Mixtures and their types
A glass of lemonade has lemon juice, sugar, salt and water. A bowl of bhel has puffed rice, onion, sev and chutney. In both, the things are mixed together, yet each one still tastes like itself.
A mixture is made when two or more substances are mixed and each keeps its own properties. The substances in it are its components. They do not react chemically with one another.
Key idea 1: Two kinds of mixtures — In a non-uniform mixture the components can be seen separately, with the eye or a magnifying glass, like onion and sev in bhel or sand in water. In a uniform mixture the components are spread evenly and cannot be told apart, even under a microscope, like sugar in water.
- Easy: Sand and water is a non-uniform mixture; salt in water is a uniform mixture.
- Medium: Oil and water form a non-uniform mixture because the oil floats as a separate layer.
- Exam-type: Table of mixture types: air is gas in gas; soda water is gas in liquid; vinegar is liquid in liquid; seawater is solid in liquid; baking powder is solid in solid.
In the figure: If you can point at the parts, it is non-uniform; if everything looks the same, it is uniform.
Key idea 2: Alloys — An alloy is a uniform mixture of a metal with other substances, often other metals. Stainless steel has iron, nickel, chromium and a little carbon. Brass is copper and zinc; bronze is copper and tin. Alloys are made because they can be stronger and more durable than a pure metal. In ancient India such a mixture of metals was called Mishraloha.
- Easy: Brass is a mixture of copper and zinc.
- Medium: Bronze is copper and tin, and ancient Indian texts mention it as Kamsya.
- Exam-type: Stainless steel looks the same all through because its iron, nickel, chromium and carbon are mixed uniformly.
In the figure: Each alloy is a blend of metals mixed evenly, so it looks like one metal.
In a mixture the components keep their properties and do not react with one another.
Thinking that a mixture whose parts cannot be seen is not a mixture. Sugar water is a uniform mixture.
Ask: can I see the parts? Yes means non-uniform; no means uniform. Memory clue: See-able = Separate-looking = Non-uniform.
Spot the difference
| Feature | Uniform mixture | Non-uniform mixture |
|---|---|---|
| parts | not visible | visible |
| spread | even | uneven |
| example | sugar in water | sand in water |
Practice questions
- 1What are the parts of a mixture called?
- 2Give one example of a uniform mixture.
- 3Name the two metals in brass.
- 4Why is bhel a non-uniform mixture?
- 5What is an alloy? Give one example with its components.
- 6Do the components of a mixture react chemically? Explain.
- 7Classify as uniform or non-uniform: vinegar, muddy water, bronze, oil and water.
- 8A friend says that salt water is not a mixture because the salt cannot be seen. Correct her.
Show answers (try them yourself first)
- 1Components.
- 2Sugar in water (also air, brass, vinegar).
- 3Copper and zinc.
- 4Its components, like puffed rice, onion and sev, can be seen separately.
- 5A uniform mixture of a metal with other substances; for example bronze, made of copper and tin.
- 6No. In a mixture each component keeps its own properties and there is no chemical reaction.
- 7Vinegar: uniform; muddy water: non-uniform; bronze: uniform; oil and water: non-uniform.
- 8Salt water is a uniform mixture; its components cannot be seen separately, but they are still there and can be separated again by evaporation.
Air as a mixture, lime water and dust
A cold glass of water kept outside soon has droplets on its outside. On a sunny morning, tiny specks dance in the beam of light coming through a window. Both tell us that air carries more than we think.
Air is a uniform mixture of mainly nitrogen, oxygen, argon, carbon dioxide and water vapour. Nitrogen is about 78% of it. Oxygen is needed for breathing and helps burning; nitrogen does not take part in burning.
Key idea 1: Lime water tests for carbon dioxide — Calcium oxide (quick lime) reacts vigorously with water to give calcium hydroxide and heat. A clear solution of calcium hydroxide is lime water. Left open to air for a few hours, it turns milky, because carbon dioxide of the air reacts with it: calcium hydroxide + carbon dioxide gives calcium carbonate + water. Calcium carbonate is a white solid that does not dissolve.
