Class 10 Science Chapter 2 Important Questions

Acids are those chemical which produce hydrogen ions in its aqueous solution for example: Citric acid, sulphuric acid, hydrochloric acid etc.
Properties of Acids are
(i) It has sour in taste.
(ii) It turns blue litmus into red.
(iii) It turns methyl orange colour into red.
(iv) Acids has corrosive in nature.

Bases are those chemical which produce hydroxide ions (OH-) in its aqueous solution for example: Sodium hydroxide, Calcium hydroxide, Magnesium hydroxide etc.
Properties of Bases are
(i) Base has bitter in taste.
(ii) Base convert red litmus into blue.
(iii) Base convert methyl orange colour into yellow.
(iv) Base convert the colour of Phenolphthalein into pink.
(v) Base does not react with metal.
(vi) Base does not react with metal carbonate and metal hydroxide carbonate due to its basic nature.
(vii) Only alkali bases are good conductor of electricity.

Litmus solution is a purple dye which is extracted from a plant lichen. This dye is used an indicator to tests for acids and bases. When litmus solution is neither acidic nor basic, its colour is purple. Acids change the colour of blue litmus to red. Bases changes the red colour to blue.

Indicator Type Its Colour Acids Base
Litmus Natural Purple Red Blue
Red cabbage leave Natural Red Red Green
Flower of hydra Natural Blue Blue Pink
Phenolphthalein Synthetic Coloueless Colourless Pink
Methyl orange Synthetic Orange Red Yellow
Turmeric Natural Yellow Yellow Reddish brown

When acid react with metal it produces metal salt and hydrogen gas.
For example:
Mg + 2HCL → MgCl₂ + H₂

2Na + 2HCL→2NaCl +H₂

Reaction of acid with metal carbonate when acid react with metal carbonate then it produces metal salt, carbon dioxide and water.

For example:
Na₂CO₃ + HCL → 2NaCl + CO₂ + H₂O

CaCO₃ + HCL → CaCl₂ + CO₂ + H₂O

(i) Na₂O + HCL → NaCl + H₂O

(ii) CaO + HCL → CaCl₂ + H₂O

(iii) MgO + H₂SO₄ → MgSO₄ + H₂O

When acid react with metallic oxide then it produces salt and water because of basic nature of metallic oxide.

When acid react with base then it produces salt and water and it is known as neutralization reaction.

For example: NaOH + HCL → NaCl + H₂O

Mg(OH)₂ + H₂SO₄ → MgSO₄ + 2H₂O

When metal hydrogen carbonate then it produces metal salt, Carbon dioxide, water.

For example:
NaHCO₃ + HCL → NaCl + CO₂ + H₂O
Ca(HCO₃)₂ + 2HCL→ CaCl₂ +2CO₂ +2H₂O

Acidic food stuff is stored in our house hold kitchen in plastic or glass container not in copper or metallic container because acid can react with these metals and can form poisonous substance.

All acids have hydrogen ions (H+) in common in their aqueous form.
Type of Acid:
Concentrated acid: Concentrated acid are those which contain less amount of water.
Dilute acid: Dilute acid are those which contain more amount of water.

When we dilute an acid with water then addition of acid in water is a correct way of dilution because when water is added into acid then exothermic reaction take place and lots of heat come out from the test tube. Due to which it can harm us while acid is added to water then endo thermic reaction take place due to assumption of heat in water.

Amphoteric oxide are those oxides which has acidic and basic both type of character.

For example:
NaOH + Zn → Na₂ZnO₃ + H₂O

pH scale is a scale for measuring hydrogen ions concentration. pH range between 0 to 14 thus, pH is a number which indicate the acidic or basic nature of a solution. The neutral value of pH is 7. A value less than 7 represent an acidic solution, while a value of more than 7 represent the basic nature of the solution.
Importance of pH in our daily life:
(i) pH of soil in the backyard: Every plant requires a specific pH for healthy growth. The optimum pH required for growth of a plant can be known by finding the pH of the place where such plant grow.

(ii) pH of our digestive system: Our stomach produce HCL which helps in the digestion of food. When the stomach has produce too much acid we feel pain and irrigation which are the indication of indigestion. To face this problem we take antacid which neutralize excess acid and bring relief.

(iii) pH changes as cause of tooth decay: Tooth enamel is calcium phosphate and is the hardest substance in the body. It does not dissolve with water buts gets corrode at pH below 5.5. Bacteria present in the mouth produce acid by dehydration of food particle left in the mouth after eating and thus spoil the teeth.

(iv) Plants and animal are pH sensitive: Living organism can survive in the narrow range (7.0 to 7.8) of pH. When acid rain flows into the river its lower the pH value and make survival of aquatic life difficult.

Different of bases are:
(i) Sodium Hydroxide: Its commercial name is caustic soda and its chemical formula is NaOH. It is used in manufacturing of soap, for making artificial fiber like rayon and in paper industries.

