Reviewing Class 8 Science Notes Chapter 8 Nature of Matter: Elements, Compounds, and Mixtures Class 8 Notes regularly helps in retaining important facts.
Class 8 Science Chapter 8 Nature of Matter: Elements, Compounds, and Mixtures Notes
Class 8 Nature of Matter: Elements, Compounds, and Mixtures Notes
Class 8 Science Chapter 8 Notes – Nature of Matter: Elements, Compounds, and Mixtures Notes Class 8
→ A mixture consists of two or more substances mixed. These substances retain their individual properties and do not react chemically with each other.
→ The individual substances that make up a mixture are called its components.
→ A pure substance consists of the same type of particles. All the constituent particles of that substance behave identically.
→ A pure substance can be either an element or a compound.
→ Elements are the simplest substances that cannot be broken down further into simpler substances. They are the building blocks of all matter.
→ Substances that are composed of two or more elements combined chemically in a fixed ratio and have different properties from their constituent elements are called compounds.
→ Minerals are natural, solid substances found on the Earth. They have a field chemical composition. Most often, they are compounds, but rarely can they also be pure elements.
Have you ever wondered what the world around you is made of? Look around! The staircase you use, the air you breathe, the water in your bottle, the food in your lunch box, the clothes and shoes you wear, the book you read, the trees outside, the ball you play with and even the stick you carry all of these are examples of matter, which you have learnt in earlier grades. You have also learnt that all these things are made up of tiny particles. Most of the things around us are not made of just one substance; rather, they are made up of two or more substances mixed. Let us now understand how different substances come together to form mixtures.
What are Mixtures? Class 8 Notes
Have you ever wondered what makes your poha so delicious or how to make the perfect sprout salad? While these dishes may seem very different, they share a commonality: they are both made by combining several ingredients. We observe the mixing of substances in everyday life. Sugar dissolved in water is also a mixture, and so are soups and lemonade.
When two or more substances are mixed, where each substance retains its properties, it is called a mixture. The individual substances that make up a mixture are called its components. The components of a mixture do not react chemically with each other. In some mixtures, the components like green gram, chickpeas, onion, and tomato in a sprout salad are easy to see. Such mixtures, where the different components are generally visible with the naked eye or with a magnifying device, are non-uniform. Can you identify a few more examples of non-uniform mixtures around you?
On the other hand, some mixtures have components that cannot be seen separately even with the help of a microscope. For example, sugar and water particles cannot be seen separately in their mixture. Such types of mixtures, where the components are evenly distributed and cannot be distinguished, are uniform. Can you list a few uniform mixtures?
Do you know that stainless steel is also a mixture?
Stainless steel contains iron, nickel, chromium, and a small amount of carbon. They are mixed so uniformly that the entire mixture appears the same throughout, and one cannot see the individual substances. Such mixtures are known as alloys. Brass, a mixture of copper and zinc, and bronze, a mixture of copper and tin, are some other examples of alloys.
Mishraloha was the name given to the mixture of two or more metals that had properties distinct from their constituent metals. Ancient Indian texts, such as the Charaka Samhita, Susruta Samhita, Rasaratna Samucchaya, Rasa Jala Nidhi, etc., mention the use of alloys for medicinal purposes. For example, Bronze, also known as Kamsya, is an alloy made up of Copper (Tamra, 4 parts) and Tin (Vanga, 1 part), which was used to improve digestion and boost immunity.
Is air a mixture?
In Curiosity, Grade 6, you learnt about air and its composition in the chapter ‘Nature’s Treasures’. Is air a mixture? What kind of mixture is it? You have learnt that air is a uniform mixture of mainly nitrogen, oxygen, argon, carbon dioxide, and water vapour. Out of these, oxygen is required by most living beings to stay alive. It also helps in combustion. Nitrogen, which constitutes about 78% of the air, does not take part in combustion. We also learnt that air has water vapour in it. When warm air touches a cool surface, the water vapour turns into liquid water, forming tiny droplets. Recall the experiment where you tested the presence of carbon dioxide in the air that we exhale. Let us confirm the presence of carbon dioxide in the air.
