5.1. Quantities and units To measu re a quant it y, we need a un it . For example, we can use the cent imetre to measu re a leng th. We w r ite the resu lt of the measu rement as a number fol lowed by the un it . 5.2. The International System of Units (SI) A lmost a l l countr ies, includ ing Spa in, use the same units of measurement : the Internationa l System of Units (SI). It was designed to ma ke commercia l and scient i f ic exchanges easier. 16 How can we describe the amount of water that is in a glass? 17 Why do we use metre, kilogram and second, and not other units of measurement? A quantit y is a proper t y of mat ter that can be measu red . It can be w r it ten as a number and a unit. The SI def ines seven f undamenta l quantities. The SI a lso def ines the units of measurement of a l l the quant it ies. A unit is a k now n amount of a quant it y that we use as a reference to measu re that quant it y. A un it must be: Constant : it is the same at any t ime and in any place. Universa l: anyone can use it . Easy to reproduce: examples of it a re easy to f ind . Measur ing a quant it y is compa r ing it w ith a un it to see how many of them it conta ins. Un l i ke the size of a hand , the cent imetre is a un it that meets these requ i rements. We cannot measu re the f lavou r of th is ju ice. However, we can say its volume is 30 cL and its temperatu re is 8 °C. The other quant it ies a re der ived quantities. Thei r un its a re def ined by the un its of the seven f undamenta l quant it ies. For example, speed is measu red in m/s. 5. Measurement 6 hands 4 hands 80 cm SI k g K s cd mol m A mass kilogram temperature kelvin luminous intensity candela time second electric current intensity ampere amount of substance mole length metre CHALLENGE 20
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