Nature of Matter : Elements, Compounds, and MixturesClass 8 Science Notes

What Are Mixtures?

Have you ever wondered what makes a dish like poha so delicious, or how to make a perfect sprout salad? These dishes, though very different, share a common element: they are both made by mixing several ingredients. The mixing of substances is something we see every day. For example, sugar dissolved in water is a mixture, just like soups and lemonade.

A mixture is formed when two or more substances are combined, with each substance retaining its individual properties. The substances that make up a mixture are called its components. Importantly, these components do not react chemically with each other.

There are two main types of mixtures:

• Non-uniform mixtures: In some mixtures, like a sprout salad with green gram, chickpeas, onion, and tomato, the components are easy to see with the naked eye or with a magnifying device. These mixtures are described as non-uniform in nature.
• Uniform mixtures: In other mixtures, like sugar and water, the components cannot be seen separately, even with a microscope. In these uniform mixtures, the components are evenly distributed and cannot be distinguished.

A step further

Stainless steel is also a mixture. It contains iron, nickel, chromium, and a small amount of carbon, mixed so uniformly that it appears the same throughout. These kinds of mixtures are known as alloys. Other examples of alloys include brass (a mixture of copper and zinc) and bronze (a mixture of copper and tin).

Our scientific heritage

Mishraloha was the name given to mixtures of two or more metals in ancient India, and these mixtures had properties different from the metals they were made from. Ancient Indian texts like the Charaka Samhita, Susruta Samhita, Rasaratna Samucchaya, and Rasa Jala Nidhi mention the use of alloys for medicinal purposes. For example, bronze, also known as Kamsya, is an alloy of copper (Tamra, four parts) and tin (Vanga, one part), and it was used to improve digestion and boost immunity.

Is air a mixture?

Air is a uniform mixture consisting mainly of nitrogen, oxygen, argon, carbon dioxide, and water vapour.

• Oxygen is vital for most living beings and supports combustion.
• Nitrogen, which makes up about 78% of the air, does not participate in combustion.
• Air also contains water vapour, which turns into liquid water droplets when warm air touches a cool surface.

To confirm the presence of carbon dioxide in the air, we can conduct a simple activity.

Activity 8.1: Let us experiment

1. Take a glass tumbler and fill it half with water.
2. Add a small amount of calcium oxide (quick lime) slowly to it.
3. Calcium oxide reacts vigorously with water to form calcium hydroxide and releases heat.
4. Stir continuously to make a solution of calcium hydroxide. This solution is called lime water.
5. Filter it and observe its colour.
6. Leave this colourless solution in a petri dish for a few hours.
7. Keep stirring the solution at regular intervals.

Lime water turns milky because carbon dioxide reacts with calcium hydroxide to form calcium carbonate (insoluble tiny white particles) and water. This demonstrates the presence of carbon dioxide in the air.

Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water

Apart from gases, dust particles are also present in the air.

Activity 8.2: Let us explore

1. Take a black sheet of paper, ensuring it's free from visible dust particles.
2. Place the black sheet of paper undisturbed near an open window, or in the garden, for a few hours.
3. Observe the surface of the paper.

You may notice tiny particles settled on its surface. This shows that dust particles are suspended in the air. They are not an integral part of the air and are considered pollutants. The amount and nature of dust particles in the air may vary from time to time and from place to place.

A step further

The major pollutants present in the air are particulate matter (dust, soot) and gases like carbon monoxide, ozone, nitrogen dioxide, and sulfur dioxide. The air quality index (AQI) is a tool used to describe the air quality.

Types of mixtures

The term 'mixture' generally refers to the mixing of two or more components. These components can be mixtures themselves (like poha and sprout salad) or pure substances (like sugar or common salt dissolved in water). In science, however, all the components of a mixture must be pure substances only.

Mixtures can be of several types, depending on the physical state of their components.

What Are Pure Substances?

Have you ever noticed the word 'pure' written on the packs of consumables like milk, ghee, and spices? The word 'pure' has slightly different meanings in common usage and in science.

In common usage, 'pure' means unadulterated products. Adulteration is an illegal process of adding cheaper or poor-quality substances to a product, usually to increase the quantity or reduce the manufacturing cost. This deteriorates the quality of the product and can make it hazardous to health.

In science, a pure substance is one that has no other substance present in it. Even products that look pure can be considered impure if they are made of more than one substance.

A pure substance cannot be separated into other kinds of matter by any physical process. It consists of the same type of particles.

What Are the Types of Pure Substances?

Recall the different states of water. Cooling water converts it into ice, and boiling water converts it into vapour. However, the particles of water remain the same during these processes.

Now, let's consider another activity where we pass electricity through water and observe its effect.

