Matter: properties and states. A summary

Transcription

1 Matter: properties and states. A summary Natural Science 1 Secondary Education

2 Matter: properties and states. (1) 1. Matter: properties and measurement Matter is everything that has mass and volume. The different materials or types of matter are also called substances: e.g. water, iron, salt. Each substance has properties that make it different from other substances General properties and specific properties a) The properties Matter has different properties: General properties are those common to all matter, like mass, volume, weight and density, and Specific properties are the characteristics that differentiate one kind of matter from another, like colour, shape, size, texture, hardness, etc. b) Measuring matter Some properties can be measured and they are called magnitudes: the distance between two cities is a magnitude, but the beauty of the landscape between them is not. To measure a magnitude, we have to compare it with a standard measurement that we call a unit. To express a measurement correctly we must indicate a number followed by its unit (20,7 km, 70 g). c) Intemational System of units (I.S.) Scientists have created a system of units so that all of us can agree on how much things measure, the Intemational System (I.S.) Length, surface area, volume and capacity Length is a magnitude which measures the distance between two points. The unit of length in I.S. is the metre (m). Surface area is the magnitude that expresses a body s extension in two dimensions (length and width). The unit of surface area in I.S. is the square metre (m 2 ). Volume is a magnitude derived from length, and it is defined as the amount of space an object occupies. The volume unit in I.S. is the cubic metre (m 3 ). Capacity is the maximum amount of liquid a container used to measure liquids can hold. The unit of capacity we use most frequently is the litre (l or L), which is equivalent to 1 dm 3. Its multiples and submultiples go from 10 to 10, so units of capacity are easy to handle. The I.S. uses some prefixes to indicate the units that are multiples and submultiples of the main unit Mass, weight and density Mass is the amount of matter a body has. This magnitude is measured with scales. Its unit in I.S. is the kilogram (kg). Weight is the force of attraction with which a body, like the Earth, attracts an object:

3 Matter: properties and states. (2) an object with a lot of mass will weigh more than an object with less mass. Mass does not vary from one place to another, but weight does vary. The density of a substance is the relationship between its mass and the volume it occupies. For example, in the case a bottle full of water with 1 L of capacity (1.000 cm 3 ), what is its mass? If you put it on the scales you will notice that it weighs 1,000 g (without counting the mass of the bottle). If you fill it with mercury, it will occupy the same volume, 1,000 cm 3 ; but if you measure its mass, you will see it is 13,600 g. We say mercury has a higher density than water because a certain volume of mercury has more mass and so it weighs more than the same volume of water. Different substances have diíferent densities. Mathematically, density is expressed as follows: Density = Mass/Volume The unit of density in I.S. is the kg/m 3, although we often use the g/cm 3.

4 Matter: properties and states. (3) 2. The states of matter 2.1. Changes of state in matter The different forms in which matter appears are called the states of aggregation and they are solid, liquid and gas. These changes of aggregation are produced by changes in temperature, allowing more or less agitation of the particles. Solids have a fixed volume and shape. Their molecules are in an ordered arrangement and they attract each other strongly which is why they are very close together. They vibrate more or less strongly depending on their temperature. The molecules of liquids are less close together and the forces of attraction between them are weaker. They do not have a shape (they adapt to the receptacle that holds them); they can flow through holes or pipes and they have a constant volume. Gases have mass and weight. Most of them are invisible. The particles are so far apart that they hardly attract each other. They move in a zig-zag motion because they

5 Matter: properties and states. (4) collide with each other and the walls of the container, that holds them at a very high speed. Gases expand. Their particles mix with the particles of other gases present; this is called diffusion. Gases do not have a fixed shape or volume and they can flow just as liquids do. Temperature is used to measure the state of agitation or movement of the particles which form objects. We usually express it in degrees centigrade ( C) Changes of state The particles of matter do not change when matter changes its state: the only thing that varies is their arrangement or energy. When matter changes state its mass does not vary. During the process of a change of state, all the heat used is used to separate and disperse their particles. There is no change in temperature until all the matter has changed its state. The temperature to change of state is a specific property that is characteristic of each substance. 3. Atoms and molecules, elements and compounds. Substances and mixtures Atoms Atoms are the simplest particles that make up matter, the bricks matter is built with. Atoms are represented by balls Molecules Molecules are made up of the union of several atoms that attract each other with strong forces in a permanent way and in fixed proportions. When atoms join together strongly, new particles are formed (molecules) with new properties. The phenomenon by which two or more atoms join together to form a molecule is a chemical reaction Elements Elements are substances formed by atoms of a single type. Elements cannot be broken down into more simple substances. Elements can be present in three ways in nature: atomic elements, molecular elements and elements with a crystal structure Compounds Compounds are substances formed by different atoms, always combined in fixed proportions. Compounds are represented by formulae formed by the symbols of their elements and subscript numbers to indicate the number of atoms of that element in the compound. So, water s formula (H2O) indicates that it is formed by two atoms of hydrogen and one atom of oxygen and the calcite formula (CaCO3) indicates that in that compound there is one atom of calcium, another atom of carbon and three atoms of oxygen. There are two types of compounds: molecular compounds and compounds with a crystal structure. Thus, water is a molecular compound, while calcite is a compound with a crystalline structure.

6 Matter: properties and states. (5) 3.5. Substances and mixtures Pure substances are formed by similar particles (atoms or molecules) but most of the matter around us is a mixture of substances. In some cases we cannot differentiate between their components, since they present a uniform aspect; this is called a homogeneous mixture or solution. For example, sea water, the air, or the steel in a spoon. If we can distinguish the components of the mixture -for example, granite rock-, then we say it is a heterogeneous mixture. The difference between pure substances and mixtures is that the first have well defined specific properties: density, fusion and boiling temperatures, etc., while the mixtures do not. References. All contents of this document have been obtained and adapted from Echevarría, C. and others (2010): Natural Science: basic concepts. Anaya-Inglés.