A mixture is a combination of two or more substances that are physically combined, not chemically bonded, and each substance keeps its own properties.
HOW CAN WE CLASSIFY MIXTURES?
Mixtures are 2 types: Homogeneous & Heterogeneous.
- Homogeneous mixture (Solution): It has a uniform composition with evenly distributed components. E.g. sugar + water, salt + water, vinegar (acetic acid + water), aerated drinks like soda (CO2 + water) etc.
- A well-stirred mixture of sugar and water is equally sweet in the first and the last sip.
- Heterogeneous mixture: It has a non-uniform composition with visibly separate parts. E.g. sand + water, oil + water, salad etc.
- The sand particles are easily visible in the water and settle with time.
Activity:
- Take three beakers A, B and C. Add 50 ml water to each.
- To beaker A: Add one spatula of common salt. Stir well.
- To beaker B: Add one spatula of chalk powder. Stir well.
- To beaker C: Add a few drops of milk. Stir well.
- Particles are not visible in beaker A (salt solution) and beaker C (milk colloid), but the chalk powder particles in beaker B are visible.
- Direct the light from a laser pointer through the beakers. The path of the laser beam is not visible in beaker A, but visible in beakers B and C because the larger particles scatter the light.
- Leave the beakers undisturbed for a few minutes. Beakers A & C remain uniform and stable. In beaker B, the chalk particles gradually settle down at the bottom.
- Set up a filtration apparatus and filter each mixture. A residue of chalk powder will be left on the filter paper from beaker B, but no residue from beakers A and C.
- Inference: These are different types of mixtures.
SOLUTIONS
When a solute (the substance that gets dissolved) is mixed with a solvent (the substance that dissolves the solute), it forms a solution. E.g. In the mixture of sugar and water, sugar is the solute, and water is the solvent.
Concentration of a solution
- The right proportion is essential to make a solution. E.g.
- Oral Rehydration Solution (ORS) requires specific amounts of salt and sugar in a fixed amount of water. If the proportion is changed, it will not be ORS, but only a solution of sugar and salt.
- The right amount of pesticide must be mixed with a fixed amount of water. Too little is ineffective and too much can harm crops, soil and the environment.
- Amount of solute dissolved in a given amount of solvent or solution is called concentration of the solution.
- Understanding concentration is important in laboratories and everyday life, including medicine, agriculture, food, cosmetics, and making tea.
Meet a Scientist
Indian paediatrician Dilip Mahalanabis developed ORS treatment for dehydration from diarrhoea and cholera. Popularized by World Health Organization (WHO), this revolutionary rehydration therapy saved millions of lives.
How do we express concentration?
A common method of expression is percentage.
A. Mass by mass percentage (% m/m or % w/w)
- It is used to express the concentration of homogeneous mixtures. It indicates the grams of solute present in 100 grams of solution.
Example: If 10 g of salt is dissolved in 90 g of water, calculate the mass-by-mass percentage of the solution formed.
Answer:
Mass of salt (solute) = 10 g
Mass of water (solvent) = 90 g
Total mass of solution = Mass of solute + Mass of solvent = 10 g + 90 g = 100 g
- This method is also used for heterogeneous mixtures, such as milk powder and spice mixtures. It is also used to label the composition of packaged foods, showing the amount of salt, sugar, or protein present.
- Industries commonly use weight-by-weight percentage (% w/w), using weight and mass interchangeably. Numerically, % m/m and % w/w are equal.
Example: If 10 g of salt is dissolved in 90 g of water, calculate the mass-by-mass percentage of the solution formed.
Answer:
Mass of salt (solute) = 10 g
Mass of water (solvent) = 90 g
Total mass of solution = Mass of solute + Mass of solvent = 10 g + 90 g = 100 g
10 g / 100 g x 100 = 10% m/m
B. Mass by volume percentage (% m/v or % w/v)
- This method is used in medicine, laboratories etc. where measuring liquid volume is easier than weighing, e.g., 5% glucose solution.
