Elements, Compounds & Mixtures
This page covers the classification of elements, compounds and mixtures, homogeneous-heterogeneous mixtures, and the methods used to separate mixtures, from the REET exam's point of view. REET repeatedly asks questions that require correctly identifying a given mixture and choosing the right separation method for it.
- 2 questions in REET 2024
- Class 6-8 level
- 10 min read
How this topic is asked in REET
REET 2024 asked two questions on this topic, and both were 'correctly identify' type questions. The first question asked which of four options is NOT an example of a homogeneous mixture (answer: oil in water, because it is heterogeneous). The second asked which is the right method for separating two miscible liquids (answer: distillation). This makes it clear that REET checks not just definitions but the ability to place a given example in the right category and to choose the correct separation method for a given situation.
Pure Substances, Elements, Compounds and Mixtures
A substance made of only one kind of particle (one chemical composition) is called a pure substance, and this includes both elements and compounds. An element is a pure substance that cannot be broken down into two or more simpler substances by chemical methods, such as hydrogen, oxygen or gold. A compound is formed by the chemical combination of two or more elements in a fixed ratio, in which the properties of the original elements are lost and the compound has its own new properties — such as water, formed from hydrogen and oxygen, whose properties are completely different from its constituent elements. A mixture, on the other hand, is formed by physically combining two or more substances in any ratio, where each component keeps its original properties and can easily be separated by physical methods (filtration, evaporation, and so on).
The biggest difference between a compound and a mixture is that forming a compound involves a fixed ratio and an energy change (a chemical reaction), producing a substance with new properties, while in a mixture the ratio of the components is variable and no new chemical entity is formed — it can be separated using only a physical method, with no chemical method needed.
Classification of Elements: Metals, Non-metals and Metalloids
| Class | Properties | Examples |
|---|---|---|
| Metal | Shiny, malleable (can be hammered into sheets), ductile (can be drawn into wires), good conductor of heat and electricity | Iron, copper, gold, aluminium |
| Non-metal | Usually dull (graphite and iodine are exceptions), brittle, poor conductor of heat and electricity (graphite is an exception) | Oxygen, carbon, sulphur, nitrogen |
| Metalloid | Show a mix of both metallic and non-metallic properties | Silicon, boron, arsenic, germanium |
Homogeneous and Heterogeneous Mixtures
- Homogeneous mixture
- One in which the components are evenly mixed throughout, with no visible boundary — such as sugar in water, ethanol in water, sulphur in carbon disulphide
- Heterogeneous mixture
- One in which the components are unevenly mixed, with distinct layers/parts visible — such as oil in water, sand in water
- Solutions, suspensions and colloids
- In a solution, the particles are extremely fine (smaller than 1 nm) and invisible; in a suspension, the particles are visible to the naked eye and settle at the bottom; in a colloid, the particles are medium-sized and show the Tyndall effect (the scattering of a beam of light)
Separation Methods and Their Uses
| Method | Suited For |
|---|---|
| Evaporation | Separating a dissolved solid from a solution (such as salt from salt water) |
| Distillation / fractional distillation | Separating mutually soluble (miscible) liquids that have different boiling points |
| Separating funnel | Separating immiscible liquids, such as oil and water |
| Sublimation | Separating a sublimable solid (such as camphor, ammonium chloride, iodine) from another solid |
| Chromatography | Separating the components of a mixture, such as ink or dyes, based on their differing properties |
| Centrifugation | Separating very fine suspended particles by spinning rapidly, such as separating cream from milk |
Comparing Examples of Compounds and Mixtures
- Water (a compound)
- Hydrogen and oxygen are joined by a chemical bond in a fixed ratio (2:1); neither the properties of hydrogen nor of oxygen show up — water has its own new properties, and it can only be broken down by a chemical method such as electrolysis
- Salt water (a mixture)
- Salt and water can mix in any ratio; both keep their original properties (the taste of salt, the fluidity of water) and can easily be separated by a simple physical method like evaporation
- Air (a mixture)
- A homogeneous mixture of nitrogen (~78%), oxygen (~21%) and other gases — its ratio can vary slightly with location and altitude, which proves it is a mixture
How to Teach This in Class
Activity
Make a mixture of sand, salt and water, and have students first separate the sand by filtration, then heat the remaining salt water to get the salt back by evaporation — this shows two separation methods in a single experiment, and students see for themselves that no new substance was formed, only the components were separated.
