Acids and alkalis, Year 7: more practice

Hydrochloric acid

提示It is kept, much weaker, in a bottle on the school lab bench.

为什么Stomach acid is hydrochloric acid, strong enough to kill many germs in food. The stomach wall is coated in mucus so that it is not damaged by it.

Alkalis

提示Acids taste sharp; these substances do the opposite and are good at lifting grease.

为什么Alkalis feel slippery or soapy and dissolve grease, which is why cleaning products often contain them. Never taste or touch unknown substances to find out.

It is far more concentrated

提示Think about how much acid is packed into each litre of liquid.

为什么Concentration means how much of the substance is dissolved in a given volume. A little acid in a lot of water is harmless, but the same acid in concentrated form can burn skin.

Green

提示This colour sits exactly halfway along the indicator colour chart.

为什么Universal indicator runs from red for strong acids, through orange and yellow, to green for neutral, then blue to purple for strong alkalis.

Blue

提示Alkalis do the opposite of what acids do to litmus paper.

为什么Red litmus paper turns blue in alkalis, and blue litmus turns red in acids. In a neutral solution neither paper changes colour.

Acids and alkalis look the same, so you need an indicator to show the difference

提示Think about how the two liquids appear before anything is added.

为什么Many acids and alkalis look like plain water. An indicator is a dye that changes colour to show which one you have, so you do not need to taste or touch it.

Danger to the environment

提示The picture shows what the substance can harm if it is poured away carelessly.

为什么This pictogram means the substance is hazardous to the environment, especially to water life. Such chemicals must be collected for disposal, not tipped down a sink.

Naked flames, sparks and heat

提示Think of the things that could set it alight.

为什么A flammable liquid gives off a vapour that catches fire easily. Bunsen burners are turned off or removed before such a liquid is used.

It poisons the body once it gets inside

提示Corrosives harm the surface; this warning is about what happens after it is swallowed or breathed in.

为什么Corrosive means it attacks skin, eyes or metal on contact. Toxic means it is poisonous if it enters the body by swallowing, breathing in or through the skin.

Rinse it under cold running water and tell your teacher

提示Dilute it as quickly as you can, and make sure an adult knows.

为什么Plenty of cold running water dilutes the chemical and washes it away. Telling the teacher means help arrives and the incident is recorded.