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When discussing substances in chemistry, it's essential to understand the term 'mixture'. A mixture is a combination of two or more substances that are not chemically bonded together, which means they retain their individual properties. Unlike chemical compounds, the constituents in a mixture can be separated by physical means, such as filtration, evaporation, or decanting.

For example, when you mix salt and pepper, you create a mixture because they can be separated easily, and they do not react to form a new substance. Mixtures come in various forms, including homogeneous mixtures where the composition is uniform throughout, and heterogeneous mixtures where the components can be visibly distinguished. Understanding mixtures is crucial because it lays the foundation for exploring more complex concepts such as colloids and the composition of everyday substances like milk.

Colloids are fascinating mixtures that display properties distinct from ordinary solutions. The hallmark of a colloid is the size of its particles - they are larger than those in a solution, typically ranging between 1 to 1000 nanometers. Despite their size, colloidal particles are still small enough that they remain dispersed and do not settle out over time.

One of the key properties of colloids is the Tyndall effect, where light is scattered by the dispersed colloidal particles, often giving a cloudy appearance. This is why colloids are sometimes understood as a middle ground between true solutions and suspensions. Additionally, colloids can exhibit interesting behavior under the influence of electrical fields or by adding certain substances, which may cause them to coagulate or separate. Common examples of colloids in addition to milk include fog, whipped cream, and jellies.

Milk, a familiar household liquid, is not just a simple fluid but a complex colloidal system. The composition of milk is primarily water, constituting about 87% of its volume. However, dispersed within this water are vital nutrients such as proteins, fats, lactose (milk sugar), minerals, and vitamins.

The proteins and fats are kind of locked in a dance where they are dispersed as tiny particles throughout the liquid medium, forming the essential properties of a colloid. The fat, which would normally separate from the water due to its lower density, is kept in suspension by the milk proteins, one of which is known as casein, creating a stable colloidal dispersion. This unique composition not only accounts for milk’s colloidal properties but also its high nutritional value, making it an essential food item around the world.