Why Sea Water Is a Mixture but Table Salt Is Not
Learn to tell homogeneous from heterogeneous mixtures, distinguish true solutions, suspensions and emulsions by particle size, compare compounds with mixtures, and classify air, brass and soil.
Why is sea water called a mixture while table salt is called a compound?
Because sea water contains several substances in no fixed proportion, each keeping its own properties, while table salt is a single substance whose two elements are chemically combined in a fixed ratio.
You can boil the salt out of sea water with no chemistry at all; you cannot get sodium out of salt that way. This page covers everything in the ICSE Class 7 Chemistry chapter's second part: types of mixtures, solutions and suspensions, compounds compared with mixtures, and why familiar substances fall where they do.
You can boil the salt out of sea water with no chemistry at all; you cannot get sodium out of salt that way. This page covers everything in the ICSE Class 7 Chemistry chapter's second part: types of mixtures, solutions and suspensions, compounds compared with mixtures, and why familiar substances fall where they do.
What is a mixture, and when is it homogeneous?
A mixture contains two or more substances physically mixed in any proportion, with each keeping its own properties.
A homogeneous mixture has the same composition throughout, and its parts cannot be told apart even with a microscope. Sugar dissolved in water, salt solution, air, brass and steel are homogeneous.
A heterogeneous mixture has an uneven composition, and its parts can be seen or separated. Sand and salt, oil and water, a mixture of iron filings and sulphur, muddy water and soil are heterogeneous.
An Indian kitchen has both on the same shelf: a cup of sweetened tea is homogeneous, while a bowl of dal with rice is plainly heterogeneous.
Two features of any mixture are worth stating together, because they appear in every comparison question. The proportion is variable — you can make tea more or less sweet — and the components can be separated by physical methods such as filtering or evaporating.
The case students misjudge is air. It looks like a single substance, but it is a homogeneous mixture, because its gases are in no fixed ratio and each keeps its own properties.
A homogeneous mixture has the same composition throughout, and its parts cannot be told apart even with a microscope. Sugar dissolved in water, salt solution, air, brass and steel are homogeneous.
A heterogeneous mixture has an uneven composition, and its parts can be seen or separated. Sand and salt, oil and water, a mixture of iron filings and sulphur, muddy water and soil are heterogeneous.
An Indian kitchen has both on the same shelf: a cup of sweetened tea is homogeneous, while a bowl of dal with rice is plainly heterogeneous.
Two features of any mixture are worth stating together, because they appear in every comparison question. The proportion is variable — you can make tea more or less sweet — and the components can be separated by physical methods such as filtering or evaporating.
The case students misjudge is air. It looks like a single substance, but it is a homogeneous mixture, because its gases are in no fixed ratio and each keeps its own properties.
How do a true solution, a suspension and an emulsion differ?
They differ mainly in particle size, and everything else follows from that.
True solution — particles are extremely small and completely dissolved. It is transparent, the particles never settle on standing, and they pass through filter paper. Sugar in water, salt in water, copper sulphate solution.
Suspension — particles are comparatively large and undissolved. It is opaque or cloudy, the particles settle on standing, and they are retained by filter paper. Chalk powder in water, muddy water, sand in water.
Emulsion — a mixture of two liquids that do not normally mix, one dispersed as fine droplets in the other. It is cloudy and separates on standing unless something stabilises it. Milk, oil shaken with water, paint.
In a solution, the substance that dissolves is the solute and the one that dissolves it is the solvent. In salt water, salt is the solute and water the solvent; in sugar syrup, sugar is the solute. The solvent is normally the component present in the larger amount, and water is called the universal solvent because it dissolves so many substances.
Muddy water from a monsoon puddle makes the contrast obvious — leave it standing and the mud settles at the bottom, which a glass of salt water will never do.
So the settling test is the quickest way to tell them apart in a practical question: a true solution never settles, a suspension always does.
True solution — particles are extremely small and completely dissolved. It is transparent, the particles never settle on standing, and they pass through filter paper. Sugar in water, salt in water, copper sulphate solution.
Suspension — particles are comparatively large and undissolved. It is opaque or cloudy, the particles settle on standing, and they are retained by filter paper. Chalk powder in water, muddy water, sand in water.