- Easy: Lime water turns milky when it meets carbon dioxide.
- Medium: Warm air touching a cool glass leaves water droplets because the water vapour in air condenses.
- Exam-type: Of two fresh lime water samples, only the one kept open turns milky, as only it can meet carbon dioxide of the air.
In the figure: The steps show how a clear liquid becomes milky only after air reaches it.
Key idea 2: Dust is not part of air — A black paper kept near an open window slowly collects tiny specks. This shows dust floating in the air. Dust and soot are particulate matter and count as pollutants, not as components of air, because their kind and number change from place to place and time to time. Carbon monoxide, ozone, nitrogen dioxide and sulfur dioxide are gaseous pollutants. The air quality index (AQI) describes how clean the air is.
- Easy: Dust and soot in air are pollutants.
- Medium: The specks seen in a sunbeam are dust particles floating in air.
- Exam-type: Nitrogen, oxygen and the other gases are always in air, but the amount of dust changes with place and time, so dust is not a component of air.
In the figure: Nitrogen is the biggest part of air and oxygen the next; all other gases together are very small.
Air is a uniform mixture of gases; lime water turns milky in carbon dioxide; dust is a pollutant.
Thinking that oxygen is the largest part of air. Nitrogen is about 78%.
Milky lime water means carbon dioxide. Memory clue: Lime turns Milky = CO2 is Mixed in.
Spot the difference
| Item | In air | Is it a component? |
|---|---|---|
| nitrogen, oxygen | always present | yes |
| carbon dioxide | present in small amount | yes |
| dust, soot | varies with place | no, a pollutant |
Practice questions
- 1Which gas forms about 78% of air?
- 2Which gas turns lime water milky?
- 3What does AQI tell us?
- 4Why does air count as a mixture?
- 5Write the word equation for lime water turning milky.
- 6How can we show that dust is present in air?
- 7Fresh lime water is kept in one closed bottle and one open dish. Which turns milky and why?
- 8Why is dust not called a component of air?
Show answers (try them yourself first)
- 1Nitrogen.
- 2Carbon dioxide.
- 3How clean or polluted the air is.
- 4Its gases are only mixed; they keep their own properties and do not react.
- 5Calcium hydroxide + carbon dioxide gives calcium carbonate + water.
- 6Keep a clean black paper near an open window for a few hours; tiny specks collect on it.
- 7The open dish, because carbon dioxide of the air reaches it; the closed bottle gets no fresh air.
- 8Its kind and amount vary from place to place and time to time; it is a pollutant.
Pure substances and elements
A packet of ghee says pure. A packet of salt says pure. In the shop, pure means that nothing cheap has been added. In the science lab, however, the word has a stricter meaning.
In science, a pure substance has no other substance in it; all its particles are of the same kind. A pure substance is either an element or a compound. An element cannot be broken into simpler substances, and each element is made of identical atoms.
Key idea 1: Two meanings of pure — In daily use, pure means unadulterated, that is, no cheap or poor-quality substance is added illegally. In science, a product that looks pure can still be impure if it has more than one substance in it. Milk, packed fruit juice and soil are mixtures; sugar and baking soda are pure substances.
- Easy: Adulteration is adding cheaper or poorer substances illegally.
- Medium: Milk looks clean but is a mixture of several substances, so a scientist calls it impure.
- Exam-type: Sugar is a pure substance; fruit juice is a mixture.
In the figure: A pure substance has one kind of particle; a mixture has several kinds.
Key idea 2: Elements, atoms and molecules — Elements are the building blocks of all matter: gold, silver, iron, sulfur, carbon, hydrogen and oxygen. The atoms of most elements cannot exist alone; two or more join to form a stable molecule, as two hydrogen atoms make one hydrogen molecule. Elements are grouped as metals, non-metals and in-between metalloids. There are 118 known elements. Most are solids, eleven are gases and only two, mercury and bromine, are liquids at room temperature.
- Easy: Gold and sulfur are elements.
- Medium: Silicon and boron are metalloids, with properties between metals and non-metals.
