Question Video: Explaining the Different Rates of Combustion in Air Compared with Pure Oxygen | Nagwa Question Video: Explaining the Different Rates of Combustion in Air Compared with Pure Oxygen | Nagwa

# Question Video: Explaining the Different Rates of Combustion in Air Compared with Pure Oxygen Science • Third Year of Preparatory School

## Join Nagwa Classes

Why is the combustion of aluminum in air slower than in pure oxygen? [A] The temperature of oxygen in air is greater than in pure oxygen. [B] The temperature of pure oxygen is greater than air. [C] The concentration of oxygen in air is less than in pure oxygen. [D] The concentration of oxygen in air is greater than in pure oxygen.

02:51

### Video Transcript

Why is the combustion of aluminum in air slower than in pure oxygen? (A) The temperature of oxygen in air is greater than in pure oxygen. (B) The temperature of pure oxygen is greater than air. (C) The concentration of oxygen in air is less than in pure oxygen. Or (D) the concentration of oxygen in air is greater than in pure oxygen.

Combustion or burning is a chemical reaction that usually involves the reaction of a substance with oxygen. In this case, the substance reacting with the oxygen is aluminum. In order for a reaction to occur between the aluminum and the oxygen, the particles will have to successfully collide with one another. If we can find a way to increase the total number of collisions, then the chance of a successful collision will increase. Another way to say this is that increasing the number of collisions will increase the number of reactions between particles. Thus, more collisions means a faster reaction.

The question describes two different conditions under which aluminum is combusted, in pure oxygen and in air. Pure oxygen is nearly 100 percent oxygen. We might describe pure oxygen as having a high concentration, where concentration is the amount of a substance in a particular volume. As pure oxygen is highly concentrated, there will be lots of collisions between the oxygen particles and aluminum particles, which should result in more reactions.

Air, on the other hand, is only composed of about 21 percent oxygen. So, we might describe air as having a low concentration of oxygen. As there are fewer oxygen particles, there will be fewer collisions, which will result in fewer reactions, so we can make the connection that decreasing the concentration decreases the number of collisions, which decreases the number of reactions. In other words, the lower the concentration, the slower the reaction.

The question asked us to explain why the combustion of aluminum in air is slower than in pure oxygen. We now know that this is the case because the concentration of oxygen in air is lower than the concentration in pure oxygen. The answer choice which correctly describes this relationship is answer choice (C). Thus, the combustion of aluminum in air is slower than in pure oxygen because the concentration of oxygen in air is less than in pure oxygen, or answer choice (C).

## Join Nagwa Classes

Attend live sessions on Nagwa Classes to boost your learning with guidance and advice from an expert teacher!

• Interactive Sessions
• Chat & Messaging
• Realistic Exam Questions