context: The density of glass depends on its chemical composition. Pure quartz glass (fused silica) is considered to have a minimum density among silicate glasses - 2200 kg / m³ (although some borosilicate glasses are less dense). On the contrary, the density of glass (crystal, lead glass, etc.) containing oxides of heavy elements - lead, bismuth, tantalum, barium - reaches 7500 kg / m³. The density of ordinary sodium-calcium-silicate glass, including window glass, ranges from 2500-2600 kg / m³. As the temperature rises from room temperature to 1300 ° С, the density of most glasses decreases by 6-12%, that is, on average for every 100 ° С the density decreases by 15 kg / m³. The Young's modulus (elastic modulus) of the glasses also depends on their chemical composition and can vary from 48 · 10³ to 12 · 10³ MPa. For example, in quartz glass, the modulus of elasticity is 71.4 · 10³ MPa. To increase the elasticity, silicon oxide is partially replaced by oxides of calcium, aluminum, magnesium, and boron. On the contrary, metal oxides reduce the elastic modulus, since the strength of the MeO bonds is much lower than the SiO bond strength. The shear modulus is 20,000–30,000 MPa, and the Poisson's ratio is 0.25. Strength: For ordinary glass, the compressive strength is 500 to 2000 MPa (for window glass about 1000 MPa). The tensile strength of glass is much less, which is why the tensile strength of glass in bending is measured by the tensile strength. This strength ranges from 35 to 100 MPa. By tempering the glass, it is possible to increase its strength by 3-4 times. Another way to increase strength is ion exchange diffusion. It also significantly increases the strength of the glass by treating their surface with chemical reagents in order to remove surface defects (the smallest cracks, scratches and so on). The hardness of glass, like many other properties, depends on impurities. On the Mohs scale, it is 6-7 units, which is between the hardness of apatite and quartz. The hardest are quartz and low alkali borosilicate glass. With an increase in the content of alkaline oxides, the hardness of the glass decreases. The softest is lead glass. Fragility. In the region of relatively low temperatures (below the melting point), glass breaks from mechanical stress without noticeable plastic deformation and, therefore, belongs to ideally brittle materials (along with diamond and quartz). This property may be reflected by the specific impact strength. As in previous cases, a change in the chemical composition makes it possible to regulate this property as well: for example, the introduction of bromine almost doubles its impact strength. For silicate glasses, the impact strength is from 1.5 to 2 kN / m, which is 100 times inferior to iron. The thermal conductivity of the glass is very insignificant and is equal to 0.0017-0.032 cal / (cm · s · deg) or from 0.711 to 1.399 W / (m · K). For window panes, this number is 0.0023 (0.96). The melting temperature. Glass is a thermoplastic material, when it is heated, it gradually softens and passes into a liquid. Melting occurs in a certain temperature range, the value of which depends on the chemical composition of the glass. Below the glass transition temperature Tc, the glass becomes brittle. For ordinary silicate glass, Tc = 425–600 ° C. Above the melting point, glass becomes liquid. At these temperatures, glass melt is processed into products.
question: What is the strength of glass?
answers.text: 500 to 2000 MPa