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A student burns a magnesium ribbon in air and dissolves the ash of the ribbon. On pouring one drop each of this solution, first in blue litmus and then in red litmus solution, he would observe that
1. blue litmus turns colourless, whereas the red litmus remains red
2. blue litmus remains blue, whereas the red litmus turns blue
3. blue litmus turns red, whereas the red litmus litmus remains red
4. blue litmus turns red, whereas the red litmus turns blue

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Correct Answer - Option 2 : blue litmus remains blue, whereas the red litmus turns blue

Concept:

Metals:

  • The elements readily form positive ions (cations) are called metals.
  • Metals are usually solid, a good conductor of heat and electricity, malleable, ductile, sonorous, and have good tensile strength. 
  • Sodium, Potassium, Magnesium, Iron, Copper, Silver etc are few examples of metals. 
  • ​Metals react with oxygen to form their respective metal oxides, this reaction goes in the manner given by the general equation.

​Metal + oxygen → Metal oxides

  • To test the acidity and basicity of these metal oxides, they are made with water and then react with water and then tested with litmus paper.
  • A basic solution turns litmus to blue whereas an acidic solution turns blue litmus to red.
  •  Metals produce basic oxides when dissolved in water.

Non-Metal:

  • The physical properties of non - metals are the opposite of metals. For example, They are insulators, have lower tensile strength, dull appearance.
  • They form negative ions by accepting or gaining electrons.
  • Generally, the Oxides of non-metals are acidic in nature.

Explanation:

Magnesium ribbon gives magnesium oxide when burnt in the air. The ribbon burns with bright white light or white flame.

  • When magnesium metal burns it reacts with oxygen present in the air to form a compound called magnesium oxide.
  • This is an exothermic reaction and intense heat is produced.
  • The reaction produces white powder or white ash as magnesium oxide.
  • The balanced chemical equation is,

2Mg(s) + O2(g) → 2MgO(s) + Energy

  • This white ash is Magnesium oxide and it can be given as

Mg + O2 → 2MgO

  • When this White ash (Magnesium oxide) is dissolved in water (H2O) it gives Magnesium Hydroxide.

MgO + H2O → Mg(OH)2

  • When magnesium ribbon is burnt in the air, then it from MgO. This MgO when dissolving in water forms Mg(OH)2 which is basic in nature (pH = 10.1).

So, this solutions blue litmus remains blue, whereas the red litmus turns blue.

  • Non-metallic oxides are acidic in nature and metal oxides are basic in nature by usual convention. 
  • Some metal oxides, for example, oxides of aluminium and zinc show both acidic and basic nature. Such oxides are called amphoteric oxides.

  • The scale to decide the acidity and basicity is the pH scale. 
  • It is a scale from 0 to 14. 0 for most acidic, 14 most basic, and 7 is neutral.
  • The pH scale is determined by the presence of hydrogen ion H+ concentration in the aqueous solution. The more its concentration more acidic the solution will be. 
  • The basic solution has more OH - ion concentration than H+ ion.

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