Emulsion — a mixture of two liquids that do not normally mix, one dispersed as fine droplets in the other. It is cloudy and separates on standing unless something stabilises it. Milk, oil shaken with water, paint.
In a solution, the substance that dissolves is the solute and the one that dissolves it is the solvent. In salt water, salt is the solute and water the solvent; in sugar syrup, sugar is the solute. The solvent is normally the component present in the larger amount, and water is called the universal solvent because it dissolves so many substances.
Muddy water from a monsoon puddle makes the contrast obvious — leave it standing and the mud settles at the bottom, which a glass of salt water will never do.
So the settling test is the quickest way to tell them apart in a practical question: a true solution never settles, a suspension always does.
How does a compound differ from a mixture?
On four points, and answers are marked point by point.
Composition. A compound's elements are in a fixed proportion by mass; a mixture's components are in any proportion.
Properties of the constituents. In a compound the constituents lose their own properties, and the compound behaves as a new substance. In a mixture each component keeps its properties.
Energy change. Forming a compound involves absorption or release of energy, often with heat or light. Making a mixture involves no noticeable energy change.
Separation. A compound's elements can be separated only by chemical means; a mixture's components separate by physical methods — filtration, evaporation, magnetic separation.
A worked comparison shows all four at once. Iron filings mixed with sulphur is a yellow-and-grey powder: a magnet still pulls the iron out, no heat is given off on mixing, and the proportions can be anything. Heat that mixture and it forms iron sulphide, a single black solid — a magnet no longer separates it, heat was given out, and the ratio is now fixed.
That pair of experiments is the chapter's best question, because the same two elements give a mixture in one case and a compound in the other. What changed is whether they merely sat together or actually combined.
Composition. A compound's elements are in a fixed proportion by mass; a mixture's components are in any proportion.
Properties of the constituents. In a compound the constituents lose their own properties, and the compound behaves as a new substance. In a mixture each component keeps its properties.
Energy change. Forming a compound involves absorption or release of energy, often with heat or light. Making a mixture involves no noticeable energy change.
Separation. A compound's elements can be separated only by chemical means; a mixture's components separate by physical methods — filtration, evaporation, magnetic separation.
A worked comparison shows all four at once. Iron filings mixed with sulphur is a yellow-and-grey powder: a magnet still pulls the iron out, no heat is given off on mixing, and the proportions can be anything. Heat that mixture and it forms iron sulphide, a single black solid — a magnet no longer separates it, heat was given out, and the ratio is now fixed.
That pair of experiments is the chapter's best question, because the same two elements give a mixture in one case and a compound in the other. What changed is whether they merely sat together or actually combined.
Why are air, brass and soil mixtures while water is a compound?
Because each mixture fails the fixed-proportion test and keeps its components' properties, while water passes both.
Air — a homogeneous mixture of nitrogen, oxygen, carbon dioxide, water vapour and noble gases. The proportions vary with place and weather, each gas keeps its own properties, and the gases can be separated physically by liquefying and then distilling the air.
Sea water — a mixture of water with dissolved salts in varying amounts. Boil it and the salts are left behind, a purely physical separation.
Brass — a mixture, specifically an alloy, of copper and zinc. Its proportion can be varied to make a harder or softer brass, which is exactly why alloys are useful.
Soil — a heterogeneous mixture of sand, clay, minerals, water, air and decayed matter, all visible as separate parts.
Water — a compound. Its hydrogen and oxygen are chemically combined in a fixed 1 : 8 ratio by mass, its properties resemble neither gas, and only an electric current can split it.
Common salt — a compound of sodium and chlorine in a fixed ratio, with properties unlike either element.
An alloy is the classification students find hardest, so it is worth fixing. Brass looks like a single gold-coloured metal, and it is homogeneous — but it is still a mixture, because there is no chemical combination and no fixed proportion.
Air — a homogeneous mixture of nitrogen, oxygen, carbon dioxide, water vapour and noble gases. The proportions vary with place and weather, each gas keeps its own properties, and the gases can be separated physically by liquefying and then distilling the air.
Sea water — a mixture of water with dissolved salts in varying amounts. Boil it and the salts are left behind, a purely physical separation.