- Exam-type: Mercury is a liquid metal and bromine is a liquid non-metal at room temperature.
In the figure: Every element belongs to one of the three groups.
A pure substance has one kind of particle; elements cannot be broken into simpler substances.
Calling a clean-looking product a pure substance. In science, pure means one kind of particle only.
Ask: how many kinds of particles? One means pure; many means mixture. Memory clue: One Kind = Pure; Many Kinds = Mixture.
Spot the difference
| Word | Everyday meaning | Science meaning |
|---|---|---|
| pure | nothing cheap added | one kind of particle |
| milk | pure if unadulterated | a mixture |
| gold | pure jewellery | an element |
Practice questions
- 1What is an element?
- 2Name two metals and two non-metals.
- 3How many elements are known at present?
- 4Why is milk called impure in science?
- 5What is a molecule of an element? Give one example.
- 6Name the two elements that are liquid at room temperature.
- 7Classify as pure substance or mixture: sugar, soil, baking soda, packed fruit juice.
- 8Explain the difference between the everyday and the scientific meaning of pure.
Show answers (try them yourself first)
- 1A pure substance that cannot be broken down into simpler substances.
- 2Metals: gold, iron; non-metals: carbon, sulfur.
- 3118.
- 4It contains more than one substance, so it is a mixture.
- 5A stable particle of two or more atoms of the same element, such as a hydrogen molecule of two hydrogen atoms.
- 6Mercury and bromine.
- 7Sugar and baking soda: pure substances; soil and packed fruit juice: mixtures.
- 8In daily use, pure means no cheap substance has been added; in science, pure means the substance has only one kind of particle.
Pure substances and elements
In the app you can move this figure with your finger.
Open in the appCompounds
Hydrogen is a fuel that burns with a pop, and oxygen helps things burn. Yet water, made from both, puts out fire. The same thing happens with salt, made from a soft metal and a harmful gas but safe on our food.
A compound is formed when different elements combine chemically in a fixed ratio to give something entirely new. Its properties are different from those of its elements, and its elements cannot be separated by any physical method.
Key idea 1: Water is a compound — When electricity is passed through water mixed with a little acid, bubbles of two gases collect, one at each end. One gas burns with a pop: hydrogen. In the other a candle burns brighter: oxygen. These gases are not water vapour, or they would have condensed back to water. In water, hydrogen and oxygen atoms are in the ratio 2:1. Sodium chloride has sodium and chlorine in the ratio 1:1.
- Easy: Water is a compound of hydrogen and oxygen.
- Medium: A pop sound shows hydrogen; a brighter flame shows oxygen.
- Exam-type: The collected gases cannot be water vapour because water vapour would condense back to liquid water.
In the figure: Water breaks into two different gases, so it is not an element.
Compounds
In the app you can move this figure with your finger.
Open in the appKey idea 2: Sugar is also a compound — When dry sugar is heated gently, it turns brown, then black, and drops of water collect near the cool open end of the tube. Black carbon is left behind. The water cannot come from air because the tube is hot, so it comes from the sugar. Water holds hydrogen and oxygen, so sugar is a compound of carbon, hydrogen and oxygen, and not an element.
- Easy: Heating dry sugar gives carbon and water.
- Medium: Sugar is not an element because it breaks into simpler substances.
- Exam-type: Evaporating salt water separates salt from water, but it cannot split salt into sodium and chlorine.
In the figure: The compound water behaves unlike either of its elements.
A compound has a fixed ratio and new properties; its elements cannot be separated by physical methods.
Calling water a mixture of hydrogen and oxygen. They are chemically combined in a fixed 2:1 ratio.
Fixed ratio plus new properties means compound. Memory clue: Compound = Combined for Keeps.
Spot the difference
| Feature | Element | Compound |
|---|---|---|
| made of | one kind of atom | two or more elements |
| breaks down | cannot | yes, into elements |
| example | oxygen | water |
Practice questions
- 1Name the two elements in water.
- 2What is the ratio of hydrogen to oxygen atoms in water?
- 3Which gas gives a pop sound near a flame?
