From the following everyday materials, which one represents a pure substance rather than a mixture?

Introduction / Context:
A very important idea in basic chemistry is the distinction between pure substances and mixtures. Pure substances have a fixed, definite composition and consistent properties, whereas mixtures contain two or more substances physically combined and can have variable composition. Everyday materials like lemonade, air and sea water may look uniform but are usually mixtures. This question asks you to identify which item in the list is a pure substance in the chemical sense, assuming that it is taken in a chemically pure form.

Given Data / Assumptions:

• Sugar in this context refers to pure crystalline sucrose, not a mixture of different ingredients.
• Lemonade is prepared by mixing water, sugar and lemon juice, and may also contain other additives.
• Air is a mixture of several gases such as nitrogen, oxygen, carbon dioxide and traces of others.
• Sea water contains dissolved salts, gases and organic matter in water.
• Milk is a complex mixture or emulsion of water, fats, proteins, lactose and minerals.

Concept / Approach:
A pure substance can be either an element or a compound. It has the same composition throughout and can be represented by a single chemical formula, for example H2O for water or C12H22O11 for sucrose. Mixtures, even if they look uniform, contain more than one chemical substance and cannot be represented by a single fixed formula. The approach is to examine each option and decide whether it is a single compound or a combination of several substances. The one that is a single compound in pure form will be the pure substance among the options.

Step-by-Step Solution:
Step 1: Consider pure crystalline sugar. Chemically, table sugar is sucrose, a single compound with a definite formula C12H22O11. Step 2: Because sucrose has a fixed composition and uniform properties when pure, it qualifies as a pure substance. Step 3: Lemonade is made by mixing water, sugar and lemon juice, and sometimes flavours and colours. Its composition can vary from one glass to another, so it is a mixture. Step 4: Air is a physical mixture of gases like nitrogen, oxygen, argon and carbon dioxide in varying proportions, so it is not a pure substance. Step 5: Sea water contains water plus dissolved salts and other substances. It is also a mixture, not a pure substance. Step 6: Milk is an emulsion of fat droplets in water with proteins and other solutes. It clearly contains several different components and is a mixture. Step 7: Therefore, the only option that can be a pure substance in the strict chemical sense is pure crystalline sugar.

Verification / Alternative check:
Textbooks often use examples like distilled water, pure sodium chloride and pure sugar to illustrate pure substances. Analytical techniques show that a sample of pure sucrose yields only sucrose molecules on analysis, while lemonade and milk yield multiple different substances. Air and sea water have compositions that vary with location and conditions, which is another sign of mixtures. Only a chemically purified, single compound has the consistent composition and properties expected of a pure substance, confirming sugar as the correct answer here.

Why Other Options Are Wrong:
Option B, lemonade, is clearly a mixture of several ingredients and does not have a fixed formula. Option C, air, is a mixture of many gases and sometimes small particles of dust or pollutants. Option D, sea water, combines water with many dissolved ions and organic materials. Option E, milk, is a complex colloidal system with proteins, fats and sugars suspended or dissolved in water. None of these can be described by a single chemical formula, which means they are mixtures, not pure substances. Only option A can represent a pure substance when sugar is taken in a purified crystalline form.

Common Pitfalls:
A common mistake is to think that anything that looks uniform to the naked eye must be a pure substance. In reality, many homogeneous looking materials are mixtures, such as air and soft drinks. Another pitfall is to confuse everyday meanings of pure, like pure milk or pure air, with the strict chemical meaning. In chemistry, pure means chemically single component, not just free from visible dirt. Remember to ask whether the sample can be represented by one chemical formula. If the answer is yes, then it is likely a pure substance; if not, it is a mixture.

Final Answer:
The pure substance among the given options is Pure crystalline sugar (sucrose) only, which is a single compound with a definite chemical composition.