Teaching aid
Bring kitchen items to class (tea leaves-water, oil-water, sugar-water, iron filings-sand) and have students classify them as elements/compounds/mixtures and as homogeneous/heterogeneous, connecting the idea to everyday experience.
Common misconception
Many students assume 'mixture' always means a mix of a solid and a liquid, or think a compound and a mixture are the same thing, or believe air is an element/compound — clear up this confusion by comparing water (a compound), salt water (a mixture) and air (a mixture).
Assessment
Give students a list of 6 everyday items and ask them to classify each as an element/compound/mixture and to name the right separation method for each mixture, then match answers through a class discussion.
Memory trick / exam tip
If you're unsure which separation method to pick, remember this order — evaporation for a dissolved solid, distillation for miscible liquids, a separating funnel for immiscible liquids, and sublimation for a sublimable solid. REET often gives a 'liquid-liquid' or 'solid-liquid' pair, so first identify which two states the mixture is made of.
Questions Asked in REET 2024 (With Solutions)
Question 127
Which of the following is not an example of homogeneous mixture ?
- Aकार्बन डाइसल्फाइड में सल्फर
- Bजल में शक्कर
- Cजल में एथेनॉल
- Dजल में तेल
Explanation
समांगी मिश्रण (homogeneous mixture) में घटक पूरे मिश्रण में एकसमान रूप से मिले रहते हैं, जैसे जल में शक्कर, जल में एथेनॉल और कार्बन डाइसल्फाइड में घुला सल्फर (विलयन)। तेल जल में नहीं घुलता और अलग परत बना लेता है, इसलिए जल में तेल विषमांगी (heterogeneous) मिश्रण है।
Question 140
The appropriate method to separate mixture of two soluble liquids is
- Aआसवन
- Bउर्ध्वपातन
- Cवाष्पीकरण
- Dपृथक्कारी कीप
Explanation
परस्पर घुलनशील (मिश्रणीय) द्रवों को पृथक् करने के लिए आसवन (distillation) — क्वथनांक का अंतर कम होने पर प्रभाजी आसवन — उपयुक्त है, क्योंकि गर्म करने पर कम क्वथनांक वाला द्रव पहले वाष्पित होकर अलग संघनित हो जाता है। पृथक्कारी कीप केवल अमिश्रणीय द्रवों (जैसे तेल और जल) के लिए है और उर्ध्वपातन ठोस पदार्थों के लिए।
Practice Questions
Question 1
Which of the following is an example of an element?
- Aजल
- Bनमक का घोल
- Cसोना
- Dवायु
Explanation
Gold (Au) is an element, which cannot be broken down further into simpler substances by a chemical method. Water is a compound, while a salt solution and air are mixtures.
Question 2
Which of the following is an example of a heterogeneous mixture?
- Aजल में शक्कर
- Bजल में बालू
- Cजल में एथेनॉल
- Dवायु
Explanation
When sand is added to water, it does not dissolve and settles at the bottom, so the two parts are clearly visible separately — this is a heterogeneous mixture. Sugar in water, ethanol in water and air are examples of homogeneous mixtures.
Question 3
The appropriate method to separate camphor from sand is:
- Aआसवन
- Bअपकेंद्रण
- Cपृथक्कारी कीप
- Dऊर्ध्वपातन
Explanation
Camphor is a sublimable solid, meaning it turns directly into gas on heating without passing through the liquid state. So it is separated from sand (which does not sublime) using sublimation.