Activity 1: Let us experiment
Take a glass tumbler and fill it half with water. Add a small amount of calcium oxide (quick lime) slowly to it. What do you observe? Calcium oxide reacts vigorously with water to form calcium hydroxide and releases heat. Stir continuously to make a solution of calcium hydroxide. This solution is called lime water. Filter it and observe its colour. Leave this colourless solution in a petri dish for a few hours. Keep stirring the solution at regular intervals. What do you observe? Does it turn milky?
Can you explain why the solution has turned milky?
You know that lime water turns milky when carbon dioxide reacts with calcium hydroxide to form calcium carbonate (insoluble, tiny white particles) and water. Since lime water turns milky when exposed to air, this activity demonstrates the presence of carbon dioxide in the air.
Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water
Apart from gases, have you ever observed anything else present in the air? Have you ever noticed tiny shining particles moving in a beam of sunlight entering a dark room through a small opening? What are these particles?
Activity 2: Let us explore
Take a black sheet of paper. Ensure that it is free from any visible dust particles. Place the black sheet of paper undisturbed near an open window, or in the garden, for a few hours. What do you observe? You may notice tiny particles settled on its surface. You may use a magnifying glass to examine the particles more closely. This shows that dust particles are suspended in the air. They are not an integral part of the air and are considered pollutants. The nature and the number of dust particles in the air may vary from time to time and from place to place.
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
You know that the term ‘mixture’ in common usage refers to the mixing of two or more components. The components of a mixture may themselves be mixtures, as in poha and sprout salad, or pure substances like sugar or common salt dissolved in water. However, in science, all the components of a mixture must be pure substances only. Mixtures could be of several types depending on the physical state of their components. Some mixtures with their examples are shown in Table 1.
You learnt in earlier grades about the separation of mixtures. It is done to separate the components of a mixture. The examples discussed were from everyday life, where separation is done to obtain the component of interest, and other components are discarded. However, in science, the purpose of separating a mixture is to obtain pure substances.
What are Pure Substances? Class 8 Notes
Have you ever noticed the word ‘pure’ written on the packs of some consumables, such as milk, ghee, and spices? The word ‘pure’ has slightly different meanings in common usage and science. In common usage, ‘pure’ means unadulterated products. Adulteration is an illegal process of adding substances that are cheaper or of poor quality to a product. This is usually done to increase the quantity or reduce the manufacturing cost. However, it deteriorates the quality of the product. It can also make the product hazardous to health.
In science, however, a pure substance has no other substance present in it. For a scientist, even these products that look pure can be considered impure if they are made of more than one substance. A pure substance is a kind of matter that cannot be separated into other kinds of matter by any physical process. When a scientist says that something is pure, it means that the substance consists of the same type of particles. According to science, how would you classify milk, packed fruit juice, baking soda, sugar, and soil as mixtures or pure substances?
What are the Types of Pure Substances? Class 8 Notes
Recall the different states of water that you studied in Curiosity, Grade 6. What happened when water was cooled or heated? We observed that on cooling, water gets converted into ice, and on boiling, it gets converted into vapour. We can get water up on heating ice or cooling water vapour. It shows that during these processes, the particles of water remain the same. Now, let us perform another activity in which we pass electricity through water and observe its effect.
Activity 3: Let us experiment (Demonstration Activity)
This activity must be performed under the supervision of the teacher. Be careful while handling sulfuric acid. Do not use a lithium-ion battery. Collect two small test tubes, a beaker or a glass tumbler, and a 9V battery. Fill 2/3rd of the beaker with water and add a few drops of dilute sulfuric acid to it. Fill both the small test tubes with water taken from the beaker. Place a 9V battery inside the beaker.
