Which of the following is NOT a pure substance?
A. milk
B. oxygen
C. water
D. carbon dioxide
To determine which of the given options is not a pure substance, we need to understand what a pure substance is. A pure substance is a form of matter that has a fixed chemical composition and distinct properties. It consists of only one type of atom or molecule.
To determine if each of the options is a pure substance, we can try to break them down into their constituent components. If they can be broken down further into other substances, they are not pure substances.
A. Milk: Milk is a complex mixture that contains water, proteins, fats, lactose, vitamins, and minerals. Since it is a mixture of various substances, it is not a pure substance.
B. Oxygen: Oxygen is an element represented by the symbol O. It exists as individual oxygen molecules (O2) and is not a mixture. Therefore, oxygen is a pure substance.
C. Water: Water is a compound made up of two hydrogen atoms bonded to one oxygen atom (H2O). It always has the same chemical composition and properties, so it is a pure substance.
D. Carbon dioxide: Carbon dioxide is a compound composed of one carbon atom bonded to two oxygen atoms (CO2). Like water, it always has the same properties and composition, making it a pure substance.
Based on this analysis, the answer is A. Milk, as it is not a pure substance.