- It indicates grams of solute present in 100 mL solution.
Example: If 5 g of glucose is dissolved in water to make 100 mL of solution, calculate its concentration in mass by volume percentage.
Answer:
Mass of glucose (solute) = 5 g
Volume of solution = 100 mL
Concentration = (Mass / Volume) x 100 = 5g / 100mL x 100 = 5% m/v
Threads of Curiosity: A saline drip in hospitals contains 0.9% (m/v) sodium chloride in water, i.e., 0.9 g salt in 100 mL solution. It is safe and replaces lost body fluids.
C. Volume by volume percentage (% v/v)
- This method is used when two miscible liquids are mixed. E.g., in perfumes, cosmetics and vinegar.
- It indicates how many millilitres of the solute is present in 100 mL solution.
Example: If 1 mL of a liquid pesticide is mixed with a sufficient amount of water to form 100 mL of a pesticide spray for rice crop, calculate its volume-by-volume percentage.
Answer:
- Volume of pesticide (solute) = 1 mL
- Total volume of solution = 100 mL
Example: If 1 mL of a liquid pesticide is mixed with a sufficient amount of water to form 100 mL of a pesticide spray for rice crop, calculate its volume-by-volume percentage.
Answer:
Volume of pesticide (solute) = 1 mL
Total volume of solution = 100 mL
= (1 mL / 100 mL) × 100 = 1% v/v
Threads of Curiosity: A saline drip in hospitals contains 0.9% (m/v) sodium chloride in water, i.e., 0.9 g salt in 100 mL solution. It is safe and replaces lost body fluids.
Solubility of substances
- The maximum amount of solute that dissolves in a fixed quantity of the solvent (100 mL or 100 g) at a given temperature is called its solubility.
- A solution that cannot dissolve any more solute at that temperature is called a saturated solution.
- Solubility of a solid solute in a liquid solvent increases with temperature.
- Solubility of gases in liquids generally decreases with an increase in temperature.
- Solubility is an important property used to separate substances from mixtures.
Activity: Graphical representation of solubility
- Consider water as the solvent, and compounds 'A' and 'B' as the solutes. Each substance has a different solubility. A graph of solubility versus temperature is called a solubility curve.
- X-axis shows temperature (°C) and the Y-axis shows the solubility of the solute in grams per 100 g of water.
- The graph shows that compound 'B' will dissolve more than Compound 'A' at a given temperature.
Solubility curves of compounds ‘A’ and ‘B’ in water
(i) The solubility of compound 'A' in water at 20 °C is less than its solubility at 60 °C.
(ii) The solubility of compound 'B' at 20 °C is less than its solubility at 60 °C.
(iii) The solubility of compound ‘B’ increases more than that of compound ‘A’ with an increase in the temperature.
METHODS OF SEPARATION OF HOMOGENEOUS MIXTURES
Crystallization
- Consider a saturated solution of compound 'B' (287 g in 100 g water) at 60 °C. If it is gradually cooled to 40 °C, some solute will separate out. This is due to the solubility of B is 241 g per 100 g water at 40 °C. So, 46 g (287g – 241 g) will separate out as pure crystals.
- A crystal is a solid made up of particles arranged in a regular geometric pattern. E.g., Rock salt crystals, Sugar crystals, Snowflakes, Frost on windows.
- The process of forming crystals from a saturated solution is called crystallization.
- This method is used to separate two solids when one is present in a small amount and both are soluble in the same solvent, and for the purification of solids. Its principle is based on the differences in the solubility of a substance at different temperatures.
Activity:
- Dissolve 1g copper sulphate in 25 mL water. Add one drop of dilute sulfuric acid (prevents unwanted reactions). Gently heat in a water bath while stirring.
- Add more CuSO4 until the solution becomes saturated.
- Filter the hot solution to remove insoluble impurities.