- 4Why can the gases collected from water not be water vapour?
- 5Why is sugar not an element?
- 6Why can the elements of a compound not be separated by physical methods?
- 7Salt is made of a soft metal and a hazardous gas but is safe to eat. What does this show?
- 8A friend says that water is a mixture of hydrogen and oxygen. Correct her.
Show answers (try them yourself first)
- 1Hydrogen and oxygen.
- 22:1.
- 3Hydrogen.
- 4Water vapour would have condensed back into water.
- 5On heating it breaks down into carbon and water, which are simpler substances.
- 6They are tightly, chemically combined.
- 7A compound has properties entirely different from its elements.
- 8They are chemically combined in a fixed 2:1 ratio and water has new properties, so it is a compound.
Mixture versus compound, and minerals
If you mix iron nails and sulfur powder in a bowl, a magnet can still pull the iron out. But if you heat them together, a new black solid forms and the magnet does nothing. The same two materials give two very different results.
A mixture keeps the properties of its components and can be separated by physical means. A compound is a new substance with its own properties. Minerals are natural solid substances found in the Earth, with a fixed chemical composition; most are compounds, a few are pure elements.
Key idea 1: Iron and sulfur, two outcomes — Sample A, iron filings and sulfur powder only mixed, is a mixture: both can be seen, a magnet pulls the iron out, and dilute hydrochloric acid gives hydrogen, a colourless gas with no smell that burns with a pop, while sulfur stays as a yellow solid. Sample B, made by heating them, is iron sulfide, a compound: a black mass the same throughout, not attracted by a magnet. Dilute hydrochloric acid gives hydrogen sulfide, a colourless gas with a rotten-egg smell.
- Easy: Sample A is a mixture and sample B is a compound.
- Medium: A magnet does not attract iron sulfide because it is a new substance, not iron.
- Exam-type: The acid test gives different gases: hydrogen from the mixture and hydrogen sulfide from the compound.
In the figure: The same two elements give a mixture or a compound depending on whether they are only mixed or heated together.
Key idea 2: Minerals and uses — Most rocks are mixtures of minerals. Native minerals such as gold and sulfur are pure elements; quartz, calcite, mica and talc are compounds. Talcum powder comes from talc, and cement is made from calcite, quartz, alumina and iron oxide. Wood, steel and concrete are mixtures used in building, and alloys like stainless steel are stronger than pure iron. Not everything is matter: light, heat and electricity are not.
- Easy: Talcum powder is made from the mineral talc.
- Medium: Most rocks are mixtures of different minerals.
- Exam-type: Magnesium, oxygen and magnesium oxide are, in order, an element, an element and a compound.
In the figure: Minerals are mostly compounds, a few are elements, and they give us everyday materials.
Mixture: properties kept, separable. Compound: new substance, fixed ratio.
Thinking that iron sulfide is only a mixture of iron and sulfur. After heating, a new compound has formed.
Test with a magnet and an acid. Memory clue: Mixed = Magnet works; Joined = Magnet fails.
Spot the difference
| Feature | Mixture | Compound |
|---|---|---|
| properties | components keep theirs | entirely new |
| ratio | any | fixed |
| separation | physical methods | not by physical methods |
Practice questions
- 1What is formed when iron filings and sulfur are only mixed?
- 2What is formed when they are heated together?
- 3From which mineral is talcum powder made?
- 4Why does a magnet not attract iron sulfide?
- 5Which gas comes from the mixture with dilute hydrochloric acid?
- 6Name two native minerals.
- 7Magnesium burns in oxygen to give magnesium oxide. Classify all three substances.
- 8Give two differences between a mixture and a compound.
Show answers (try them yourself first)
- 1A mixture.
- 2A compound, iron sulfide.
- 3Talc.
- 4It is a new substance with properties different from iron.
- 5Hydrogen gas.
- 6Gold and sulfur.
- 7Magnesium: element; oxygen: element; magnesium oxide: compound.
- 8A mixture keeps the properties of its components and can be separated physically; a compound has new properties and a fixed ratio and cannot be split by physical methods.