Without spilling the water, carefully place the water-filled test tubes on each of the terminals of the battery. Wait for a few minutes. Do you observe the formation of any gas bubbles at both terminals inside the test tubes? Let it continue for 10-15 minutes. Observe the volume of gas collected in each test tube. Is the volume of the gas collected the same in both the test tubes? Remove these test tubes one by one carefully. Test these gases one by one by bringing a burning candle close to the mouth of the test tubes. What happens in each case? Which gas is present in each test tube?
Perform gas testing with care. Maintain a safe distance from the setup. 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 3, we can infer that water is composed of two different constituents, hydrogen and oxygen.
Water → Hydrogen + Oxygen
When an electric current is passed through water, it breaks down into hydrogen and oxygen. Is this a chemical change or a physical change?
Elements
The two substances, hydrogen and oxygen, formed in Activity 3, are pure 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. They are the building blocks of all matter. Some other examples of elements are gold, silver, sulfur, carbon, etc.
The atoms of most of the 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. You have already studied that gold, silver, magnesium, iron, and aluminium are metals, whereas carbon, sulfur, hydrogen, and oxygen are non-metals. Isn’t it interesting to know that some elements like silicon and boron have intermediate properties between those of metals and non-metals? They are called metalloids, about which you will learn in higher grades.
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) and turn into liquid. More than 45 different elements, like aluminium, copper, silicon, cobalt, lithium, gold, silver, etc., are used in manufacturing a mobile phone, including its screen, battery, and other components.
Compounds
Why cannot we separate hydrogen and oxygen present in water by physical means?
In water, the particles of hydrogen and oxygen are so tightly attached 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 the elements that form that compound. The constituent elements of a compound cannot be separated by any physical method. From Activity 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 is 2:1.
Are common salt and sugar elements or compounds? Let us find out.
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. We learnt that 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 us now explore if we can separate the elements in sugar!
Activity 4: Let us experiment
Put a teaspoon of sugar in a boiling tube. Heat it gently as shown in Figure a. What do you observe? 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. Where did this water come from? Was it in the dry sugar, or did it come by the condensation of water vapour in the air? Since we are heating the tube, the water must have come from the dry sugar and not from the air. Can you predict what is left behind? Charcoal (carbon) is left behind in the boiling tube. You can scoop it out in a watch glass and explore if it burns like coal. 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 5: Let us experiment (Demonstration activity)
This activity may be demonstrated under the supervision of the teacher. It may be performed in a fume hood or a well-ventilated area. Do not inhale the gases. Take 5.6 grams of iron filings and 3.2 grams of sulfur powder on a watch glass. Observe them.
Mix them thoroughly in a watch glass. Label this mixture as Sample A. Observe it.
Is this a uniform or a non-uniform mixture? Can you still observe both iron and sulfur as separate substances? Take half of Sample A in a china dish and gently heat it with continuous stirring until a black mass is formed. Let the content of the china dish cool. Place this black mass in a mortar and grind it with the help of a pestle. Now, put it on another watch glass and label it as Sample B.
Now, you have two samples Sample A and Sample B (Fig. 8.16a and 8.16b).
Compare both Samples A and B step by step and record your observations in Table 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 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.
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 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. The reaction can be represented as
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 does not affect 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. The reaction can be represented as
Iron Sulfide + Dilute Hydrochloric Acid → Iron Chloride + Hydrogen Sulfide
How Do We Use Elements, Compounds, and Mixtures? Class 8 Notes
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 recognising what surrounds us; it is also the key to innovation. For instance, 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 fertilisers, 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. You learnt that various metals are obtained from minerals. Let us learn about these minerals.
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? Class 8 Notes
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 of the 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.
Use of elements, compounds, and mixtures in Indian Art
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 mold. 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 matters and what it is not helps us better understand the world around us.
The post Nature of Matter: Elements, Compounds, and Mixtures Class 8 Notes Science Chapter 8 appeared first on Learn CBSE.
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