- Cool it slowly without disturbing. It forms large, shiny, well-shaped, blue-coloured crystals.
- Filter the crystals, rinse with cold water and dry on a watch glass.
Think as a Scientist
Rapid cooling of a hot, saturated copper sulphate solution produces smaller, less well-formed crystals than slow cooling at room temperature.
Test it: Prepare a hot saturated copper sulphate solution and divide it into two equal parts. Cool one slowly at room temperature and the other rapidly in ice-cold water. Compare the size, shape, and number of crystals formed.
Distillation
- It is the separation of a homogeneous mixture of two miscible liquids by heating until the lower-boiling liquid vaporises and condensing the vapour back into liquid.
- It recovers the solvent or separates liquids with boiling-point differences of at least 25°C.
- It is also used to separate a liquid from a solution containing dissolved solids. Here, vapours of the lower-boiling liquid are passed through a condenser cooled by circulating water or air and condensed as a pure liquid. The pure liquid is collected in a separate vessel. The solid or other liquid from the mixture remains in the distillation flask.
- A mixture of acetone and water can be separated by distillation because acetone (56°C) boils much earlier than water (100°C). It allows acetone to vaporise before significant amount of water vapour forms.
Distillation set-up
India's Scientific Contributions
- The distillation process has long been used for various purposes, including the extraction of floral fragrances for perfumes.
- The pleasant earthy scent rising from the ground after the first rain is captured in Kannauj, Uttar Pradesh, to create a natural perfume called Mitti ka Ittar.
- This perfume is produced using the traditional, generational Deg-Bhapka distillation method.
- Mitti ka Ittar is in high demand both within India and internationally.
- The Fragrance and Flavour Development Centre in Kannauj provides facilities for separating fragrance components from various plant parts.
- This centre also assists farmers in cultivating fragrant plants and provides business expertise to aspiring perfumers.
Ready to Go Beyond
- A petroleum refinery is an industrial unit that processes crude oil to produce petroleum products (mixtures of gases and liquids, like petroleum gas, petrol, kerosene, and diesel).
- Crude petroleum is extracted from the Earth's crust and separated into fractions using fractional distillation. It is the separation of components of a mixture that have boiling point differences of less than 25 °C.
- The gaseous fraction is pressurized into steel cylinders to form Liquefied Petroleum Gas (LPG).
Paper Chromatography
When a drop of water falls on sketch-pen writing, the colour spreads. With black ink, different colours may separate out.
Activity:
- Draw a horizontal line 2 cm from the bottom of a 3 cm wide chromatographic paper strip. Mark a black ink spot at its centre.
- Add a thin layer of water to a gas jar, measuring cylinder, or beaker.
- Place the paper vertically with its lower end in water, keeping the ink spot above the water level.
- As water rises through the paper, the ink separates into different coloured spots.
- This method of separating the components of a mixture is called paper chromatography.
- It separates mixture components based on their different interactions with the solvent and paper. They are carried by liquid at different speeds.
- To separate green food colour, 2% m/v salt solution is used as the solvent. It can also separate spinach leaf pigments, and flower pigments.
- In some cases, it needs to use a different solvent, such as alcohol or a mixture of solvents instead of water.
Paper chromatography
Threads of Curiosity:
The word 'Chromatography' means 'writing with colour' - Greek chroma (colour) and graphein (to write). It was first used to separate dyes and inks.
Pause and Ponder
State whether the following statements are True or False. Also, correct the False statements.
- (i) Salt can be separated from a salt solution by evaporation or distillation: True
- (ii) Distillation can be used for separation of two liquids even when these have the same boiling point: False
Correction: Distillation requires a significant difference in boiling points (at least 25 °C) to separate two liquids. - (iii) In paper chromatography, the solvent level should be above the sample spot at the beginning of the experiment: False
Correction: The solvent level must be below the sample spot at the beginning of the experiment. - (iv) Evaporation and crystallization are the same processes: False
Correction: They are different processes. Evaporation turns a liquid into vapor to leave a dry solid residue. Crystallization slowly cools a hot saturated solution to form pure, solid crystals.
