Question

Classify each of the following as a pure substance or a mixture. If a mixture, indicate whether it is homogeneous or heterogeneous: (a) rice pudding, (b) seawater, (c) magnesium, (d) crushed ice.

Short answer

(a) Rice pudding is a heterogeneous mixture. (b) Seawater is a homogeneous mixture. (c) Magnesium is a pure substance. (d) Crushed ice is a pure substance.

Step-by-step solution

Definitions

A pure substance is a type of matter that has a constant composition and distinct properties, meaning it consists of only one element or compound. A mixture is a combination of two or more pure substances that can be physically separated. A homogeneous mixture, also called a solution, is a mixture that has a uniform composition throughout, meaning it appears consistent and uniform when observed. A heterogeneous mixture is a mixture that does not have a uniform composition and can be visually separated into its components. Now we can classify each item as a pure substance or a mixture and determine the type of mixture when applicable. (a) Rice pudding

Classification - Rice pudding

Rice pudding is made up of multiple components, such as rice, sugar, milk, and spices. These parts can be separated through physical means. Thus, rice pudding is a mixture. Since we can visually see the different components, it is a heterogeneous mixture. (b) Seawater

Classification - Seawater

Seawater is composed of water and various dissolved salts. Because these components are mixed uniformly throughout the water, it is hard to see the separate components. Thus, seawater is a mixture. Since it appears consistent and uniform, it is a homogeneous mixture. (c) Magnesium

Classification - Magnesium

Magnesium is an element found on the periodic table. It is a type of matter that consists of only one kind of atom. Therefore, magnesium is a pure substance. (d) Crushed ice

Classification - Crushed ice

Crushed ice is made up of ice, which is essentially frozen water. Water is a compound, and crushed ice is just the same compound in a different physical state. Therefore, crushed ice is a pure substance.

Key concepts

Homogeneous Mixture

Imagine making a cup of saltwater. When you dissolve salt in water, you create a uniform mixture that's the same throughout. This is called a homogeneous mixture or solution. You can't see where the water ends and the salt begins; it's all mixed together seamlessly.

To understand homogeneous mixtures further, consider seawater as an example from the exercise. Seawater is a classic case of a homogeneous mixture because it contains water and dissolved salts and minerals. No matter where you sample the water from, you're likely to find a consistent composition—uniform and indistinguishable. Homogeneous mixtures are prime candidates for physical separation techniques like distillation, where you can recover the salt by evaporating the water.

Heterogeneous Mixture

Now, switch gears and think about a bowl of rice pudding. It's a delicious dessert, but if you look closely, you'll see grains of rice, maybe some raisins, and creamy milk. Unlike the saltwater, you can actually see the different parts of rice pudding. This is what we call a heterogeneous mixture. It's a physical blend where the different components remain distinct and can often be seen with the naked eye.

Other examples include things like salads or a box of assorted nuts. Heterogeneous mixtures don't look the same throughout, and this makes them easier to separate by physical means, like picking out nuts or sifting flour from sugar. Understanding the heterogeneous nature helps you know how to approach separating the mixture if needed.

Classification of Matter

Matter is all around us and it comes in different forms. Speaking scientifically, matter can be broadly classified into pure substances and mixtures.

Pure substances include elements, like magnesium from our exercise, which is made up of only one type of atom. Compounds, such as water, are also pure substances; even though they contain more than one element, they're always found together in the same fixed proportion.

Mixing pure substances together can create mixtures, which might be either homogeneous or heterogeneous, depending on whether the composition is the same or varies throughout the mixture. This classification is fundamental in chemistry because it determines how we study and manipulate different types of matter.

Physical Separation of Components

After classifying matter, you might wonder how to separate a mixture back into its pure substances. This is where physical separation techniques come into play. These methods take advantage of the physical properties of components, like their state, boiling point, or size.

For example, filtration can separate solids from liquids, as could be done with the rice from rice pudding. Distillation separates components with different boiling points, which is how you'd get fresh water from seawater. Other methods like chromatography can separate pigments based on how quickly they move across a surface. These techniques are essential in chemistry and are often used not only in laboratories but also in industries for purification processes.