Introducing Science Summary Sheet

Transcription

1 Introducing Science Summary Sheet Acids in the laboratory Dilute acids You will have used some dilute acids at school, such as hydrochloric acid, sulphuric acid and nitric acid. Their bottles are labelled with the warning symbol for 'irritant'. This means that if any of them makes contact with your skin, it will become red or blistered. You must wash off any spills with plenty of water, otherwise your skin will soon feel as if it is burning. Concentrated acids You are unlikely to have used concentrated acids but your teacher might have shown you some experiments with them. This is because concentrated acids are corrosive. They can attack metals and destroy skin if spilled. Acids in the home Laboratory acids are far too dangerous to taste, but you will have swallowed some dilute weak acids. Acids have a sour taste, like vinegar, which contains ethanoic acid, and lemons, which contain citric acid. These are safe to use in food, but they can still hurt if they get into a cut or into your eyes. Other acids you will find at home are carbonic acid in fizzy drinks, tannic acid in tea and ascorbic acid which is vitamin C, found in fruit and vegetables. Bases and alkalis Bases v alkalis Bases are substances that react with acids and neutralise them. They are usually metal oxides, metal hydroxides, metal carbonates or metal hydrogen carbonates. Many bases are insoluble - they do not dissolve in water. If a base does dissolve in water, we call it an alkali. Here are two examples: Copper oxide is a base because it will react with acids and neutralise them, but it is not an alkali because it does not dissolve in water. Sodium hydroxide is a base because it will react with acids and neutralise them. It's also an alkali because it dissolves in water.

2 Introducing Science Summary Sheet Bases in the laboratory You will have used some strong bases and alkalis at school, such as sodium hydroxide solution. Like acids, their bottles are labelled with the warning symbol for 'irritant'. This means that they will make your skin red or blistered unless you wash off any spills with plenty of water. Alkalis feel soapy when they get on your skin, so it is easy to tell when you have had an accident and must wash your hands. SOLID Concentrated alkalis are corrosive. They can attack metals and destroy skin if spilled. They are just as Solids are made up of particles that are very close dangerous as concentrated acids, but many people do not realise this. together and are held tightly together by strong Bases in the home bonds. Solids cannot be squashed, do not flow, have Bases react with oils and fats, so they are often used a in fixed strong shape household and volume, cleaners. and are Drain dense cleaners (are heavy and oven cleaners usually contain sodium hydroxide for example. for And their ammonia size). (Volume is also commonly is the amount used of in cleaners. space Ammonia can be recognised by its choking smell. something takes up. It is measured in units such as cubic centimetres (cm 3 ).) It is wise to wear gloves when using these substances, otherwise they will react with your skin and burn it. LIQUID Weak bases and alkalis are found in toothpaste, antacid tablets (to help cure an upset stomach) and baking Liquids are made up of particles that are fairly powder. close together; the bonds between the particles are Indicators and the ph scale weaker than the bonds in solids. This allows the particles to move past each other. When an acid is dissolved in water we get an acidic solution, Liquids and cannot alkalis be squashed, make alkaline flow solutions. quite easily If and a have a solution is neither acidic nor alkaline we call it neutral. fixed Pure volume water is but neutral, no fixed and shape. so is paraffin. Although they are dense, liquids are usually less dense Indicators are substances that change colour when they are added to acidic or alkaline solutions. You can than solids. prepare homemade indicators from red cabbage or beetroot juice - these will help you see if a solution is acidic or alkaline. AS Litmus and universal indicator are two indicators that are commonly used in the laboratory. 7 Litmus Litmus indicator solution turns red in acidic solutions and blue in alkaline solutions - and it turns purple in neutral solutions.

3 7 Introducing Science Summary Sheet Universal indicator and the ph scale Universal indicator is a mixture of several different indicators. Unlike litmus, universal indicator can show us exactly how strongly acidic or alkaline a solution is. This is measured using the ph scale. The ph scale runs from ph 0 to ph 14. Universal indicator has many different colour changes, from red for strong acids to dark purple for strong bases. In the middle, neutral ph 7 is indicated by green. 7 Universal indicator shows how acidic or alkaline a solution is These are the important points about the ph scale: neutral solutions are ph 7 exactly acidic solutions have ph values less than 7 alkaline solutions have ph values more than 7 the closer to ph 0 you go, the more strongly acidic a solution is the closer to ph 14 you go, the more strongly alkaline a solution is A chemical reaction happens if you mix together an acid and a base. The reaction is called neutralisation, AS and a neutral solution is made if you add just the right amount of acid and base together. Using neutralisation Farmers use lime (calcium oxide) to neutralise acid soils. Your stomach contains hydrochloric acid, and too much of this causes indigestion. Antacid tablets contain bases such as magnesium hydroxide and magnesium carbonate to neutralise the extra acid. Bee stings are acidic. They can be neutralised using baking powder, which contains sodium hydrogen carbonate.

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