HOW CAN WE SEPARATE THE COMPONENTS OF HETEROGENEOUS MIXTURES?
Separation of two immiscible liquids
- Oil and water are immiscible liquids and form separate layers. Similarly, sand and water, and iron filings and sulfur, do not mix.
Activity:
- Pour a mixture of 5 mL mustard oil and 20 mL water into a 50 mL separating funnel.
- Yellow mustard oil forms the upper layer and water forms the lower layer.
- Open the stopcock to collect the lower water layer, discard the small mixed portion, then collect the upper oil layer separately.
Separation of immiscible liquids
Think as a Scientist
What if two immiscible liquids of the same density are mixed in a separating funnel, how will the layers form?
Answer: Since their densities are identical, gravity acts on them equally, preventing them from separating into top and bottom layers. Instead, they will break into droplets and stay suspended in each other, forming emulsion or suspension.
- Gas particles move freely in all directions, making most gas mixtures homogeneous.
- Smoke (solid particles in air), fog (liquid water droplets in air), and dust in air are heterogeneous mixtures involving gases.
- Most solid-solid mixtures are heterogeneous.
Sublimation
Activity:
- Place a mixture of crushed camphor and sand in a china dish on a tripod stand.
- Plug the nozzle of a dry glass funnel with cotton and invert it over the dish.
- Heat the dish gently using a burner.
- Solid camphor deposits as white powder on the inner walls of the funnel, while sand stays in the dish.
- On heating (below its melting point), a solid changes directly into vapour without becoming a liquid is called Sublimation.
- On cooling, vapours condense back into a solid without becoming a liquid. This is called Deposition.
- Camphor separates from sand because it sublimes upon heating, while sand does not.
Sublimation of camphor
Alloys
- Metals do not dissolve in each other at room temperature. However, when molten, they mix to form a solution. On cooling, it solidifies into a new material appearing as a single metal.
- A homogeneous mixture of two or more metals, or a metal and a non-metal, is called an alloy. Its components cannot be separated by physical methods.
- Examples of alloys:
- Brass: 80% copper + 20% zinc.
- Bronze: 80% copper + 20% tin.
- Stainless steel: Usually a mixture of iron, carbon (0.03 – 0.8%), chromium (16 – 18%), nickel (10.0 – 14.0%) and molybdenum (2.0 – 3.0%).
Pause and Ponder
- 6. Why do immiscible liquids form two separate layers in a separating funnel?
Answer: They form two separate layers according to their densities, with the denser liquid at the bottom and the lighter liquid on top. - 7. Is sublimation different from evaporation? Justify.
Answer: Yes. In sublimation, a solid changes directly into a gas without becoming a liquid. In evaporation, a liquid changes into a gas from its surface.
Suspensions
- In a mixture of sand and water, sand particles remain visible even after stirring.
- Such heterogeneous mixtures where solid particles do not dissolve but remain suspended in the medium are called suspensions.
- Suspension particles are visible to the naked eye.
A. Centrifugation
- When left undisturbed, heavier mud particles in water settle at the bottom. Filtering muddy water often leaves it cloudy.
- Techniques like centrifugation and coagulation are used to remove these fine suspended particles.
- Centrifugation involves spinning a mixture in a tube at high speed. During this, the tubes become horizontal. Centrifugal force moves heavier particles outward to the bottom of the tube, leaving lighter liquid at the top.
- Centrifugation is used in laboratories to separate blood components and in many chemical industries.
Centrifugation machine (a) outer view (b) inner view
Bridging Science and Society: The Paperfuge
A paperfuge is a hand-powered low-cost device based on a common toy that performs centrifugation without electricity. By spinning blood samples at high speed, it separates heavier components from lighter ones, like a laboratory centrifuge. This helps detect diseases like malaria and anaemia in remote areas.