Activity 8.3: Let us experiment (Demonstration activity)

1. Collect two small test tubes, a beaker or a glass tumbler, and a 9 V battery.
2. Fill 2/3rd of the beaker with water and add a few drops of dilute sulfuric acid to it.
3. Fill both the small test tubes completely with water taken from the beaker.
4. Place a 9 V battery inside the beaker.
5. Without spilling the water, carefully place the water-filled test tubes on each of the terminals of the battery.
6. Wait for a few minutes.
7. Observe the formation of any gas bubbles at both the terminals inside the test tubes.
8. Let it continue for 10-15 minutes.
9. Observe the volume of gas collected in each test tube.
10. Remove these test tubes one-by-one carefully.
11. Test these gases one-by-one by bringing a burning candle close to the mouth of the test tubes.

To test the gases present in the two test tubes, bring a burning candle near the mouth of each test tube.

• A pop sound can be heard from one, indicating the presence of hydrogen gas.
• In the other test tube, the flame of the burning candle will glow brighter, confirming the presence of oxygen gas.

From Activity 8.3, we can infer that water is composed of two different constituents: hydrogen and oxygen.

Water → Hydrogen + Oxygen

Elements

The two substances, hydrogen and oxygen, formed in Activity 8.3, are pure substances and are termed as elements. Each element is made up of identical particles called atoms. These particles are different from the particles of any other element. Elements are substances that cannot be further broken down into simpler substances and are the building blocks of all matter. Some other examples of elements are gold, silver, sulfur, and carbon.

A step further

The atoms of most elements cannot exist independently. Two or more such atoms combine and form a stable particle of that element called a molecule. For example, two atoms of hydrogen combine to form one molecule of hydrogen. Similarly, two atoms of oxygen combine to form one molecule of oxygen.

Elements can be classified into metals and non-metals.

• Examples of metals include gold, silver, magnesium, iron, and aluminium.
• Examples of non-metals include carbon, sulfur, hydrogen, and oxygen.

Some elements, like silicon and boron, have intermediate properties between those of metals and non-metals. These are called metalloids.

• The number of elements known at present is 118, and most of them exist in a solid state.
• Eleven elements exist in a gaseous state at room temperature, all of which are non-metals like oxygen, helium, nitrogen, etc.
• Only two elements are liquid at room temperature: mercury, which is a metal, and bromine, which is a non-metal.
• Although gallium and caesium are solid elements, they become liquid at a temperature around 30°C (303 K).

A step further

More than 45 different elements, like aluminium, copper, silicon, cobalt, lithium, gold, and silver, are used in manufacturing a mobile phone, including its screen, battery, and other components.

Compounds

Why can't we separate hydrogen and oxygen present in water by physical means?

In water, the particles of hydrogen and oxygen are so tightly attached to each other that it is generally impossible to separate them using physical methods. That is why water is a compound. Compounds are formed when different elements combine in fixed ratios to form something entirely new. The properties of compounds are different from those of elements forming that compound. The constituent elements of a compound cannot be separated by any physical method.

From Activity 8.3, we find that molecules of water are made of two different elements: hydrogen and oxygen, combined chemically in a fixed ratio. The ratio of the number of atoms of hydrogen to oxygen in water has been found to be 2:1.

Are common salt and sugar elements or compounds?

Sodium, a soft metal, and chlorine, a hazardous gas, combine to form a harmless yet taste-enhancing substance that is essential for our lives. This substance is known as sodium chloride, which is made up of particles of sodium and chlorine in a 1:1 ratio. Dissolved sodium chloride (common salt) may be separated from water by the physical process of evaporation.

Is it possible to separate sodium chloride into its elements by physical processes?

Let's explore if we can separate the elements in sugar!

Activity 8.4: Let us experiment

1. Put a teaspoon of sugar in a boiling tube.
2. Heat it gently.

As you heat the sugar, it turns brown. Later, it begins to char, i.e., it turns blackish.

You will find small droplets of water inside the boiling tube near its open end. This water comes from the dry sugar. Charcoal (carbon) is left behind in the boiling tube.

Sugar decomposes on heating and gives carbon and water. As you know, water consists of hydrogen and oxygen. Hence, sugar cannot be an element. It may be stated that sugar is a chemical compound consisting of the elements carbon, hydrogen, and oxygen.

Let us explore more about compounds.

Activity 8.5: Let us experiment (Demonstration activity)

1. Take 5.6 grams of iron filings and 3.2 grams of sulfur powder on a watch glass. Observe them carefully.
2. Mix them thoroughly in a watch glass. Label this mixture as Sample A.
3. Observe it carefully.
4. Is this a uniform or a non-uniform mixture? Can you still observe both iron and sulfur as separate substances?
5. Take half of Sample A in a china dish and gently heat it with continuous stirring until a black mass is formed.
6. Let the content of the china dish cool.
7. Place this black mass in a mortar and grind it with the help of a pestle.
8. Now, put it on another watch glass and label it as Sample B.
9. Now, you have two samples: Sample A and Sample B. Compare both the Samples A and B step by step and record your observations in Table 8.2.

