Okay, I will solve the tasks one by one and provide detailed explanations.

Task 1

The question asks which gas reacts with iron nails and causes them to corrode.

• Corrosion of iron (rusting) is a chemical reaction that involves iron, oxygen, and water.

Therefore, the correct answer is:

• A oxygen

Task 2

The question asks about the color change of the Universal Indicator when dilute hydrochloric acid is neutralized by aqueous sodium hydroxide.

• Hydrochloric acid (HCl) is an acid.
• Sodium hydroxide (NaOH) is a base.
• Neutralization is the reaction between an acid and a base.
• The Universal Indicator shows different colors depending on the pH of the solution. In acidic solutions, it is red, and in basic solutions, it is blue/purple. Neutralization will move the pH towards neutral (pH 7), which is green. However, since we are starting with an acid and neutralizing it with a base, the color will change from red to blue/purple as the solution becomes more alkaline.

Therefore, the correct answer is:

• A red to blue

Task 3

The question asks which of the listed household items contains a base.

1. Oranges: Contain citric acid (an acid).
2. Toothpaste: Often contains bases like sodium bicarbonate or calcium carbonate to neutralize acids in the mouth.
3. Salt: Sodium chloride (NaCl) is neutral.
4. Washing powder: Contains bases to help remove dirt and grease.

Therefore, the item(s) containing a base are toothpaste and washing powder.

The correct answer is:

• D 2 and 4 only