat the experiment in the same way using KCl solution, NH₄Cl solution, CaCl₂ solution, and hydrochloric acid. Are the observations the same?
Chloride (Cl⁻) ion from any chloride salt reacts with silver nitrate to produce the same product- silver chloride, which exists as the white precipitate.
• What is observed when silver nitrate solution is added to sodium chloride solution?
A white precipitate of silver chloride (AgCl) is formed.
• What happens when silver nitrate solution is added to KCl, NH₄Cl, CaCl₂, or HCl solutions?
The same observation — a white precipitate of silver chloride (AgCl) — is formed.
• Why are the observations the same in all cases?
Because all these solutions contain chloride ions (Cl⁻), which react with Ag⁺ ions from silver nitrate to form insoluble AgCl.
| Chloride(Cl⁻) ion react with silver nitrate to form white precipitate of silver chloride. |
|---|
♦ Complete the chemical equation for the reaction between dilute sulphuric acid (H₂SO₄) and magnesium hydroxide (Mg(OH)₂) solution.
Mg(OH)₂ + H₂SO₄ → MgSO₄ + 2H₂O
♦ What is the salt formed?
Magnesium sulphate (MgSO₄).
♦ Find and write the acid and alkali/base that must be reacted to obtain the salts given in the table below.
Step 1: Take a pinch of ammonium sulphate in a test tube and mix it well with some water. Pour barium chloride solution into it.
Observation: A white precipitate is formed.
Step 2: Pour dilute hydrochloric acid into it.
Observation: No visible change. The precipitate does not dissolve in HCl.
♦ Repeat the experiment using solutions of sulphate salts and sulphuric acid available in the school laboratory.
For any solution of sulphate salts, the same observations are obtained. So this can be used to identify sulphate salts.
| The white precipitate formed in the experiments is due to the barium sulphate formed by the reaction between sulphate (SO₄²⁻) ion and barium chloride solution. Barium sulphate does not react with dilute hydrochloric acid. |
|---|
♦ What happens when sulphate ions react with barium chloride solution?
A white precipitate of barium sulphate (BaSO₄) is formed.
♦ What is the cause of the white precipitate in this reaction?
The combination of SO₄²⁻ ions from the sulphate solution and Ba²⁺ ions from barium chloride produces insoluble BaSO₄.
♦ Does barium sulphate react with dilute hydrochloric acid?
No, barium sulphate does not react with dilute hydrochloric acid.
♦ What are the products formed when sodium hydroxide and dilute nitric acid react?
Sodium nitrate and water.
NaOH + HNO₃ → NaNO₃ + H₂O
♦ What is the salt formed here?
Sodium nitrate
♦ How about an experiment to identify nitrate salts?
Experiment
Take a little ammonium nitrate salt solution in a test tube. Add freshly prepared ferrous sulphate solution to it. Add concentrated sulphuric acid drop by drop very slowly through the sides of the test tube.
Observation: A brown ring has appeared at the surface of separation of the solution mixture and sulphuric acid.
This test, used for the identification of nitrates, is called the Brown ring test.
♦ What observations do you make when you repeat the previous experiment using the nitrate salts and nitric acid?
All nitrate salts and nitric acid give the same positive Brown Ring Test.
| When nitrate (NO₃⁻)ion, freshly prepared ferrous sulphate solution and concentrated sulphuric acid react, a brown ring forms between the two liquids. |
|---|
♦ What observation is made in the Brown Ring Test?
A brown ring appears at the junction of the two liquid layers.
♦ What does the formation of the brown ring indicate?
It confirms the presence of nitrate ions (NO₃⁻) in the solution.
♦ Let's do an experiment to identify carbonate salts.
Experiment
Take a little sodium carbonate salt solution in a test tube. Add 2mL of dilute hydrochloric acid to it. Pass the gas released from the test tube through clear lime water.
Observation: Clear lime water turns milky.
♦ Repeat the previous experiment using other carbonate salts available in the laboratory.
Other carbonate salts also give the same observation in this test.
| The formation of bubbles in the test tube is due to the liberation of carbon dioxide as a result of the reaction between carbonate salt and hydrochloric acid. Clear lime water turns milky when carbon dioxide passes through it. |
|---|
Let's Assess
1. Complete the table.
| Substance | The colour change that occurs when exposed to moist red litmus | The colour change that occurs when exposed to moist blue litmus |
|---|---|---|
| Lemon juice | No colour change | Turns red |
| Lime water | Turns blue | No colour change |
| Sodium hydroxide | Turns blue | No colour change |
| Vinegar | No colour change | Turns red |
| Dilute hydrochloric acid | No colour change | Turns red |
i. Nitrogen dioxide dissolves in water to form nitric acid.
ii. Phosphorus pentoxide (P₂O₅) is a non-metal oxide.
iii. Sulphur trioxide turns moist red litmus blue.
Answer: i, ii
3. Explain any two cases where the importance of neutralisation reaction is used in daily life.
• Antacids neutralise excess stomach acid, giving relief from acidity and indigestion.
• Used in agricultural fields to maintain the acidity/basicity of soil.
4. Solutions of some chemicals used in the laboratory are given in Box 1 below. Using these, write which salts given in Box 2 you can identify. Explain the method of experiment and observation.
The reaction between ammonium carbonates and dilute HCl results the liberation of CO₂ gas.
Experiment
Take a little ammonium carbonate salt solution in a test tube. Add 2mL of dilute hydrochloric acid to it. Pass the gas released from the test tube through clear lime water.
Observation: Clear lime water turns milky.
• Silver nitrate solution - Ammonium chloride
Ammonium chloride react with silver nitrate to form silver chloride.
Experiment
Take 5 mL of ammonium chloride solution in a test tube. Add 2 mL of silver nitrate solution to it.
Observation: A white precipitate is formed. The white precipitate is the result of a combination of Cl⁻ ions from NaCl and Ag⁺ ion from AgNO₃, to form AgCl (Silver chloride).
5. A bottle of salt in the lab had its label on it disintegrated. The details of the experiments conducted to identify the salt are given below.
• When a little barium chloride solution was added to the salt solution, a white precipitate formed.
• When dilute hydrochloric acid was added to the white precipitate, no observable change occurred.
Which salt is it? (Ammonium sulphate, Ammonium carbonate)
Answer: Ammonium sulphate
6. A colourless acid solution is in a bottle in the laboratory. 5 mL of it is taken in a test tube. 2 mL of dilute hydrochloric acid is added to it, no observable changes occur. After taking 5 mL of the solution in another test tube, when 2 mL of silver nitrate solution is added, a white precipitate is formed.
(i) Which acid is the solution in the bottle?
When NaOH solution is added dropwise to 5 mL of the solution, it is
observed that the pH gradually increases.
(ii) When the pH reaches 7, what are the possible compounds present in the solution?
(iii) Write the chemical equation for the reaction between NaOH and the acid solution.
Answer:
i) Hydrochloric acid
ii) NaCl, H₂O
iii) NaOH + HCl → NaCl + H₂O
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