Exam practice

formic acid (in an ant’s sting; a bee’s venom is mostly proteins, but this is the usual answer)

base

salt

citric acid

acid

The hairs of a nettle (sisnu) are tiny hollow needles. When they touch the skin, their tips break off and inject formic acid and other irritating chemicals, which make the skin burn and itch.

Common salt (sodium chloride) is a neutral salt: its solution is neither acidic nor basic. Litmus changes color only in an acid or a base.

A lemon contains citric acid, and acids taste sour.

Strong acids are corrosive: they burn the skin and eyes, make holes in clothes and eat into metals. Even a drop can cause a serious burn, so they are handled carefully, with the bottle closed and away from the face.

Ash contains bases (mostly potassium carbonate). Bases feel slippery and soapy: they react with the oils on your skin and turn them into a kind of soap.

An acid is a substance that tastes sour and turns blue litmus red (and methyl orange red). Examples from home: citric acid in lemons and oranges, lactic acid in curd (also acetic acid in vinegar, tartaric acid in tamarind).

Hydrochloric acid, sulphuric acid and nitric acid.

A base is a substance that tastes bitter, feels soapy and turns red litmus blue (and phenolphthalein pink). A base that dissolves in water is an alkali. Bases in daily life: soap, ash water, slaked lime (chuna), antacid tablets (aluminium and magnesium hydroxides), toothpaste, and washing soda and baking soda solutions (basic salts).

An organic acid is an acid obtained from plants or animals. Organic acids are weak, and many are eaten in food. Examples: citric acid (lemon), lactic acid (curd), acetic acid (vinegar), tartaric acid (tamarind), malic acid (apple), ascorbic acid (amala).

Add two drops of methyl orange to each tube.

• The tube in which it turns red holds the acid.
• In the other two it stays yellow, because methyl orange is yellow in both a base and a neutral salt. To tell them apart, add a drop of phenolphthalein to each (or dip red litmus): the one that turns pink (or turns red litmus blue) holds the base, and the one that shows no change holds the salt.

Acids taste sour; turn blue litmus red; turn methyl orange red; leave phenolphthalein colorless; and strong acids are corrosive.

Bases taste bitter; feel soapy; turn red litmus blue; turn methyl orange yellow; and turn phenolphthalein pink.

A salt is the substance formed, along with water, when an acid and a base cancel each other out. Common salt (sodium chloride) is an example. Salts are of three types: neutral (common salt), acidic (alum, ammonium chloride) and basic (washing soda, baking soda).

Crush the petals of a red flower (rose or hibiscus) and soak them in a little warm water. Filter the colored juice. Soak strips of white filter paper in the juice and hang them up to dry. The dried strips turn pink-red in an acid and green in a base, like litmus paper (Figure 10.2).

On the change in their color. An indicator has one color in an acid and another in a base, and doesn’t change in a neutral salt solution: for example, blue litmus turns red in an acid, and phenolphthalein turns pink in a base.

Dip a red and a blue litmus strip into each.

• Orange juice turns blue litmus red (red litmus doesn’t change): it is an acid (citric acid).
• Washing soda solution turns red litmus blue (blue doesn’t change): it is basic. (Washing soda is a basic salt, so it acts as a base here.)
• Salt solution changes neither strip: it is a neutral salt.

Any three of: red cabbage leaves, turmeric, red rose or hibiscus petals, red onion skins, beetroot.

bad conductors

iodine

potassium chlorate (in the head of a safety match, with sulphur)

zinc

chalcopyrite

Our bodies need a little iodine to keep the thyroid gland healthy; without it, the gland swells (goiter), and babies’ brains may not develop properly. Everyone eats salt every day, so adding iodine to salt makes sure everyone gets enough.

Gold is very unreactive: it doesn’t combine with oxygen, water or most other substances. So, unlike most metals, it stays as pure metal in the rocks and river sand.

Aluminium is light (about a third the density of iron), strong when mixed with a little of other metals, and doesn’t rust, because a thin oxide layer protects it. A lighter vehicle or airplane needs less fuel.

Plastic is a bad conductor of heat. The metal body of the cooker gets very hot, but the plastic handle stays cool enough to hold safely.

Sulphur burns in air with a blue flame when it is hot enough. Near volcanoes, very hot sulphur gases come out of the ground and catch fire as they meet the air. The blue flame is faint, so it is seen only at night.

Gold

Iron

Sulphur

Iodine

Metals are usually hard and shiny; they conduct heat and electricity well; they can be beaten into sheets (malleable) and drawn into wires (ductile); they make a ringing sound when struck; and most have high melting points and are solid at room temperature (mercury is the exception).

In a metal, the atoms hold their outermost electrons only loosely. These electrons are free to move about through the whole metal. When a metal is joined to a cell, the free electrons drift along it, and that flow is an electric current. When one end is heated, the free electrons pick up energy and carry it quickly to the cooler parts. Non-metals have almost no free electrons, so they conduct badly.

Iodine (tincture of iodine, an antiseptic for wounds) and sulphur (ointments for skin diseases).
Other uses: iodine in iodized salt; sulphur for sulphuric acid, matches and gunpowder; carbon as charcoal fuel and graphite in pencils; oxygen for breathing (in hospitals too); nitrogen to make fertilizers; chlorine to make drinking water safe.