LEARN · PRACTISE · DISCOVER
Materials, Acids, Bases and Changes
This course covers the second half of EVS science: natural phenomena, materials, transport and services, our environment, and science methodology. We begin with objects and materials: their properties and classification, metals and non-metals and the reactivity order, methods of separation, saturated and unsaturated solutions, acids, bases, salts and indicators, neutralisation, physical and chemical changes, rusting, galvanisation and crystallisation.
Materials, metals and non-metals
Materials are grouped by properties: appearance (shiny or dull), hardness, solubility in water, whether they float or sink, whether light passes through them (transparent, translucent, opaque), and whether they conduct heat and electricity.
| Property | Metals | Non-metals |
|---|---|---|
| Lustre | shiny | dull (iodine is an exception) |
| Malleability | can be beaten into sheets | brittle |
| Ductility | can be drawn into wires | not ductile |
| Sonority | produce a ringing sound | not sonorous |
| Conduction | good conductors of heat and electricity | poor conductors (graphite conducts electricity) |
| State | solid, except mercury (liquid) | solid, liquid (bromine) or gas |
| Oxides | basic (turn red litmus blue) | acidic (turn blue litmus red) |
Reactivity order (most to least reactive, common school list): potassium > sodium > calcium > magnesium > aluminium > zinc > iron > lead > copper > mercury > silver > gold. A more reactive metal displaces a less reactive one from its salt solution: an iron nail in blue copper sulphate solution turns the solution green (iron sulphate) and gets coated with brown copper. Copper cannot displace iron. Sodium is stored in kerosene because it reacts vigorously with air and water; gold stays shiny because it is very unreactive.
Separation, solutions and crystallisation
| Method | Used to separate | Example |
|---|---|---|
| Handpicking, sieving | larger pieces or particles of different sizes | stones from rice; bran from flour |
| Threshing, winnowing | grain from stalks; grain from husk (by wind) | paddy |
| Sedimentation and decantation | heavier insoluble solid from a liquid | sand and water |
| Loading | speeding up sedimentation by adding alum | muddy water |
| Filtration | insoluble solid from a liquid | tea leaves from tea |
| Evaporation | dissolved solid from a liquid | salt from sea water |
| Distillation (evaporation + condensation) | pure liquid from a solution | distilled water |
| Churning | lighter particles from a liquid | butter from curd |
| Magnet | magnetic from non-magnetic | iron filings from sand |
A solution has a solute dissolved in a solvent (sugar in water). A saturated solution cannot dissolve any more solute at that temperature; an unsaturated one can. Most solids dissolve more in hot water.
Crystallisation obtains pure, regular crystals from a hot saturated solution by cooling it slowly (copper sulphate crystals; pure salt). Evaporation gives salt, but crystallisation gives purer crystals.
Acids, bases, salts and indicators
| Acids | Bases | |
|---|---|---|
| Taste and feel | sour | bitter, soapy to touch |
| Litmus | blue → red | red → blue |
| Turmeric | no change | yellow → red |
| China rose (hibiscus) | dark pink (magenta) | green |
| Phenolphthalein | colourless | pink |
| Examples | citric (lemon, orange), acetic (vinegar), lactic (curd), tartaric (tamarind), formic (ant sting), hydrochloric (stomach) | soap, baking soda, lime water (calcium hydroxide), milk of magnesia |
Indicators change colour in acids and bases. Litmus comes from lichens; turmeric and China rose are natural indicators.
Neutralisation: acid + base → salt + water (heat is released).
- Indigestion (too much stomach acid): an antacid such as milk of magnesia.
- Ant sting (formic acid): rub baking soda solution or calamine.
- Acidic soil: add quicklime or slaked lime; basic soil: add organic matter (compost).
- Factory wastes are neutralised before release.
Physical and chemical changes
| Physical change | Chemical change |
|---|---|
| no new substance; often reversible | new substance formed; usually irreversible |
| melting ice, dissolving sugar, cutting paper, stretching a rubber band | burning, rusting, cooking, curdling of milk, ripening of fruit |
Rusting happens when iron is exposed to both oxygen and water (moisture), forming iron oxide (rust). It is faster near the sea because of salty, humid air. Prevention: painting, oiling or greasing, galvanisation (coating iron with zinc), chromium plating, and making stainless steel (an alloy of iron with chromium, nickel and carbon).
Exam focus
- Mercury is a liquid metal; bromine is a liquid non-metal.
- More reactive metals displace less reactive ones: iron + copper sulphate → green solution.
- Butter by churning; salt by evaporation; pure crystals by crystallisation.
- Turmeric turns red in bases; China rose turns green in bases.
- Rusting needs oxygen and water; galvanisation uses zinc.
Example
Mrs Leela gives Class 5 groups turmeric paper and asks them to dab lemon juice, soap water and baking soda solution on it. The red spots on soap and baking soda surprise everyone. Children then make their own “turmeric indicator” at home and test kitchen items.
Key takeaways
- Metals are lustrous, malleable, ductile, sonorous and conductive; non-metals generally are not.
- A more reactive metal displaces a less reactive metal from its salt.
- Choose a separation method by the properties of the mixture’s parts.
- Saturated solutions dissolve no more solute; crystallisation gives pure crystals.
- Acids turn blue litmus red, bases turn red litmus blue; acid + base → salt + water.
- Rusting is a chemical change that needs air and moisture; galvanisation prevents it.
Practice
- Which metal is a liquid at room temperature?
- (A) sodium
- (B) iron
- (C) mercury
- (D) copper
- The property of metals that allows them to be drawn into wires is:
- (A) ductility
- (B) malleability
- (C) sonority
- (D) lustre
- When an iron nail is placed in copper sulphate solution, the colour of the solution changes from:
- (A) blue to red
- (B) colourless to blue
- (C) green to blue
- (D) blue to green
- Butter is separated from curd by:
- (A) filtration
- (B) churning
- (C) sieving
- (D) winnowing
- China rose indicator turns ___ in a basic solution.
- (A) dark pink
- (B) green
- (C) blue
- (D) colourless
- Which of these is a chemical change?
- (A) melting of ice
- (B) dissolving salt in water
- (C) cutting paper
- (D) rusting of iron
- Galvanisation is the coating of iron with:
- (A) zinc
- (B) tin
- (C) copper
- (D) chromium
- An ant sting contains formic acid. It can be treated by rubbing:
- (A) vinegar
- (B) lemon juice
- (C) baking soda solution
- (D) curd
Answers and explanations (tap to show)
- (C) Mercury is the only metal that is liquid at room temperature.
- (A) Ductility means a metal can be drawn into wires.
- (D) Iron displaces copper, forming green iron sulphate solution.
- (B) Churning separates the lighter butter from curd or cream.
- (B) China rose turns green in bases and dark pink in acids.
- (D) Rust is a new substance (iron oxide).
- (A) Galvanised iron has a protective layer of zinc.
- (C) Baking soda is a base and neutralises the acid. Vinegar, lemon juice and curd are acidic.