Step 1 - Appearance

Compare the appearance of Sample A and Sample B, like colour and texture.

Step 2 - Magnet test

Take a magnet and move it over the Samples A and B, one by one.

What do you observe?

Step 3 - Gas test

Take a small amount of Sample A in a test tube and add a few drops of dilute hydrochloric acid.

What do you observe?

Gently smell the evolved gas by wafting it towards your nose.

Test the evolved gas by bringing a burning splinter or a lighted candle near the mouth of the test tube.

What do you observe?

Repeat the above steps with Sample B as well.

Some discussion points

• Do the Samples, A and B, look the same?
• Which sample exhibits magnetic properties?
• Can we separate the components of Samples A and B?
• On adding dilute hydrochloric acid, do gases evolve in both Samples A and B?
• In both the cases, do the gases smell the same or different?
• Also, categorise the substances used in this activity into mixtures, compounds, and elements.

Sample A: We can say that Sample A is a mixture of the two elements—iron and sulfur. Its components retain their properties, and their black and yellow coloured particles can be seen. On bringing a magnet near Sample A, the iron filings get attracted towards the magnet. Hence, iron and sulfur can be separated.

You might have observed that in Sample A, iron in the mixture reacts with dilute hydrochloric acid to form iron chloride and hydrogen gas. The gas is colourless, has no smell, and burns with a 'pop' sound.

Iron + Dilute Hydrochloric acid → Iron chloride + Hydrogen gas

Sulfur, on the other hand, is left as a yellow solid at the bottom of the test tube. This shows that sulfur does not react with hydrochloric acid.

Sample B: The black mass obtained in Sample B is iron sulfide. We observe that the texture and the colour are the same throughout. It is formed by heating the two elements, iron and sulfur. It is not attracted by a magnet. The new substance has completely different properties, and iron and sulfur can no longer be separated. Hence, we can say that a compound has been formed. Can you explain now why the magnet has no effect on Sample B?

Also, Sample B, iron sulfide, reacts with dilute hydrochloric acid to form iron chloride and hydrogen sulfide gas. The gas is colourless and has a rotten egg-like odour.

Iron sulfide + Dilute Hydrochloric acid → Iron chloride + Hydrogen sulfide

How Do We Use Elements, Compounds, and Mixtures?

Elements, compounds, and mixtures are all around us. The air we breathe is a mixture of gases like oxygen, nitrogen, and carbon dioxide. Water, which is essential for life, is a compound made of elements, hydrogen and oxygen. Elements like iron and aluminium are used to construct bridges, buildings, and vehicles.

Understanding these concepts is not just about recognizing what surrounds us; it is also the key to innovation.

• Chemists study how elements combine to create compounds, enabling them to invent life-saving medicines and vaccines to fight diseases.
• This knowledge also helps in the creation of fertilizers, thereby enhancing crop production that feeds the ever-increasing human population globally.

Engineers and material scientists rely on their understanding of compounds and mixtures to design materials with unique properties. For example, they have developed alloys like stainless steel, which is stronger and more durable than pure iron. Wood, steel, and concrete, which are used as building materials, are all mixtures.

Various metals are obtained from minerals.

A step further

An example of a 'wonder' material developed by material scientists is graphene aerogel. This is made from carbon and is said to be the lightest material on earth. It is so light that even grass can hold it. It is highly porous and, therefore, has a high absorbing capacity. For this reason, it can potentially be used as an environmental cleaner, for example, to clean up oil spills in both seas and on land. It is useful in fabricating energy-saving devices and special coatings for buildings.

What Are Minerals?

Most rocks are a mixture of minerals, which can be viewed with the eyes, or by using a magnifying glass or a microscope. Some of the minerals are called native minerals, which are pure elements and not compounds. These can be metals, such as gold, silver, copper, etc., or non-metals like sulfur, carbon, etc.

Most minerals are compounds made up of more than one element. Some common examples of minerals include quartz, calcite, mica, pyroxene, and olivine. Many things that we use in our everyday life are made up of minerals or elements extracted from minerals. For example, cement is made from calcite, quartz, alumina, and iron oxide, which are minerals or are obtained from minerals. Talcum powder is made from the mineral talc.

Our scientific heritage

The Dhokra art is an old craft from Bihar and Odisha that uses different metals to create beautiful figures inspired by nature. The process begins with shaping a design in beeswax. This wax model is covered with clay to make a mould. After the clay hardens, the wax is melted out, leaving a hollow space. This space is then filled with molten brass or bronze which makes Dhokra art strong and gives it a shiny golden colour. The figures often show animals, people, and nature, reflecting tribal creativity and tradition.

Elements and compounds are the building blocks of matter—everything that has mass and takes up space. They make the materials we see and use every day. However, not everything around us is matter. Light, heat, electricity, and even thoughts and emotions are important parts of our world, but they are not made of matter. Understanding what matter is—and what it is not—helps us better understand the world around us.