8.2 What Are Pure Substances?
- Have you noticed the word ‘pure’ written on packs of some consumables, such as milk, ghee, and spices?
- The word ‘pure’ has slightly different meanings in common usage and in science.
- In common usage, ‘pure’ means unadulterated products.
🔑 Key Term — Adulteration
- Adulteration is an illegal process of adding substances which are cheaper, or of a poor quality, to a product.
- It is usually done to increase the quantity or reduce the manufacturing cost.
- It deteriorates the quality of the product and can also make the product hazardous to health.
- In science, a pure substance is the one that has no other substance present in it.
- For a scientist, even 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 something is pure, it means the substance consists of the same type of particles.
🪜 A step further
- According to science, how would you classify milk, packed fruit juice, baking soda, sugar, and soil — as mixtures or pure substances?
Show Answer
Mixtures: milk, packed fruit juice, soil (each contains more than one substance). Pure substances: baking soda and sugar (each consists of only one kind of substance/particles).
8.3 What Are the Types of Pure Substances?
- Recall the different states of water: on cooling, water gets converted into ice; on boiling, it gets converted into vapour.
- We can get back water by heating ice or cooling water vapour.
- This shows that during these processes, the particles of water remain the same.
- Now let us perform an activity in which we pass electricity through water and observe its effect.
🔬 Activity 8.3: Let us experiment (Demonstration activity)
⚠️ Safety first
This activity must be performed under the supervision of the teacher. Be careful while handling sulfuric acid. Do not use lithium-ion battery.
- Collect two small test tubes, a beaker or a glass tumbler, and a 9 V 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 completely with water taken from the beaker.
- Place the 9 V 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 the 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?
Fig. 8.8: Passing electricity through water — gases collected in the test tubes
- Can these collected gases be water vapour?
- No — these gases are not water vapour, otherwise they would have condensed back to form water.
⚠️ Safety first
Perform gas testing with care. Maintain a safe distance from the set-up.
- To test the gases, 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.
Fig. 8.9: Testing the nature of gas (a) hydrogen; (b) oxygen
- From Activity 8.3, we can infer that water is composed of two different constituents — hydrogen and oxygen.
👂 Ever heard of …
- When electric current is passed through water, it breaks down into hydrogen and oxygen. Is this a chemical change or a physical change? Recall Curiosity, Grade 7, chapter ‘Changes Around Us: Physical and Chemical’.
Show Answer
It is a chemical change — water breaks down into two entirely new substances (hydrogen and oxygen) with completely different properties, and the change cannot be reversed by simple physical means.
8.3.1 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.
- They are the building blocks of all matter.
- Some other examples of elements are gold, silver, sulfur, carbon, etc.
🪜 A step further
- 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.
Fig. 8.10: Depiction of molecules of (a) hydrogen; (b) oxygen
- Elements can be classified into metals and non-metals.
- Metals: gold, silver, magnesium, iron, and aluminium.
- Non-metals: carbon, sulfur, hydrogen, and oxygen.
- 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.
🪜 A step further
- 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.
🪜 A step further
- 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.
📝 Practice Questions — Pure Substances & Elements
LOTS What is a pure substance according to science?
Show Answer
A pure substance is a kind of matter that cannot be separated into other kinds of matter by any physical process. It consists of the same type of particles.
Medium Name the gases collected at the two terminals in Activity 8.3, and state the test for each.
Show Answer
Hydrogen and oxygen. Hydrogen burns with a ‘pop’ sound when a burning candle is brought near it. Oxygen makes the flame of the burning candle glow brighter.
HOTS Why can the gases collected in Activity 8.3 not be water vapour?
Show Answer
If the gases were water vapour, they would have condensed back into liquid water inside the test tubes. Since they remained as gases and gave different results in the candle test (pop sound and brighter flame), they must be two different gases — hydrogen and oxygen.
HOTS Mercury and bromine are both liquids at room temperature, yet they belong to different classes of elements. Explain.
Show Answer
Mercury is a metal that is liquid at room temperature, while bromine is a non-metal that is liquid at room temperature. This shows that the physical state of an element does not decide whether it is a metal or non-metal — classification depends on its properties.