(ii) Potassium Hydroxide: Its commercial name is caustic soda and its chemical formula is KOH. It is used in manufacturing of nickel iron storage battery or absorbing CO₂ and in soap industries.

(iii) Calcium Hydroxide: Its commercial name is slaked lime and its chemical formula is Ca(OH)₂. It is used in manufacturing of bleaching powder in white washing and testing of carbon dioxide gas.

(iv) Magnesium Hydroxide: Its commercial name is milk of lime and its chemical formula is Mg(OH)₂. It is used in manufacturing of antacid or neutralization of acid in the stomach for removing Greece from cloth in dry cleaning and in cleaning window pans.

(v) Ammonium Hydroxide: Its commercial name is Ammonia water and its chemical name is NH4OH. It is used in as a cleaning agent and sanitizer in many household and industry cleaner.

Salt are the ionic compound consisting of two parts one part carrying a positive charge cation and the negative charge anion.

It is aqueous mixture of sodium chloride and water. When we pass electricity through this solution then it produces NaOH + H₂O → NaOH + H₂ + Cl. This process is known as Chloro alkali process.

Colour and taste: It is a colour less mixture with melting point 820 oC
Solubility: It is soluble at room temperature.
Action of heat: At room temperature it exists in the form of dehydrate sodium chloride. When we heat these crystals then the water molecules come out in the form of water and a black residue of NaCl remain.
Hygroscopic Nature: It is hygroscopic in nature because it absorbs moisture from the surrounding.
Action with Concentrated Sulphuric Acid:
H₂SO₄ + NaCl → NaHSO₄ + HCl
Action with silver nitrate:
AgNO₃ + NaCl → NaNO₃ + AgCl

(i)Chemical Name: Sodium: Bi carbonate
(ii) Chemical Formula: NaHCO₃
(iii) Formation: NaCl + CO₂ + H₂O → NH₄Cl + NaHCO₃
(iv) Colour and state: It is a white crystalline solid.
(v) Action of air: It is stable in air.
(vi) Solubility in water: It is soluble in water the solution will be alkali.
NOTE: We use baking soda in bakery or in kitchen because when we add baking soda in any substance it makes the food fluffy. Their carbon dioxide is responsible for the fluffiness of food like in cake. In bakery Baking powder is used instead of baking soda because baking powder contain mild acid like tartaric acid.
(vii) Uses of Baking soda: It is used in kitchen.
It is also used in medicine as antacid.
It is used in soda acid fire extinguisher.

Base which are soluble in water and produce OH- are called alkalis. Some examples are:
Sodium hydroxide NaOH → Na+ + OH-.
Potassium hydroxide KOH → K+ + OH-.
All alkalis are base but all base are not alkalis. For examples A(OH)₃ is a base but not a alkalis.

The gas evolved is hydrogen gas which is used in the hydrogenation to produce Vanaspati. The equation of the reaction is:
Zn + 2HCL → ZnCl₂ + H₂.
Hydrogen gas burs with pop sound 2H₂ + O₂ → 2H₂O.

Lemon juice contain citric acid.
Vinegar contain acetic acid.
Vitamin C tablets contain ascorbic acid.
Tamarind contains tartaric acid.
Sour milk contains lactic acid.
Orange contains citric acid.

All living things are very sensitive to acids and too much acid can kill cells or stop proper working of cells. Concentrated acid are very dangerous and should never be handled without protection.
The following projection must be observed while dealing with acids.
(i) Never try to touch or taste acid which are used in the laboratory.
(ii) Never add water into concentrated acid otherwise bottle will break. Always add conc. Acid to water very slowly with continuous cooling under running water.

(i) Hydrochloric acid is released in stomach to make medium acidic in nature. It leads to coagulation of protein and helps in their digestion.
(ii) HCL kills bacteria coming to stomach along with the food.
(iii) Vinegar (Acetic acid) is used as preservation in pickles and in chines food. It gives sour taste to the food.
(iv) Cold drinks contain carbonic acid.
(v) Lemon contain citric acid which is used in case of indigestion.
(vi) Orange and Amla contain ascorbic acids (Vitamin C) while prevent scurvy.
Q30 What do you understand by the Strong and Weak acids?
Ans. Strong Acid: Those acids which dissociate into ions completely is called Strong acids. For example H₂SO₄, HCL, HBr, HI, HNO₃, HCIO₄ are strong acids.
Weak acids: Those acids which do not dissociate into ions completely are called weak acids. For example: Citric acid, Acetic acid, Ascorbic acid, Tartaric acid, Formic acid.

Salts are formed by reaction of base with acid. Sulphide are regarded as salts because they are formed from acid (H₂S) and base (like NaOH,KOH)

2NaOH + H₂S → Na₂S + 2H₂O