Paperfuge
B. Coagulation
- It is a process in which a substance is added to a liquid mixture to make tiny, suspended particles clump together into larger, solid masses.
- These clumps settle by gravity (sedimentation) and can be separated from water by decantation or filtration.
- The substance that causes coagulation is called coagulant. E.g.,
- Adding powdered alum (fitkari) to muddy water causes fine suspended particles to clump together. Alum is a white crystalline substance used in water purification.
- Adding an acid (lemon juice / vinegar) into milk causes coagulation of milk proteins to form cheese (paneer).
Process of coagulation
Colloids
- A colloid is a mixture in which very small particles of one substance are uniformly dispersed throughout another substance and do not settle down over time. E.g., blood, milk, tomato sauce and ice cream.
- They can be coagulated and separated by centrifugation.
- Solutions, suspensions and colloids differ in the size of the dissolved or the suspended particles:
- In a solution: Smallest particles of solute (< 1 nm).
- In colloids: Larger particles (1 – 1000 nm).
- In suspensions: Much larger particles (> 1000 nm).
Bridging Science and Society
- Blood donation saves lives by providing blood for emergencies, surgeries, and serious illnesses.
- Donated blood is tested, grouped, and separated into plasma, platelets, white blood cells, and red blood cells, which are stored in blood banks.
- The body naturally replaces donated blood within a few weeks, making blood donation safe.
Components of blood: a) before centrifugation b) after centrifugation
TYNDALL EFFECT
- The scattering of light by particles is called the Tyndall effect (first explained by John Tyndall).
- Examples:
- When laser light is passed through mixture of chalk powder or milk in water, the path is visible due to Tyndall effect.
- Light beams entering a dark room through a hole are visible because dust and smoke particles in the air scatter the light. It is also observed in the floodlights in sports stadium.
- Tyndall effect occurs when it passes through a colloid or suspension, but not through a transparent solution.
Components of a colloid:
- Dispersed phase: It is the solute-like component or the dispersed particles in a colloid.
- Dispersion medium: It is the component in which the dispersed phase is suspended.
Threads of Curiosity:
Some colloids have both the dispersed phase and dispersion medium as liquids. They are called emulsions.
Based on the nature of the dispersed phase and the dispersion medium, emulsions containing oil and water are 2 types:
- Oil-in-water: E.g., Milk and vanishing creams.
- Water-in-oil: E.g., butter, body lotions, cold cream.
Some medicines are prepared as emulsions to disperse them in water and make them easily palatable. This also reduces the greasy feeling of the liquid medicine.
Presence of emulsifying agents stabilises emulsions. E.g., proteins in milk and butter.
An emulsion can be made by shaking a few drops of cooking oil with water containing a few drops of soap solution.
Pause and Ponder
- 8. Clouds are made up of tiny water droplets or ice crystals floating in the air. Based on what you know about solutions, suspensions and colloids, what type of mixture do you think clouds are and why?
Answer: Clouds are colloids. Reason: Tiny water droplets or ice crystals stay suspended evenly in the air without settling under gravity or being caught by ordinary filters. - 9. Why do cities with a lot of smoke and dust in the air often look hazy?
Answer: Tyndall effect. Suspended smoke and dust particles act as colloids that scatter passing sunlight in all directions, creating a blurry, uniform haze.
Properties of different types of mixtures
| S. No. | Property | Solution | Suspension | Colloid |
|---|---|---|---|---|
| 1 | Nature | Homogeneous | Heterogeneous | Heterogeneous |
| 2 | Particle size | < 1 nm | > 1000 nm | 1 - 1000 nm |
| 3 | Visibility | Not visible | Visible to naked eye | Not visible to naked eye |
| 4 | Separation by filtration | Cannot be separated | Can be separated | Cannot be separated |
| 5 | Settling | Does not settle | Settles down | Does not settle |
| 6 | Tyndall effect | Does not scatter light | Scatters light | Scatters light |