8.3.2 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 to each other that it is generally impossible to separate them apart 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.
- 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.
Fig. 8.11: Depiction of molecules of water
💬 Oh! This is really fascinating…
- Hydrogen is a fuel, oxygen supports combustion, whereas water extinguishes fire! A compound’s properties are completely different from its elements.
- 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 — sodium chloride.
- Sodium chloride 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? No — because it is a compound.
- Let us now explore if we can separate the elements in sugar!
🔬 Activity 8.4: Let us experiment
⚠️ Safety first
This activity must be performed in the presence of a teacher.
- Put a teaspoon of sugar in a boiling tube.
- Heat it gently.
- What do you observe?
- Observation: 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.
- 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.
- Inference: Sugar decomposes on heating and gives carbon and water.
- Water consists of hydrogen and oxygen — hence, sugar cannot be an element.
- Conclusion: Sugar is a chemical compound consisting of the elements carbon, hydrogen, and oxygen.
Fig. 8.12: Heating sugar in a boiling tube
🔬 Activity 8.5: Let us experiment (Demonstration activity)
⚠️ Safety first
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 carefully.
- Mix them thoroughly in a watch glass. Label this mixture as Sample A.
- Observe it carefully.
- 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. Compare both 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.
⚠️ Safety first
Be careful while handling hydrochloric acid. Never smell anything directly.
Fig. 8.14–8.16: Sample A (mixture of iron filings and sulfur powder) and Sample B (black mass — iron sulfide)
| S.No. | Experiment | Sample A | Sample B |
|---|---|---|---|
| 1. | Appearance (i) Colour (ii) Texture | Black and yellow particles visible separately; non-uniform texture | Uniform black colour; same texture throughout |
| 2. | Magnet test | Iron filings get attracted towards the magnet | Not attracted by the magnet |
| 3. | Gas test (i) Odour (ii) Burning | Colourless gas, no smell; burns with a ‘pop’ sound (hydrogen) | Colourless gas with rotten egg-like odour (hydrogen sulfide) |
Table 8.2: Comparison of Samples A and B (observations filled for exam practice)
Fig. 8.17: Responses of Samples A and B to a magnet
- 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 — A Mixture
- 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.
- 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.
- 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 — A Compound (Iron Sulfide)
- The black mass obtained in Sample B is iron sulfide.
- 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, a compound has been formed.
- 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.
📝 Practice Questions — Compounds
LOTS What are compounds? Give two examples.
Show Answer
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 forming them. Examples: water (hydrogen + oxygen in 2:1 ratio) and sodium chloride (sodium + chlorine in 1:1 ratio).
Medium Sugar turns black on heating and leaves water droplets in the tube. What does this prove about sugar?
Show Answer
It proves sugar is a compound, not an element. On heating, sugar decomposes to give carbon (the black charcoal) and water. Since water itself consists of hydrogen and oxygen, sugar must be a chemical compound of carbon, hydrogen, and oxygen.
HOTS Why does the magnet attract iron in Sample A but has no effect on Sample B, even though both contain iron?
Show Answer
In Sample A, iron is present as a free element in a mixture, retaining its own magnetic property. In Sample B, iron has chemically combined with sulfur to form the compound iron sulfide — a completely new substance with entirely different properties. Iron no longer exists as free iron, so the magnet has no effect.
HOTS Hydrogen is a fuel and oxygen supports combustion, yet water extinguishes fire. Explain this apparent contradiction.
Show Answer
Water is a compound of hydrogen and oxygen, and the properties of a compound are entirely different from those of its constituent elements. When hydrogen and oxygen combine chemically in a fixed 2:1 ratio, they form a completely new substance whose properties (extinguishing fire) do not resemble either element.
8.4 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 recognising 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 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, 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.
🪜 A step further
- An example of a ‘wonder’ material developed by material scientists is graphene aerogel.
- It is made from carbon and is said to be the lightest material on earth — so light that even grass can hold it.
- It is highly porous and therefore has a high absorbing capacity.
- 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.
Fig. 8.21: Graphene aerogel — light enough for grass to hold
8.5 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 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.
- 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.
