Please answer the following questions: 25) Why is magnesium ribbon cleaned before burning? 26) Identify compound X and the reaction type. Write the balanced equation. 27) Difference between arteries a

Here are the answers to your questions:

25. A magnesium ribbon should be cleaned before burning in air to remove the layer of magnesium oxide ($\text{MgO}$) that forms on its surface. This oxide layer is relatively unreactive and prevents the magnesium from readily reacting with oxygen during combustion.

i) Compound 'X' is iron(III) oxide ($\text{Fe}_2\text{O}_3$).

ii) The reaction is called the Thermite reaction.
The balanced chemical equation is:

$$\text{Fe}_2\text{O}_3(\text{s}) + 2\text{Al}(\text{s}) \rightarrow 2\text{Fe}(\text{l}) + \text{Al}_2\text{O}_3(\text{s}) + \text{Heat}$$

• Arteries carry oxygenated blood away from the heart to the body, while veins carry deoxygenated blood back to the heart.
• Arteries have thick, elastic, muscular walls to withstand high pressure, whereas veins have thinner walls and contain valves to prevent backflow of blood.

28. B) Four organs of the Human excretory system are:

• Kidneys
• Ureters
• Urinary bladder
• Urethra

i) Amoeba: Binary fission

ii) Planaria: Regeneration

30. Planets do not twinkle because they are much closer to Earth than stars and appear as extended sources of light. The light from a planet is considered a collection of many point sources. Any atmospheric turbulence that might cause one part of the planet's light to dim is compensated by another part brightening, resulting in a steady, non-twinkling appearance.