In basic water treatment, which two chemicals are most commonly used together to purify and disinfect drinking water supplies?

Introduction / Context:
Safe drinking water usually requires chemical treatment to remove harmful microorganisms. This general chemistry and environmental science question asks which pair of chemicals is commonly used to purify and disinfect water, especially in treatment plants or emergency disinfection situations.

Given Data / Assumptions:
- The question refers to purification and disinfection of drinking water, not industrial processes.
- Only one pair of chemicals listed is widely recognised as a practical, controllable disinfectant combination.
- The focus is on typical oxidising disinfectants that kill bacteria and other pathogens.

Concept / Approach:
Water disinfection is usually carried out with strong oxidising agents. The most common is chlorine, often added as chlorine gas, sodium hypochlorite, or bleaching powder. Hydrogen peroxide is another powerful oxidising agent and is used in some advanced treatment systems and point of use products. When combined in carefully controlled doses, chlorine and hydrogen peroxide can help in advanced oxidation processes, improving microbial kill and breakdown of some organic contaminants.

Step-by-Step Solution:
Step 1: Recall that chlorine is the classic disinfectant used worldwide in water treatment plants. Step 2: Recognise that hydrogen peroxide is also an oxidising disinfectant used in many water treatment and sanitation applications. Step 3: Check each option and identify which pair contains two realistic disinfectants for drinking water. Step 4: Note that combinations involving fluorine or nitrogen gas are not practically used to disinfect drinking water. Step 5: Conclude that the correct pair is chlorine and hydrogen peroxide, because both are established oxidising disinfectants that can be used to purify water.

Verification / Alternative check:
A quick check with typical water treatment methods shows chlorine, chlorine dioxide, ozone, and sometimes hydrogen peroxide as disinfection tools. Fluorine is used mainly for fluoridation in some regions, not for primary disinfection. Nitrogen and ozone together are not a standard pair for drinking water purification, and fluorine based pairs are not common disinfectant combinations. This confirms that the realistic pair among the options is chlorine and hydrogen peroxide.

Why Other Options Are Wrong:
- Chlorine and fluorine: Fluorine is too reactive and hazardous for routine water disinfection, and it is not used in this way.
- Fluorine and iodine: This is not a standard treatment pair; iodine is sometimes used in tablets for emergency disinfection, but fluorine is not paired with it for water treatment.
- Fluorine and ozone: Ozone can disinfect water, but it is not usually combined with elemental fluorine, which is extremely dangerous and impractical.
- Ozone and nitrogen: Nitrogen gas is inert under normal conditions and does not disinfect water, so this pair is not suitable.

Common Pitfalls:
Many learners see fluorine and think of water fluoridation and mistakenly assume that any fluorine containing pair must be correct. Another mistake is to assume that any strong oxidant combination is used in practice without considering safety and practicality. It is important to remember that realistic disinfection agents for drinking water must be controllable, safe at low doses, and commonly used in treatment systems.

Final Answer:
The correct pair of chemicals commonly used to help purify and disinfect drinking water is Chlorine and hydrogen peroxide.