Fig. 8.22: Some minerals — Quartz, Calcite, Mica, Pyroxene, Olivine, Talc
🏛️ Our scientific heritage — 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 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.
Fig. 8.23: Dhokra art
- 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.
📸 Snapshots
- A mixture consists two or more substances mixed together. 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 the same type of particles. All the constituent particles of that substance behave identically.
- 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 which 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 fixed chemical composition. Most often they are compounds but rarely, they can also be pure elements.
🧠 Keep the curiosity alive
1. Consider the following reaction where two substances, A and B, combine to form a product C: A + B ⟶ C. Assume that A and B cannot be broken down into simpler substances by chemical reactions. Based on this information, which of the following statements is correct?
(i) A, B, and C are all compounds and only C has a fixed composition.
(ii) C is a compound, and A and B have a fixed composition.
(iii) A and B are compounds, and C has a fixed composition.
(iv) A and B are elements, C is a compound, and has a fixed composition.
Show Answer
Correct answer: (iv) A and B are elements, C is a compound, and has a fixed composition. Since A and B cannot be broken down into simpler substances, they are elements. When elements combine chemically they form a compound (C), and compounds always have a fixed composition.
2. Assertion: Air is a mixture.
Reason: A mixture is formed when two or more substances are mixed, without undergoing any chemical change.
(i) Both Assertion and Reason are true and Reason is the correct explanation for Assertion.
(ii) Both Assertion and Reason are true, but Reason is not the correct explanation for Assertion.
(iii) Assertion is true, but Reason is false.
(iv) Assertion is false, but Reason is true.
Show Answer
Correct answer: (i) Both Assertion and Reason are true and Reason is the correct explanation for Assertion. Air is indeed a mixture of gases (nitrogen, oxygen, argon, carbon dioxide, water vapour) that are mixed together without any chemical change, and each gas retains its own properties.
3. Water, a compound, has different properties compared to those of the elements oxygen and hydrogen from which it is formed. Justify this statement.
Show Answer
When elements combine chemically in a fixed ratio to form a compound, the compound has entirely new properties, different from its constituent elements. Hydrogen is a fuel and burns with a pop sound; oxygen supports combustion and makes a flame glow brighter. But water, formed by their chemical combination in a 2:1 ratio, extinguishes fire instead. This proves that the properties of a compound are completely different from the properties of the elements forming it.
4. In which of the following cases are all the examples correctly matched? Give reasons in support of your answers.
(i) Elements — water, nitrogen, iron, air.
(ii) Uniform mixtures — minerals, seawater, bronze, air.
(iii) Pure substances — carbon dioxide, iron, oxygen, sugar.
(iv) Non-uniform mixtures — air, sand, brass, muddy water.
Show Answer
Correct answer: (iii) Pure substances — carbon dioxide, iron, oxygen, sugar. Carbon dioxide and sugar are compounds; iron and oxygen are elements — all four are pure substances.
Why the others are wrong: (i) Water is a compound and air is a mixture, not elements. (ii) Minerals as found in rocks are generally non-uniform; the rest (seawater, bronze, air) are uniform — so all examples are not correctly matched. (iv) Air and brass are uniform mixtures, not non-uniform.
5. Iron reacts with moist air to form iron oxide, and magnesium burns in oxygen to form magnesium oxide. Classify all the substances involved in the above reactions as elements, compounds or mixtures, with justification.
Show Answer
Elements: Iron, magnesium, and oxygen — they cannot be broken down into simpler substances.
Compounds: Iron oxide and magnesium oxide — each is formed when two elements combine chemically in a fixed ratio, producing new substances with different properties.
Mixture: Moist air — it is a mixture of gases (nitrogen, oxygen, carbon dioxide, argon) along with water vapour, where each component retains its properties.
6. Classify the following as elements, compounds, or mixtures in Table 8.3: Carbon dioxide, sand, seawater, magnesium oxide, muddy water, aluminium, gold, oxygen, rust, iron sulfide, glucose, air, water, fruit juice, nitrogen, sodium chloride, sulfur, hydrogen, baking soda. Identify pure substances amongst these and list them below.