Question 4
Silicon is classified as a:
- Aधातु
- Bअधातु
- Cउपधातु
- Dयौगिक
Explanation
Silicon is a metalloid, which shows a mix of both metallic and non-metallic properties and is used in electronics as a semiconductor.
Question 5
Which method is used to separate cream from milk?
- Aवाष्पीकरण
- Bपृथक्कारी कीप
- Cऊर्ध्वपातन
- Dअपकेंद्रण
Explanation
Cream is separated from milk using centrifugation, in which rapid spinning separates heavier and lighter particles into different layers.
Question 6
What is the main difference between a compound and a mixture?
- Aयौगिक निश्चित अनुपात में बनता है और नए गुण रखता है, मिश्रण में घटक अपने मूल गुण बनाए रखते हैं
- Bयौगिक भौतिक विधि से अलग होता है, मिश्रण नहीं
- Cमिश्रण को रासायनिक विधि से ही अलग किया जा सकता है
- Dदोनों में कोई अंतर नहीं है
Explanation
A compound forms in a fixed ratio and has new properties, while in a mixture the components keep their original properties and can be separated by physical methods.
Question 7
The most suitable method to separate different colours present in ink is:
- Aवर्णलेखन (chromatography)
- Bआसवन
- Cऊर्ध्वपातन
- Dअपकेंद्रण
Explanation
The different colours mixed in ink differ in solubility and absorption, so they are separated into distinct bands using chromatography.
Question 8
The Tyndall effect (scattering of a light beam) is observed in:
- Aसच्चे विलयन में
- Bकोलॉइड में
- Cशुद्ध जल में
- Dगैसों के मिश्रण में
Explanation
Colloid particles are larger than those in a true solution, so a beam of light scatters off them — this is called the Tyndall effect. A true solution's particles are too fine to show this effect.
Question 9
The most suitable activity for a teacher to clarify the difference between element, compound and mixture in class 7 is:
- Aकेवल परिभाषाएँ श्यामपट्ट पर लिखना
- Bपाठ्यपुस्तक के प्रश्न-उत्तर रटवाना
- Cरसोई की वस्तुओं को कक्षा में लाकर वर्गीकृत करवाना
- Dकेवल चित्र दिखाना
Explanation
Bringing familiar kitchen items (water, salt, oil-water, tea, and so on) into class for classification lets students connect abstract definitions to everyday examples, making the concept stick.
Question 10
Which method is used to separate components like petrol, diesel and kerosene from petroleum?
- Aसरल आसवन
- Bप्रभाजी आसवन
- Cऊर्ध्वपातन
- Dपृथक्कारी कीप
Explanation
Petroleum is a mixture of many miscible liquids (hydrocarbons) with slightly different boiling points. Fractional distillation, an extended form of simple distillation, is used to separate them.
Frequently Asked Questions
What is the difference between an element, a compound and a mixture?+
An element cannot be broken down further by a chemical method; a compound is formed by the chemical combination of two or more elements in a fixed ratio and has new properties; a mixture is a physical combination of two or more substances, where each component keeps its original properties.
What are homogeneous and heterogeneous mixtures?+
In a homogeneous mixture, the components are evenly mixed throughout (such as sugar in water), while in a heterogeneous mixture the components are unevenly mixed and distinct parts are clearly visible (such as oil in water).
How would you separate two miscible liquids?+
Mutually soluble (miscible) liquids are separated by distillation, in which the liquid with the lower boiling point vaporises first and condenses separately.
What is the difference between metals and non-metals?+
Metals are shiny, malleable, ductile and good conductors of heat and electricity (such as iron, copper), while non-metals are usually dull, brittle and poor conductors of heat and electricity (such as sulphur, oxygen).
What are metalloids?+
Metalloids are elements that show a mix of both metallic and non-metallic properties, such as silicon and boron — they are useful in electronics as semiconductors.
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Change Log
- 17 September 2026 — English page published — mirrors the fact-checked Hindi page, independently bilingual-accuracy checked