Brass — a mixture, specifically an alloy, of copper and zinc. Its proportion can be varied to make a harder or softer brass, which is exactly why alloys are useful.
Soil — a heterogeneous mixture of sand, clay, minerals, water, air and decayed matter, all visible as separate parts.
Water — a compound. Its hydrogen and oxygen are chemically combined in a fixed 1 : 8 ratio by mass, its properties resemble neither gas, and only an electric current can split it.
Common salt — a compound of sodium and chlorine in a fixed ratio, with properties unlike either element.
An alloy is the classification students find hardest, so it is worth fixing. Brass looks like a single gold-coloured metal, and it is homogeneous — but it is still a mixture, because there is no chemical combination and no fixed proportion.
Exam tip
Exam tip: proving your classification with two reasons
"Is it a compound or a mixture?" questions are marked on the reason, so give two.
Use the fixed-proportion test and the properties test together: air is a mixture because its gases occur in no fixed proportion and each retains its own properties. One reason earns half the mark.
For comparison questions, answer property by property — composition, properties of constituents, energy change, separation — in the same order each time.
For a solution-versus-suspension question, name the settling behaviour and the filter paper result, since those are the two observable tests.
And always identify the solute and solvent when a solution is named. In sea water the salts are the solute and water the solvent — the larger amount is the solvent.
Use the fixed-proportion test and the properties test together: air is a mixture because its gases occur in no fixed proportion and each retains its own properties. One reason earns half the mark.
For comparison questions, answer property by property — composition, properties of constituents, energy change, separation — in the same order each time.
For a solution-versus-suspension question, name the settling behaviour and the filter paper result, since those are the two observable tests.
And always identify the solute and solvent when a solution is named. In sea water the salts are the solute and water the solvent — the larger amount is the solvent.
Did you know
Why does a magnet stop working on iron once it is heated with sulphur?
Because the iron is no longer present as iron.
In the unheated mixture, the iron filings sit beside the sulphur untouched, keeping every property they had — including being attracted to a magnet. Heating makes the two elements combine into iron sulphide.
In that compound the atoms are chemically bound into a new substance, and iron sulphide is not magnetic. Nothing was added or taken away; the same atoms simply stopped being two separate substances.
In the unheated mixture, the iron filings sit beside the sulphur untouched, keeping every property they had — including being attracted to a magnet. Heating makes the two elements combine into iron sulphide.
In that compound the atoms are chemically bound into a new substance, and iron sulphide is not magnetic. Nothing was added or taken away; the same atoms simply stopped being two separate substances.
Key takeaways
Mixtures, solutions and compounds: quick revision
- A mixture holds two or more substances physically in any proportion, each keeping its own properties, separable by physical methods.
- Homogeneous mixtures are uniform throughout (salt solution, air, brass); heterogeneous ones are not (soil, sand and salt, oil and water).
- A true solution is transparent and never settles; a suspension is cloudy, settles on standing and is held back by filter paper; an emulsion is one liquid dispersed in another.
- The solute dissolves and the solvent does the dissolving, with the solvent normally the larger amount.
- A compound has a fixed proportion by mass, its constituents lose their properties, energy is involved in its formation, and only chemical means separate it.
- Air, sea water, brass and soil are mixtures; water and common salt are compounds — and iron with sulphur is a mixture until heated, when it becomes non-magnetic iron sulphide.
You will remember all of this far better after answering five questions on it than after reading it twice.
- Homogeneous mixtures are uniform throughout (salt solution, air, brass); heterogeneous ones are not (soil, sand and salt, oil and water).
- A true solution is transparent and never settles; a suspension is cloudy, settles on standing and is held back by filter paper; an emulsion is one liquid dispersed in another.
- The solute dissolves and the solvent does the dissolving, with the solvent normally the larger amount.
- A compound has a fixed proportion by mass, its constituents lose their properties, energy is involved in its formation, and only chemical means separate it.
- Air, sea water, brass and soil are mixtures; water and common salt are compounds — and iron with sulphur is a mixture until heated, when it becomes non-magnetic iron sulphide.
You will remember all of this far better after answering five questions on it than after reading it twice.