Show Answer
Elements: Aluminium, gold, oxygen, nitrogen, sulfur, hydrogen.
Compounds: Carbon dioxide, magnesium oxide, rust, iron sulfide, glucose, water, sodium chloride, baking soda.
Mixtures: Sand, seawater, muddy water, air, fruit juice.
Pure substances: All elements and compounds — aluminium, gold, oxygen, nitrogen, sulfur, hydrogen, carbon dioxide, magnesium oxide, rust, iron sulfide, glucose, water, sodium chloride, baking soda.
7. What new substance is formed when a mixture of iron filings and sulfur powder is heated, and how is it different from the original mixture? Also, write the word equation for the reaction.
Show Answer
When a mixture of iron filings and sulfur powder is heated, a new compound called iron sulfide is formed. It is different from the original mixture because: (1) it has a uniform black colour and texture throughout, while the mixture showed separate black and yellow particles; (2) it is not attracted by a magnet, while iron in the mixture was; (3) iron and sulfur can no longer be separated from it; (4) with dilute hydrochloric acid it gives hydrogen sulfide gas (rotten egg smell), while the mixture gave odourless hydrogen gas.
Word equation: Iron + Sulfur ⟶ Iron sulfide (on heating).
8. Is it possible for a substance to be classified as both an element and a compound? Explain why or why not.
Show Answer
No, it is not possible. An element is made up of only one kind of atom and cannot be broken down into simpler substances. A compound is made up of two or more different elements chemically combined in a fixed ratio. Since a substance either consists of one type of atom (element) or more than one type chemically combined (compound), it can never be both at the same time.
9. How would our daily lives be changed if water were not a compound but a mixture of hydrogen and oxygen?
Show Answer
If water were a mixture of hydrogen and oxygen, the two gases would retain their individual properties. Hydrogen is a fuel and oxygen supports combustion — so ‘water’ would catch fire or support burning instead of extinguishing it! It would also exist as a gas rather than a liquid, so there would be no rivers, oceans, rain, or drinking water. The components could be easily separated, and life as we know it would be impossible, since living beings depend on liquid water.
10. Analyse Fig. 8.24. Identify Gas A. Also, write the word equation of the chemical reaction. (Iron filings with dilute hydrochloric acid in a test tube.)
Show Answer
Gas A is hydrogen gas. When iron filings react with dilute hydrochloric acid, iron chloride is formed and hydrogen gas is released. The gas is colourless, has no smell, and burns with a ‘pop’ sound.
Word equation: Iron + Dilute Hydrochloric acid ⟶ Iron chloride + Hydrogen gas.
11. Write the names of any two compounds made only from non-metals, and also mention two uses of each of them.
Show Answer
1. Water (hydrogen + oxygen, both non-metals) — Uses: (i) essential for drinking and sustaining life; (ii) extinguishing fires.
2. Carbon dioxide (carbon + oxygen, both non-metals) — Uses: (i) used by plants for photosynthesis; (ii) used in fire extinguishers and in aerated (soda) water.
12. How can gold be classified as both a mineral and a metal?
Show Answer
Gold is a metal because of its properties — it is lustrous, malleable, and a good conductor. At the same time, gold is found naturally in the Earth as a native mineral — a mineral that is a pure element and not a compound. Since minerals are natural, solid substances found on the Earth with a fixed chemical composition, and gold occurs naturally in pure elemental form, it is both a mineral and a metal.
- Prepare some questions based on your learnings so far … (Why? When? How long? Where? Why not?)
- Reflect on the questions framed by your friends and try to answer … (I think … But we thought … Maybe … Shouldn’t it be …)
💡 Discover, design, and debate
- Design and create comic strips from real-life examples to differentiate between elements, compounds, and mixtures with diagrams and illustrate their properties and uses.
- Search for discoveries of some elements (such as phosphorus, sodium), compounds (such as penicillin) and mixtures (such as brass, bronze, stainless steel). Present your findings in the class.
- Let us search: Read labels on items like detergents or snacks, and try to list the mixtures and compounds they contain.
- Work in groups: Each group will pretend to be in the role of either an element, a compound, or a mixture. Debate which category among them is the most important.